6 Basic concepts [basic]

Every name is introduced in some portion of program text called a declarative region, which is the largest part of the program in which that name is valid, that is, in which that name may be used as an unqualified name to refer to the same entity. In general, each particular name is valid only within some possibly discontiguous portion of program text called its scope. To determine the scope of a declaration, it is sometimes convenient to refer to the potential scope of a declaration. The scope of a declaration is the same as its potential scope unless the potential scope contains another declaration of the same name. In that case, the potential scope of the declaration in the inner (contained) declarative region is excluded from the scope of the declaration in the outer (containing) declarative region.

[ Example: In

int j = 24;
int main() {
  int i = j, j;
  j = 42;
}

the identifier j is declared twice as a name (and used twice). The declarative region of the first j includes the entire example. The potential scope of the first j begins immediately after that j and extends to the end of the program, but its (actual) scope excludes the text between the , and the }. The declarative region of the second declaration of j (the j immediately before the semicolon) includes all the text between { and }, but its potential scope excludes the declaration of i. The scope of the second declaration of j is the same as its potential scope.  — end example ]

The names declared by a declaration are introduced into the scope in which the declaration occurs, except that the presence of a friend specifier, certain uses of the elaborated-type-specifier, and using-directives alter this general behavior.

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,

• (4.1)

  they shall all refer to the same entity, or all refer to functions and function templates; or

• (4.2)

  exactly one declaration shall declare a class name or enumeration name that is not a typedef name and the other declarations shall all refer to the same variable, non-static data member, or enumerator, or all refer to functions and function templates; in this case the class name or enumeration name is hidden. [ Note: A namespace name or a class template name must be unique in its declarative region ([namespace.alias], Clause [temp]).  — end note ]

[ Note: These restrictions apply to the declarative region into which a name is introduced, which is not necessarily the same as the region in which the declaration occurs. In particular, elaborated-type-specifiers and friend declarations may introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to that region. Local extern declarations ([basic.link]) may introduce a name into the declarative region where the declaration appears and also introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to both regions.  — end note ]

[ Note: The name lookup rules are summarized in [basic.lookup].  — end note ]

The point of declaration for a name is immediately after its complete declarator and before its initializer (if any), except as noted below. [ Example:

unsigned char x = 12;
{ unsigned char x = x; }

Here the second x is initialized with its own (indeterminate) value.  — end example ]

[ Note: A name from an outer scope remains visible up to the point of declaration of the name that hides it. [ Example:

const int  i = 2;
{ int  i[i]; }

declares a block-scope array of two integers.  — end example ]  — end note ]

The point of declaration for a class or class template first declared by a class-specifier is immediately after the identifier or simple-template-id (if any) in its class-head. The point of declaration for an enumeration is immediately after the identifier (if any) in either its enum-specifier or its first opaque-enum-declaration, whichever comes first. The point of declaration of an alias or alias template immediately follows the type-id to which the alias refers.

The point of declaration of a using-declarator that does not name a constructor is immediately after the using-declarator.

The point of declaration for an enumerator is immediately after its enumerator-definition. [ Example:

const int x = 12;
{ enum { x = x }; }

Here, the enumerator x is initialized with the value of the constant x, namely 12.  — end example ]

After the point of declaration of a class member, the member name can be looked up in the scope of its class. [ Note: This is true even if the class is an incomplete class. For example,

struct X {
  enum E { z = 16 };
  int b[X::z];      // OK
};

 — end note ]

The point of declaration of a class first declared in an elaborated-type-specifier is as follows:

• (7.1)

  for a declaration of the form

  class-key attribute-specifier-seq${}_{opt}$ identifier ;
  

  the identifier is declared to be a class-name in the scope that contains the declaration, otherwise

• (7.2)

  for an elaborated-type-specifier of the form

  class-key identifier
  

  if the elaborated-type-specifier is used in the decl-specifier-seq or parameter-declaration-clause of a function defined in namespace scope, the identifier is declared as a class-name in the namespace that contains the declaration; otherwise, except as a friend declaration, the identifier is declared in the smallest namespace or block scope that contains the declaration. [ Note: These rules also apply within templates.  — end note ] [ Note: Other forms of elaborated-type-specifier do not declare a new name, and therefore must refer to an existing type-name. See [basic.lookup.elab] and [dcl.type.elab].  — end note ]

The point of declaration for an injected-class-name is immediately following the opening brace of the class definition.

The point of declaration for a function-local predefined variable ([dcl.fct.def]) is immediately before the function-body of a function definition.

The point of declaration for a template parameter is immediately after its complete template-parameter. [ Example:

typedef unsigned char T;
template<class T
  = T     // lookup finds the typedef name of unsigned char
  , T     // lookup finds the template parameter
    N = 0> struct A { };

 — end example ]

[ Note: Friend declarations refer to functions or classes that are members of the nearest enclosing namespace, but they do not introduce new names into that namespace ([namespace.memdef]). Function declarations at block scope and variable declarations with the extern specifier at block scope refer to declarations that are members of an enclosing namespace, but they do not introduce new names into that scope.  — end note ]

[ Note: For point of instantiation of a template, see [temp.point]. — end note ]

A name declared in a block is local to that block; it has block scope. Its potential scope begins at its point of declaration and ends at the end of its block. A variable declared at block scope is a local variable.

The potential scope of a function parameter name (including one appearing in a lambda-declarator) or of a function-local predefined variable in a function definition begins at its point of declaration. If the function has a function-try-block the potential scope of a parameter or of a function-local predefined variable ends at the end of the last associated handler, otherwise it ends at the end of the outermost block of the function definition. A parameter name shall not be redeclared in the outermost block of the function definition nor in the outermost block of any handler associated with a function-try-block.

The name declared in an exception-declaration is local to the handler and shall not be redeclared in the outermost block of the handler.

Names declared in the init-statement, the for-range-declaration, and in the condition of if, while, for, and switch statements are local to the if, while, for, or switch statement (including the controlled statement), and shall not be redeclared in a subsequent condition of that statement nor in the outermost block (or, for the if statement, any of the outermost blocks) of the controlled statement; see [stmt.select].

In a function declaration, or in any function declarator except the declarator of a function definition, names of parameters (if supplied) have function prototype scope, which terminates at the end of the nearest enclosing function declarator.

Labels have function scope and may be used anywhere in the function in which they are declared. Only labels have function scope.

