The Adapter Pattern

This post is a cross-post from www.ModernesCpp.com.

The idea of the adapter pattern is straightforward: It converts the interface of a class into another interface.

Imagine you have a class that implements the functionality required by the client. There is only one issue: the interface does not follow your company's naming policy. Thanks to the Adapter Pattern, you can pretty easily support the required interface using the existing class.

The Adapter Pattern is the only pattern inside the book "Design Patterns: Elements of Reusable Object-Oriented Software" (Design Pattern) that is implemented on a class level but also on an object level. Here are the facts before I show you both ways to implement this structural pattern.

The Adapter Pattern

Purpose

• Translate one interface into another one

Also known as

• Wrapper

Use Case

• A class does not have the required interface
• Definition of a general interface for a set of similar classes

Example

Container Adapter

The container adapters std::stack, std::queue, and std::priority_queue provide a different interface for the sequence containers. The following code snippet shows the template signature of the three container adapters:

template<typename T, typename Container = std::deque<T>> 
class stack;

template<typename T, typename Container = std::deque<T>> 
class queue;

template<typename T, typename Container = std::vector<T>, 
         typename Compare = std::less<typename Container::value_type>> 
class priority_queue;

By default, std::stack, and std::queue use std::deque as sequence container, but std::priority_queue uses std::vector. Additionally, std::priority_queue also requires a comparator that is defaulted to std::less of the sequence container elements.

 C++ has more adapters.

Iterator Adapter

C++ supports insert iterators and streams iterators.

• Insert Iterators

With the three insert iterators std::front_inserter, std::back_inserter, and