The declarative region of a namespace-definition is its namespace-body. Entities declared in a namespace-body are said to be members of the namespace, and names introduced by these declarations into the declarative region of the namespace are said to be member names of the namespace. A namespace member name has namespace scope. Its potential scope includes its namespace from the name's point of declaration onwards; and for each using-directive that nominates the member's namespace, the member's potential scope includes that portion of the potential scope of the using-directive that follows the member's point of declaration. [ Example:

namespace N {
  int i;
  int g(int a) { return a; }
  int j();
  void q();
}
namespace { int l=1; }
// the potential scope of l is from its point of declaration to the end of the translation unit

namespace N {
  int g(char a) {   // overloads N​::​g(int)
    return l+a;     // l is from unnamed namespace
  }

  int i;            // error: duplicate definition
  int j();          // OK: duplicate function declaration

  int j() {         // OK: definition of N​::​j()
    return g(i);    // calls N​::​g(int)
  }
  int q();          // error: different return type
}

 — end example ]

A namespace member can also be referred to after the ​::​ scope resolution operator ([expr.prim]) applied to the name of its namespace or the name of a namespace which nominates the member's namespace in a using-directive; see [namespace.qual].

The outermost declarative region of a translation unit is also a namespace, called the global namespace. A name declared in the global namespace has global namespace scope (also called global scope). The potential scope of such a name begins at its point of declaration and ends at the end of the translation unit that is its declarative region. A name with global namespace scope is said to be a global name.

The potential scope of a name declared in a class consists not only of the declarative region following the name's point of declaration, but also of all function bodies, default arguments, noexcept-specifiers, and brace-or-equal-initializers of non-static data members in that class (including such things in nested classes).

A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.

A name declared within a member function hides a declaration of the same name whose scope extends to or past the end of the member function's class.

The potential scope of a declaration that extends to or past the end of a class definition also extends to the regions defined by its member definitions, even if the members are defined lexically outside the class (this includes static data member definitions, nested class definitions, and member function definitions, including the member function body and any portion of the declarator part of such definitions which follows the declarator-id, including a parameter-declaration-clause and any default arguments).

[ Example:

typedef int  c;
enum { i = 1 };

class X {
  char  v[i];                       // error: i refers to ​::​i but when reevaluated is X​::​i
  int  f() { return sizeof(c); }    // OK: X​::​c
  char  c;
  enum { i = 2 };
};

typedef char*  T;
struct Y {
  T  a;                             // error: T refers to ​::​T but when reevaluated is Y​::​T
  typedef long  T;
  T  b;
};

typedef int I;
class D {
  typedef I I;                      // error, even though no reordering involved
};

 — end example ]

The name of a class member shall only be used as follows:

• (6.1)

  in the scope of its class (as described above) or a class derived from its class,

• (6.2)

  after the . operator applied to an expression of the type of its class ([expr.ref]) or a class derived from its class,

• (6.3)

  after the -> operator applied to a pointer to an object of its class ([expr.ref]) or a class derived from its class,

• (6.4)

  after the ​::​ scope resolution operator ([expr.prim]) applied to the name of its class or a class derived from its class.

The name of a scoped enumerator has enumeration scope. Its potential scope begins at its point of declaration and terminates at the end of the enum-specifier.

The declarative region of the name of a template parameter of a template template-parameter is the smallest template-parameter-list in which the name was introduced.

The declarative region of the name of a template parameter of a template is the smallest template-declaration in which the name was introduced. Only template parameter names belong to this declarative region; any other kind of name introduced by the declaration of a template-declaration is instead introduced into the same declarative region where it would be introduced as a result of a non-template declaration of the same name. [ Example:

namespace N {
  template<class T> struct A { };               // #1
  template<class U> void f(U) { }               // #2
  struct B {
    template<class V> friend int g(struct C*);  // #3
  };
}

The declarative regions of T, U and V are the template-declarations on lines #1, #2, and #3, respectively. But the names A, f, g and C all belong to the same declarative region — namely, the namespace-body of N. (g is still considered to belong to this declarative region in spite of its being hidden during qualified and unqualified name lookup.)  — end example ]

The potential scope of a template parameter name begins at its point of declaration and ends at the end of its declarative region. [ Note: This implies that a template-parameter can be used in the declaration of subsequent template-parameters and their default arguments but cannot be used in preceding template-parameters or their default arguments. For example,

template<class T, T* p, class U = T> class X { /* ... */ };
template<class T> void f(T* p = new T);

This also implies that a template-parameter can be used in the specification of base classes. For example,

template<class T> class X : public Array<T> { /* ... */ };
template<class T> class Y : public T { /* ... */ };

The use of a template parameter as a base class implies that a class used as a template argument must be defined and not just declared when the class template is instantiated.  — end note ]

The declarative region of the name of a template parameter is nested within the immediately-enclosing declarative region. [ Note: As a result, a template-parameter hides any entity with the same name in an enclosing scope. [ Example:

typedef int N;
template<N X, typename N, template<N Y> class T> struct A;

Here, X is a non-type template parameter of type int and Y is a non-type template parameter of the same type as the second template parameter of A.  — end example ]  — end note ]

[ Note: Because the name of a template parameter cannot be redeclared within its potential scope ([temp.local]), a template parameter's scope is often its potential scope. However, it is still possible for a template parameter name to be hidden; see [temp.local].  — end note ]

A name can be hidden by an explicit declaration of that same name in a nested declarative region or derived class ([class.member.lookup]).

A class name or enumeration name can be hidden by the name of a variable, data member, function, or enumerator declared in the same scope. If a class or enumeration name and a variable, data member, function, or enumerator are declared in the same scope (in any order) with the same name, the class or enumeration name is hidden wherever the variable, data member, function, or enumerator name is visible.

In a member function definition, the declaration of a name at block scope hides the declaration of a member of the class with the same name; see [basic.scope.class]. The declaration of a member in a derived class hides the declaration of a member of a base class of the same name; see [class.member.lookup].

During the lookup of a name qualified by a namespace name, declarations that would otherwise be made visible by a using-directive can be hidden by declarations with the same name in the namespace containing the using-directive; see [namespace.qual].

If a name is in scope and is not hidden it is said to be visible.

In all the cases listed in [basic.lookup.unqual], the scopes are searched for a declaration in the order listed in each of the respective categories; name lookup ends as soon as a declaration is found for the name. If no declaration is found, the program is ill-formed.

The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive; see [namespace.udir]. For the purpose of the unqualified name lookup rules described in [basic.lookup.unqual], the declarations from the namespace nominated by the using-directive are considered members of that enclosing namespace.

The lookup for an unqualified name used as the postfix-expression of a function call is described in [basic.lookup.argdep]. [ Note: For purposes of determining (during parsing) whether an expression is a postfix-expression for a function call, the usual name lookup rules apply. The rules in [basic.lookup.argdep] have no effect on the syntactic interpretation of an expression. For example,

typedef int f;
namespace N {
  struct A {
    friend void f(A &);
    operator int();
    void g(A a) {
      int i = f(a);  // f is the typedef, not the friend function: equivalent to int(a)
    }
  };
}

Because the expression is not a function call, the argument-dependent name lookup does not apply and the friend function f is not found.  — end note ]

A name used in global scope, outside of any function, class or user-declared namespace, shall be declared before its use in global scope.

A name used in a user-declared namespace outside of the definition of any function or class shall be declared before its use in that namespace or before its use in a namespace enclosing its namespace.

In the definition of a function that is a member of namespace N, a name used after the function's declarator-id29 shall be declared before its use in the block in which it is used or in one of its enclosing blocks ([stmt.block]) or shall be declared before its use in namespace N or, if N is a nested namespace, shall be declared before its use in one of N's enclosing namespaces. [ Example:

namespace A {
  namespace N {
    void f();
  }
}
void A::N::f() {
  i = 5;
  // The following scopes are searched for a declaration of i:
  // 1) outermost block scope of A​::​N​::​f, before the use of i
  // 2) scope of namespace N
  // 3) scope of namespace A
  // 4) global scope, before the definition of A​::​N​::​f
}

 — end example ]

A name used in the definition of a class X outside of a member function body, default argument, noexcept-specifier, brace-or-equal-initializer of a non-static data member, or nested class definition30 shall be declared in one of the following ways:

• (7.1)

  before its use in class X or be a member of a base class of X ([class.member.lookup]), or

• (7.2)

  if X is a nested class of class Y, before the definition of X in Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class),31 or

• (7.3)

  if X is a local class or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or

• (7.4)

  if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the definition of class X in namespace N or in one of N's enclosing namespaces.

[ Example:

namespace M {
  class B { };
}

namespace N {
  class Y : public M::B {
    class X {
      int a[i];
    };
  };
}

// The following scopes are searched for a declaration of i:
// 1) scope of class N​::​Y​::​X, before the use of i
// 2) scope of class N​::​Y, before the definition of N​::​Y​::​X
// 3) scope of N​::​Y's base class M​::​B
// 4) scope of namespace N, before the definition of N​::​Y
// 5) global scope, before the definition of N

 — end example ] [ Note: When looking for a prior declaration of a class or function introduced by a friend declaration, scopes outside of the innermost enclosing namespace scope are not considered; see [namespace.memdef].  — end note ] [ Note: [basic.scope.class] further describes the restrictions on the use of names in a class definition. [class.nest] further describes the restrictions on the use of names in nested class definitions. [class.local] further describes the restrictions on the use of names in local class definitions.  — end note ]

For the members of a class X, a name used in a member function body, in a default argument, in a noexcept-specifier, in the brace-or-equal-initializer of a non-static data member, or in the definition of a class member outside of the definition of X, following the member's declarator-id32, shall be declared in one of the following ways:

• (8.1)

  before its use in the block in which it is used or in an enclosing block ([stmt.block]), or

• (8.2)

  shall be a member of class X or be a member of a base class of X ([class.member.lookup]), or

• (8.3)

  if X is a nested class of class Y, shall be a member of Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class),33 or

• (8.4)

  if X is a local class or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or

• (8.5)

  if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the use of the name, in namespace N or in one of N's enclosing namespaces.

[ Example:

class B { };
namespace M {
  namespace N {
    class X : public B {
      void f();
    };
  }
}
void M::N::X::f() {
  i = 16;
}

// The following scopes are searched for a declaration of i:
// 1) outermost block scope of M​::​N​::​X​::​f, before the use of i
// 2) scope of class M​::​N​::​X
// 3) scope of M​::​N​::​X's base class B
// 4) scope of namespace M​::​N
// 5) scope of namespace M
// 6) global scope, before the definition of M​::​N​::​X​::​f

 — end example ] [ Note: [class.mfct] and [class.static] further describe the restrictions on the use of names in member function definitions. [class.nest] further describes the restrictions on the use of names in the scope of nested classes. [class.local] further describes the restrictions on the use of names in local class definitions.  — end note ]

Name lookup for a name used in the definition of a friend function defined inline in the class granting friendship shall proceed as described for lookup in member function definitions. If the friend function is not defined in the class granting friendship, name lookup in the friend function definition shall proceed as described for lookup in namespace member function definitions.

In a friend declaration naming a member function, a name used in the function declarator and not part of a template-argument in the declarator-id is first looked up in the scope of the member function's class ([class.member.lookup]). If it is not found, or if the name is part of a template-argument in the declarator-id, the look up is as described for unqualified names in the definition of the class granting friendship. [ Example:

struct A {
  typedef int AT;
  void f1(AT);
  void f2(float);
  template <class T> void f3();
};
struct B {
  typedef char AT;
  typedef float BT;
  friend void A::f1(AT);      // parameter type is A​::​AT
  friend void A::f2(BT);      // parameter type is B​::​BT
  friend void A::f3<AT>();    // template argument is B​::​AT
};

 — end example ]

During the lookup for a name used as a default argument in a function parameter-declaration-clause or used in the expression of a mem-initializer for a constructor ([class.base.init]), the function parameter names are visible and hide the names of entities declared in the block, class or namespace scopes containing the function declaration. [ Note: [dcl.fct.default] further describes the restrictions on the use of names in default arguments. [class.base.init] further describes the restrictions on the use of names in a ctor-initializer.  — end note ]

During the lookup of a name used in the constant-expression of an enumerator-definition, previously declared enumerators of the enumeration are visible and hide the names of entities declared in the block, class, or namespace scopes containing the enum-specifier.

A name used in the definition of a static data member of class X (after the qualified-id of the static member) is looked up as if the name was used in a member function of X. [ Note: [class.static.data] further describes the restrictions on the use of names in the definition of a static data member.  — end note ]

If a variable member of a namespace is defined outside of the scope of its namespace then any name that appears in the definition of the member (after the declarator-id) is looked up as if the definition of the member occurred in its namespace. [ Example:

namespace N {
  int i = 4;
  extern int j;
}

int i = 2;

int N::j = i;       // N​::​j == 4

 — end example ]

A name used in the handler for a function-try-block is looked up as if the name was used in the outermost block of the function definition. In particular, the function parameter names shall not be redeclared in the exception-declaration nor in the outermost block of a handler for the function-try-block. Names declared in the outermost block of the function definition are not found when looked up in the scope of a handler for the function-try-block. [ Note: But function parameter names are found.  — end note ]

[ Note: The rules for name lookup in template definitions are described in [temp.res].  — end note ]

When the postfix-expression in a function call is an unqualified-id, other namespaces not considered during the usual unqualified lookup may be searched, and in those namespaces, namespace-scope friend function or function template declarations ([class.friend]) not otherwise visible may be found. These modifications to the search depend on the types of the arguments (and for template template arguments, the namespace of the template argument). [ Example:

namespace N {
  struct S { };
  void f(S);
}

void g() {
  N::S s;
  f(s);     // OK: calls N​::​f
  (f)(s);   // error: N​::​f not considered; parentheses prevent argument-dependent lookup
}

 — end example ]

For each argument type T in the function call, there is a set of zero or more associated namespaces and a set of zero or more associated classes to be considered. The sets of namespaces and classes are determined entirely by the types of the function arguments (and the namespace of any template template argument). Typedef names and using-declarations used to specify the types do not contribute to this set. The sets of namespaces and classes are determined in the following way:

• (2.1)

  If T is a fundamental type, its associated sets of namespaces and classes are both empty.

• (2.2)

  If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the innermost enclosing namespaces of its associated classes. Furthermore, if T is a class template specialization, its associated namespaces and classes also include: the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces of which any template template arguments are members; and the classes of which any member templates used as template template arguments are members. [ Note: Non-type template arguments do not contribute to the set of associated namespaces. — end note ]

• (2.3)

  If T is an enumeration type, its associated namespace is the innermost enclosing namespace of its declaration. If it is a class member, its associated class is the member's class; else it has no associated class.

• (2.4)

  If T is a pointer to U or an array of U, its associated namespaces and classes are those associated with U.

• (2.5)

  If T is a function type, its associated namespaces and classes are those associated with the function parameter types and those associated with the return type.

• (2.6)

  If T is a pointer to a member function of a class X, its associated namespaces and classes are those associated with the function parameter types and return type, together with those associated with X.

• (2.7)

  If T is a pointer to a data member of class X, its associated namespaces and classes are those associated with the member type together with those associated with X.

If an associated namespace is an inline namespace, its enclosing namespace is also included in the set. If an associated namespace directly contains inline namespaces, those inline namespaces are also included in the set. In addition, if the argument is the name or address of a set of overloaded functions and/or function templates, its associated classes and namespaces are the union of those associated with each of the members of the set, i.e., the classes and namespaces associated with its parameter types and return type. Additionally, if the aforementioned set of overloaded functions is named with a template-id, its associated classes and namespaces also include those of its type template-arguments and its template template-arguments.

Let X be the lookup set produced by unqualified lookup and let Y be the lookup set produced by argument dependent lookup (defined as follows). If X contains

• (3.1)

  a declaration of a class member, or

• (3.2)

  a block-scope function declaration that is not a using-declaration, or

• (3.3)

  a declaration that is neither a function nor a function template

then Y is empty. Otherwise Y is the set of declarations found in the namespaces associated with the argument types as described below. The set of declarations found by the lookup of the name is the union of X and Y. [ Note: The namespaces and classes associated with the argument types can include namespaces and classes already considered by the ordinary unqualified lookup.  — end note ] [ Example:

namespace NS {
  class T { };
  void f(T);
  void g(T, int);
}
NS::T parm;
void g(NS::T, float);
int main() {
  f(parm);                      // OK: calls NS​::​f
  extern void g(NS::T, float);
  g(parm, 1);                   // OK: calls g(NS​::​T, float)
}

 — end example ]

When considering an associated namespace, the lookup is the same as the lookup performed when the associated namespace is used as a qualifier ([namespace.qual]) except that:

• (4.1)

  Any using-directives in the associated namespace are ignored.

• (4.2)

  Any namespace-scope friend functions or friend function templates declared in associated classes are visible within their respective namespaces even if they are not visible during an ordinary lookup ([class.friend]).

• (4.3)

  All names except those of (possibly overloaded) functions and function templates are ignored.

The name of a class or namespace member or enumerator can be referred to after the ​::​ scope resolution operator ([expr.prim]) applied to a nested-name-specifier that denotes its class, namespace, or enumeration. If a ​::​ scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that ​::​ considers only namespaces, types, and templates whose specializations are types. If the name found does not designate a namespace or a class, enumeration, or dependent type, the program is ill-formed. [ Example:

class A {
public:
  static int n;
};
int main() {
  int A;
  A::n = 42;        // OK
  A b;              // ill-formed: A does not name a type
}

 — end example ]

[ Note: Multiply qualified names, such as N1​::​N2​::​N3​::​n, can be used to refer to members of nested classes ([class.nest]) or members of nested namespaces.  — end note ]

In a declaration in which the declarator-id is a qualified-id, names used before the qualified-id being declared are looked up in the defining namespace scope; names following the qualified-id are looked up in the scope of the member's class or namespace. [ Example:

class X { };
class C {
  class X { };
  static const int number = 50;
  static X arr[number];
};
X C::arr[number];   // ill-formed:
                    // equivalent to ​::​X C​::​arr[C​::​number];
                    // and not to C​::​X C​::​arr[C​::​number];

 — end example ]

A name prefixed by the unary scope operator ​::​ ([expr.prim]) is looked up in global scope, in the translation unit where it is used. The name shall be declared in global namespace scope or shall be a name whose declaration is visible in global scope because of a using-directive ([namespace.qual]). The use of ​::​ allows a global name to be referred to even if its identifier has been hidden.

A name prefixed by a nested-name-specifier that nominates an enumeration type shall represent an enumerator of that enumeration.

If a pseudo-destructor-name ([expr.pseudo]) contains a nested-name-specifier, the type-names are looked up as types in the scope designated by the nested-name-specifier. Similarly, in a qualified-id of the form:

nested-name-specifier${}_{opt}$ class-name :: ~ class-name

the second class-name is looked up in the same scope as the first. [ Example:

struct C {
  typedef int I;
};
typedef int I1, I2;
extern int* p;
extern int* q;
p->C::I::~I();      // I is looked up in the scope of C
q->I1::~I2();       // I2 is looked up in the scope of the postfix-expression

struct A {
  ~A();
};
typedef A AB;
int main() {
  AB* p;
  p->AB::~AB();     // explicitly calls the destructor for A
}

 — end example ] [ Note: [basic.lookup.classref] describes how name lookup proceeds after the . and -> operators.  — end note ]

An elaborated-type-specifier may be used to refer to a previously declared class-name or enum-name even though the name has been hidden by a non-type declaration.

If the elaborated-type-specifier has no nested-name-specifier, and unless the elaborated-type-specifier appears in a declaration with the following form:

class-key attribute-specifier-seq${}_{opt}$ identifier ;

the identifier is looked up according to [basic.lookup.unqual] but ignoring any non-type names that have been declared. If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:

class-key attribute-specifier-seq${}_{opt}$ identifier ;

the elaborated-type-specifier is a declaration that introduces the class-name as described in [basic.scope.pdecl].

If the elaborated-type-specifier has a nested-name-specifier, qualified name lookup is performed, as described in [basic.lookup.qual], but ignoring any non-type names that have been declared. If the name lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. [ Example:

struct Node {
  struct Node* Next;            // OK: Refers to Node at global scope
  struct Data* Data;            // OK: Declares type Data
                                // at global scope and member Data
};

struct Data {
  struct Node* Node;            // OK: Refers to Node at global scope
  friend struct ::Glob;         // error: Glob is not declared, cannot introduce a qualified type ([dcl.type.elab])
  friend struct Glob;           // OK: Refers to (as yet) undeclared Glob at global scope.
  /* ... */
};

struct Base {
  struct Data;                  // OK: Declares nested Data
  struct ::Data*     thatData;  // OK: Refers to ​::​Data
  struct Base::Data* thisData;  // OK: Refers to nested Data
  friend class ::Data;          // OK: global Data is a friend
  friend class Data;            // OK: nested Data is a friend
  struct Data { /* ... */ };    // Defines nested Data
};

struct Data;                    // OK: Redeclares Data at global scope
struct ::Data;                  // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Data;              // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Datum;             // error: Datum undefined
struct Base::Data* pBase;       // OK: refers to nested Data

 — end example ]

In a class member access expression, if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list ([temp.names]) or a less-than operator. The identifier is first looked up in the class of the object expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class template.

If the id-expression in a class member access is an unqualified-id, and the type of the object expression is of a class type C, the unqualified-id is looked up in the scope of class C. For a pseudo-destructor call, the unqualified-id is looked up in the context of the complete postfix-expression.

If the unqualified-id is ~type-name, the type-name is looked up in the context of the entire postfix-expression. If the type T of the object expression is of a class type C, the type-name is also looked up in the scope of class C. At least one of the lookups shall find a name that refers to cv T. [ Example:

struct A { };

struct B {
  struct A { };
  void f(::A* a);
};

void B::f(::A* a) {
  a->~A();                      // OK: lookup in *a finds the injected-class-name
}

 — end example ]

If the id-expression in a class member access is a qualified-id of the form

class-name-or-namespace-name::...

the class-name-or-namespace-name following the . or -> operator is first looked up in the class of the object expression and the name, if found, is used. Otherwise it is looked up in the context of the entire postfix-expression. [ Note: See [basic.lookup.qual], which describes the lookup of a name before ​::​, which will only find a type or namespace name.  — end note ]

If the qualified-id has the form

::class-name-or-namespace-name::...

the class-name-or-namespace-name is looked up in global scope as a class-name or namespace-name.

If the nested-name-specifier contains a simple-template-id, the names in its template-arguments are looked up in the context in which the entire postfix-expression occurs.

If the id-expression is a conversion-function-id, its conversion-type-id is first looked up in the class of the object expression and the name, if found, is used. Otherwise it is looked up in the context of the entire postfix-expression. In each of these lookups, only names that denote types or templates whose specializations are types are considered. [ Example:

struct A { };
namespace N {
  struct A {
    void g() { }
    template <class T> operator T();
  };
}

int main() {
  N::A a;
  a.operator A();               // calls N​::​A​::​operator N​::​A
}

 — end example ]

In a using-directive or namespace-alias-definition, during the lookup for a namespace-name or for a name in a nested-name-specifier only namespace names are considered.

A program shall contain a global function called main. Executing a program starts a main thread of execution ([intro.multithread], [thread.threads]) in which the main function is invoked, and in which variables of static storage duration might be initialized ([basic.start.static]) and destroyed ([basic.start.term]). It is implementation-defined whether a program in a freestanding environment is required to define a main function. [ Note: In a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration.  — end note ]

An implementation shall not predefine the main function. This function shall not be overloaded. Its type shall have C++ language linkage and it shall have a declared return type of type int, but otherwise its type is implementation-defined. An implementation shall allow both

• (2.1)

  a function of () returning int and

• (2.2)

  a function of (int, pointer to pointer to char) returning int

as the type of main ([dcl.fct]). In the latter form, for purposes of exposition, the first function parameter is called argc and the second function parameter is called argv, where argc shall be the number of arguments passed to the program from the environment in which the program is run. If argc is nonzero these arguments shall be supplied in argv[0] through argv[argc-1] as pointers to the initial characters of null-terminated multibyte strings (ntmbs s) ([multibyte.strings]) and argv[0] shall be the pointer to the initial character of a ntmbs that represents the name used to invoke the program or "". The value of argc shall be non-negative. The value of argv[argc] shall be 0. [ Note: It is recommended that any further (optional) parameters be added after argv.  — end note ]

The function main shall not be used within a program. The linkage of main is implementation-defined. A program that defines main as deleted or that declares main to be inline, static, or constexpr is ill-formed. The main function shall not be declared with a linkage-specification. A program that declares a variable main at global scope or that declares the name main with C language linkage (in any namespace) is ill-formed. The name main is not otherwise reserved. [ Example: Member functions, classes, and enumerations can be called main, as can entities in other namespaces.  — end example ]

Terminating the program without leaving the current block (e.g., by calling the function std​::​exit(int)) does not destroy any objects with automatic storage duration ([class.dtor]). If std​::​exit is called to end a program during the destruction of an object with static or thread storage duration, the program has undefined behavior.

A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std​::​exit with the return value as the argument. If control flows off the end of the compound-statement of main, the effect is equivalent to a return with operand 0 (see also [except.handle]).

Variables with static storage duration are initialized as a consequence of program initiation. Variables with thread storage duration are initialized as a consequence of thread execution. Within each of these phases of initiation, initialization occurs as follows.

A constant initializer for a variable or temporary object o is an initializer whose full-expression is a constant expression, except that if o is an object, such an initializer may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types. [ Note: Such a class may have a non-trivial destructor.  — end note ] Constant initialization is performed if a variable or temporary object with static or thread storage duration is initialized by a constant initializer for the entity. If constant initialization is not performed, a variable with static storage duration or thread storage duration is zero-initialized. Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. All static initialization strongly happens before ([intro.races]) any dynamic initialization. [ Note: The dynamic initialization of non-local variables is described in [basic.start.dynamic]; that of local static variables is described in [stmt.dcl].  — end note ]

An implementation is permitted to perform the initialization of a variable with static or thread storage duration as a static initialization even if such initialization is not required to be done statically, provided that

• (3.1)

  the dynamic version of the initialization does not change the value of any other object of static or thread storage duration prior to its initialization, and

• (3.2)

  the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.

[ Note: As a consequence, if the initialization of an object obj1 refers to an object obj2 of namespace scope potentially requiring dynamic initialization and defined later in the same translation unit, it is unspecified whether the value of obj2 used will be the value of the fully initialized obj2 (because obj2 was statically initialized) or will be the value of obj2 merely zero-initialized. For example,

inline double fd() { return 1.0; }
extern double d1;
double d2 = d1;     // unspecified:
                    // may be statically initialized to 0.0 or
                    // dynamically initialized to 0.0 if d1 is
                    // dynamically initialized, or 1.0 otherwise
double d1 = fd();   // may be initialized statically or dynamically to 1.0

 — end note ]

Dynamic initialization of a non-local variable with static storage duration is unordered if the variable is an implicitly or explicitly instantiated specialization, is partially-ordered if the variable is an inline variable that is not an implicitly or explicitly instantiated specialization, and otherwise is ordered. [ Note: An explicitly specialized non-inline static data member or variable template specialization has ordered initialization. — end note ]

Dynamic initialization of non-local variables V and W with static storage duration are ordered as follows:

• (2.1)

  If V and W have ordered initialization and V is defined before W within a single translation unit, the initialization of V is sequenced before the initialization of W.

• (2.2)

  If V has partially-ordered initialization, W does not have unordered initialization, and V is defined before W in every translation unit in which W is defined, then

  • (2.2.1)

    if the program starts a thread ([intro.multithread]) other than the main thread ([basic.start.main]), the initialization of V strongly happens before the initialization of W;

  • (2.2.2)

    otherwise, the initialization of V is sequenced before the initialization of W.

• (2.3)

  Otherwise, if the program starts a thread other than the main thread before either V or W is initialized, it is unspecified in which threads the initializations of V and W occur; the initializations are unsequenced if they occur in the same thread.

• (2.4)

  Otherwise, the initializations of V and W are indeterminately sequenced.

[ Note: This definition permits initialization of a sequence of ordered variables concurrently with another sequence.  — end note ]

A non-initialization odr-use is an odr-use ([basic.def.odr]) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.

It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized.36 It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs. [ Note: Such points should be chosen in a way that allows the programmer to avoid deadlocks.  — end note ] [ Example:

// - File 1 -
#include "a.h"
#include "b.h"
B b;
A::A(){
  b.Use();
}

// - File 2 -
#include "a.h"
A a;

// - File 3 -
#include "a.h"
#include "b.h"
extern A a;
extern B b;

int main() {
  a.Use();
  b.Use();
}

It is implementation-defined whether either a or b is initialized before main is entered or whether the initializations are delayed until a is first odr-used in main. In particular, if a is initialized before main is entered, it is not guaranteed that b will be initialized before it is odr-used by the initialization of a, that is, before A​::​A is called. If, however, a is initialized at some point after the first statement of main, b will be initialized prior to its use in A​::​A.  — end example ]

It is implementation-defined whether the dynamic initialization of a non-local inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of that variable. It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized. It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

If the initialization of a non-local variable with static or thread storage duration exits via an exception, std​::​terminate is called.

Destructors for initialized objects (that is, objects whose lifetime has begun) with static storage duration, and functions registered with std​::​atexit, are called as part of a call to std​::​exit. The call to std​::​exit is sequenced before the invocations of the destructors and the registered functions. [ Note: Returning from main invokes std​::​exit ([basic.start.main]).  — end note ]

Destructors for initialized objects with thread storage duration within a given thread are called as a result of returning from the initial function of that thread and as a result of that thread calling std​::​exit. The completions of the destructors for all initialized objects with thread storage duration within that thread strongly happen before the initiation of the destructors of any object with static storage duration.

If the completion of the constructor or dynamic initialization of an object with static storage duration strongly happens before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first. If the completion of the constructor or dynamic initialization of an object with thread storage duration is sequenced before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first. If an object is initialized statically, the object is destroyed in the same order as if the object was dynamically initialized. For an object of array or class type, all subobjects of that object are destroyed before any block-scope object with static storage duration initialized during the construction of the subobjects is destroyed. If the destruction of an object with static or thread storage duration exits via an exception, std​::​terminate is called.

If a function contains a block-scope object of static or thread storage duration that has been destroyed and the function is called during the destruction of an object with static or thread storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed block-scope object. Likewise, the behavior is undefined if the block-scope object is used indirectly (i.e., through a pointer) after its destruction.

If the completion of the initialization of an object with static storage duration strongly happens before a call to std​::​atexit (see <cstdlib>, [support.start.term]), the call to the function passed to std​::​atexit is sequenced before the call to the destructor for the object. If a call to std​::​atexit strongly happens before the completion of the initialization of an object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std​::​atexit. If a call to std​::​atexit strongly happens before another call to std​::​atexit, the call to the function passed to the second std​::​atexit call is sequenced before the call to the function passed to the first std​::​atexit call.

If there is a use of a standard library object or function not permitted within signal handlers ([support.runtime]) that does not happen before completion of destruction of objects with static storage duration and execution of std​::​atexit registered functions ([support.start.term]), the program has undefined behavior. [ Note: If there is a use of an object with static storage duration that does not happen before the object's destruction, the program has undefined behavior. Terminating every thread before a call to std​::​exit or the exit from main is sufficient, but not necessary, to satisfy these requirements. These requirements permit thread managers as static-storage-duration objects.  — end note ]

Calling the function std​::​abort() declared in <cstdlib> terminates the program without executing any destructors and without calling the functions passed to std​::​atexit() or std​::​at_­quick_­exit().

All variables which do not have dynamic storage duration, do not have thread storage duration, and are not local have static storage duration. The storage for these entities shall last for the duration of the program ([basic.start.static], [basic.start.term]).

If a variable with static storage duration has initialization or a destructor with side effects, it shall not be eliminated even if it appears to be unused, except that a class object or its copy/move may be eliminated as specified in [class.copy].

The keyword static can be used to declare a local variable with static storage duration. [ Note: [stmt.dcl] describes the initialization of local static variables; [basic.start.term] describes the destruction of local static variables.  — end note ]

The keyword static applied to a class data member in a class definition gives the data member static storage duration.

All variables declared with the thread_­local keyword have thread storage duration. The storage for these entities shall last for the duration of the thread in which they are created. There is a distinct object or reference per thread, and use of the declared name refers to the entity associated with the current thread.

A variable with thread storage duration shall be initialized before its first odr-use and, if constructed, shall be destroyed on thread exit.

Block-scope variables not explicitly declared static, thread_­local, or extern have automatic storage duration. The storage for these entities lasts until the block in which they are created exits.

[ Note: These variables are initialized and destroyed as described in [stmt.dcl].  — end note ]

If a variable with automatic storage duration has initialization or a destructor with side effects, an implementation shall not destroy it before the end of its block nor eliminate it as an optimization, even if it appears to be unused, except that a class object or its copy/move may be eliminated as specified in [class.copy].

Objects can be created dynamically during program execution, using new-expressions, and destroyed using delete-expressions. A C++ implementation provides access to, and management of, dynamic storage via the global allocation functions operator new and operator new[] and the global deallocation functions operator delete and operator delete[]. [ Note: The non-allocating forms described in [new.delete.placement] do not perform allocation or deallocation.  — end note ]

The library provides default definitions for the global allocation and deallocation functions. Some global allocation and deallocation functions are replaceable ([new.delete]). A C++ program shall provide at most one definition of a replaceable allocation or deallocation function. Any such function definition replaces the default version provided in the library ([replacement.functions]). The following allocation and deallocation functions ([support.dynamic]) are implicitly declared in global scope in each translation unit of a program.

void* operator new(std::size_t);
void* operator new(std::size_t, std::align_val_t);

void operator delete(void*) noexcept;
void operator delete(void*, std::size_t) noexcept;
void operator delete(void*, std::align_val_t) noexcept;
void operator delete(void*, std::size_t, std::align_val_t) noexcept;

void* operator new[](std::size_t);
void* operator new[](std::size_t, std::align_val_t);

void operator delete[](void*) noexcept;
void operator delete[](void*, std::size_t) noexcept;
void operator delete[](void*, std::align_val_t) noexcept;
void operator delete[](void*, std::size_t, std::align_val_t) noexcept;

These implicit declarations introduce only the function names operator new, operator new[], operator delete, and operator delete[]. [ Note: The implicit declarations do not introduce the names std, std​::​size_­t, std​::​align_­val_­t, or any other names that the library uses to declare these names. Thus, a new-expression, delete-expression or function call that refers to one of these functions without including the header <new> is well-formed. However, referring to std or std​::​size_­t or std​::​align_­val_­t is ill-formed unless the name has been declared by including the appropriate header.  — end note ] Allocation and/or deallocation functions may also be declared and defined for any class ([class.free]).

Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in [basic.stc.dynamic.allocation] and [basic.stc.dynamic.deallocation].

The storage duration of subobjects and reference members is that of their complete object ([intro.object]).

Objects declared as characters (char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, collectively called narrow character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements; that is, they have the same object representation. For narrow character types, all bits of the object representation participate in the value representation. [ Note: A bit-field of narrow character type whose length is larger than the number of bits in the object representation of that type has padding bits; see [class.bit].  — end note ] For unsigned narrow character types, each possible bit pattern of the value representation represents a distinct number. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined. For each value i of type unsigned char in the range 0 to 255 inclusive, there exists a value j of type char such that the result of an integral conversion from i to char is j, and the result of an integral conversion from j to unsigned char is i.

There are five standard signed integer types : “signed char”, “short int”, “int”, “long int”, and “long long int”. In this list, each type provides at least as much storage as those preceding it in the list. There may also be implementation-defined extended signed integer types. The standard and extended signed integer types are collectively called signed integer types. Plain ints have the natural size suggested by the architecture of the execution environment47; the other signed integer types are provided to meet special needs.

For each of the standard signed integer types, there exists a corresponding (but different) standard unsigned integer type: “unsigned char”, “unsigned short int”, “unsigned int”, “unsigned long int”, and “unsigned long long int”, each of which occupies the same amount of storage and has the same alignment requirements as the corresponding signed integer type48; that is, each signed integer type has the same object representation as its corresponding unsigned integer type. Likewise, for each of the extended signed integer types there exists a corresponding extended unsigned integer type with the same amount of storage and alignment requirements. The standard and extended unsigned integer types are collectively called unsigned integer types. The range of non-negative values of a signed integer type is a subrange of the corresponding unsigned integer type, the representation of the same value in each of the two types is the same, and the value representation of each corresponding signed/unsigned type shall be the same. The standard signed integer types and standard unsigned integer types are collectively called the standard integer types, and the extended signed integer types and extended unsigned integer types are collectively called the extended integer types. The signed and unsigned integer types shall satisfy the constraints given in the C standard, section 5.2.4.2.1.

Unsigned integers shall obey the laws of arithmetic modulo $2^n$ where n is the number of bits in the value representation of that particular size of integer.49

Type wchar_­t is a distinct type whose values can represent distinct codes for all members of the largest extended character set specified among the supported locales. Type wchar_­t shall have the same size, signedness, and alignment requirements as one of the other integral types, called its underlying type. Types char16_­t and char32_­t denote distinct types with the same size, signedness, and alignment as uint_­least16_­t and uint_­least32_­t, respectively, in <cstdint>, called the underlying types.

Values of type bool are either true or false.50 [ Note: There are no signed, unsigned, short, or long bool types or values.  — end note ] Values of type bool participate in integral promotions.

Types bool, char, char16_­t, char32_­t, wchar_­t, and the signed and unsigned integer types are collectively called integral types.51 A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system.52 [ Example: This International Standard permits two's complement, ones' complement and signed magnitude representations for integral types.  — end example ]

There are three floating-point types: float, double, and long double. The type double provides at least as much precision as float, and the type long double provides at least as much precision as double. The set of values of the type float is a subset of the set of values of the type double; the set of values of the type double is a subset of the set of values of the type long double. The value representation of floating-point types is implementation-defined. [ Note: This International Standard imposes no requirements on the accuracy of floating-point operations; see also [support.limits].  — end note ] Integral and floating types are collectively called arithmetic types. Specializations of the standard library template std​::​numeric_­limits shall specify the maximum and minimum values of each arithmetic type for an implementation.

A type cv void is an incomplete type that cannot be completed; such a type has an empty set of values. It is used as the return type for functions that do not return a value. Any expression can be explicitly converted to type cv void. An expression of type cv void shall be used only as an expression statement, as an operand of a comma expression, as a second or third operand of ?: ([expr.cond]), as the operand of typeid, noexcept, or decltype, as the expression in a return statement for a function with the return type cv void, or as the operand of an explicit conversion to type cv void.

A value of type std​::​nullptr_­t is a null pointer constant. Such values participate in the pointer and the pointer to member conversions ([conv.ptr], [conv.mem]). sizeof(std​::​nullptr_­t) shall be equal to sizeof(void*).

[ Note: Even if the implementation defines two or more basic types to have the same value representation, they are nevertheless different types.  — end note ]

Compound types can be constructed in the following ways:

• (1.1)

  arrays of objects of a given type, [dcl.array];

• (1.2)

  functions, which have parameters of given types and return void or references or objects of a given type, [dcl.fct];

• (1.3)

  pointers to cv void or objects or functions (including static members of classes) of a given type, [dcl.ptr];

• (1.4)

  references to objects or functions of a given type, [dcl.ref]. There are two types of references:

  • (1.4.1)

    lvalue reference

  • (1.4.2)

    rvalue reference

• (1.5)

  classes containing a sequence of objects of various types (Clause [class]), a set of types, enumerations and functions for manipulating these objects ([class.mfct]), and a set of restrictions on the access to these entities (Clause [class.access]);

• (1.6)

  unions, which are classes capable of containing objects of different types at different times, [class.union];

• (1.7)

  enumerations, which comprise a set of named constant values. Each distinct enumeration constitutes a different enumerated type, [dcl.enum];

• (1.8)

  pointers to non-static class members,53 which identify members of a given type within objects of a given class, [dcl.mptr].

These methods of constructing types can be applied recursively; restrictions are mentioned in [dcl.ptr], [dcl.array], [dcl.fct], and [dcl.ref]. Constructing a type such that the number of bytes in its object representation exceeds the maximum value representable in the type std​::​size_­t ([support.types]) is ill-formed.

The type of a pointer to cv void or a pointer to an object type is called an object pointer type. [ Note: A pointer to void does not have a pointer-to-object type, however, because void is not an object type.  — end note ] The type of a pointer that can designate a function is called a function pointer type. A pointer to objects of type T is referred to as a “pointer to T”. [ Example: A pointer to an object of type int is referred to as “pointer to int” and a pointer to an object of class X is called a “pointer to X”.  — end example ] Except for pointers to static members, text referring to “pointers” does not apply to pointers to members. Pointers to incomplete types are allowed although there are restrictions on what can be done with them ([basic.align]). Every value of pointer type is one of the following:

• (3.1)

  a pointer to an object or function (the pointer is said to point to the object or function), or

• (3.2)

  a pointer past the end of an object ([expr.add]), or

• (3.3)

  the null pointer value ([conv.ptr]) for that type, or

• (3.4)

  an invalid pointer value.

A value of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory ([intro.memory]) occupied by the object54 or the first byte in memory after the end of the storage occupied by the object, respectively. [ Note: A pointer past the end of an object ([expr.add]) is not considered to point to an unrelated object of the object's type that might be located at that address. A pointer value becomes invalid when the storage it denotes reaches the end of its storage duration; see [basic.stc].  — end note ] For purposes of pointer arithmetic and comparison ([expr.rel], [expr.eq]), a pointer past the end of the last element of an array x of n elements is considered to be equivalent to a pointer to a hypothetical element x[n]. The value representation of pointer types is implementation-defined. Pointers to layout-compatible types shall have the same value representation and alignment requirements. [ Note: Pointers to over-aligned types have no special representation, but their range of valid values is restricted by the extended alignment requirement. — end note ]

Two objects a and b are pointer-interconvertible if:

• (4.1)

  they are the same object, or

• (4.2)

  one is a standard-layout union object and the other is a non-static data member of that object ([class.union]), or

• (4.3)

  one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, the first base class subobject of that object ([class.mem]), or

• (4.4)

  there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast. [ Note: An array object and its first element are not pointer-interconvertible, even though they have the same address.  — end note ]

A pointer to cv-qualified or cv-unqualified void can be used to point to objects of unknown type. Such a pointer shall be able to hold any object pointer. An object of type cv void* shall have the same representation and alignment requirements as cv char*.

A type mentioned in [basic.fundamental] and [basic.compound] is a cv-unqualified type. Each type which is a cv-unqualified complete or incomplete object type or is void ([basic.types]) has three corresponding cv-qualified versions of its type: a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version. The type of an object ([intro.object]) includes the cv-qualifiers specified in the decl-specifier-seq, declarator, type-id, or new-type-id when the object is created.

• (1.1)

  A const object is an object of type const T or a non-mutable subobject of such an object.

• (1.2)

  A volatile object is an object of type volatile T, a subobject of such an object, or a mutable subobject of a const volatile object.

• (1.3)

  A const volatile object is an object of type const volatile T, a non-mutable subobject of such an object, a const subobject of a volatile object, or a non-mutable volatile subobject of a const object.

The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements.55

A compound type is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded. Any cv-qualifiers applied to an array type affect the array element type ([dcl.array]).

See [dcl.fct] and [class.this] regarding function types that have cv-qualifiers.

There is a partial ordering on cv-qualifiers, so that a type can be said to be more cv-qualified than another. Table 10 shows the relations that constitute this ordering.

In this International Standard, the notation cv (or cv1, cv2, etc.), used in the description of types, represents an arbitrary set of cv-qualifiers, i.e., one of {const}, {volatile}, {const, volatile}, or the empty set. For a type cv T, the top-level cv-qualifiers of that type are those denoted by cv. [ Example: The type corresponding to the type-id const int& has no top-level cv-qualifiers. The type corresponding to the type-id volatile int * const has the top-level cv-qualifier const. For a class type C, the type corresponding to the type-id void (C​::​* volatile)(int) const has the top-level cv-qualifier volatile.  — end example ]

Cv-qualifiers applied to an array type attach to the underlying element type, so the notation “cv T”, where T is an array type, refers to an array whose elements are so-qualified. An array type whose elements are cv-qualified is also considered to have the same cv-qualifications as its elements. [ Example:

typedef char CA[5];
typedef const char CC;
CC arr1[5] = { 0 };
const CA arr2 = { 0 };

The type of both arr1 and arr2 is “array of 5 const char”, and the array type is considered to be const-qualified.  — end example ]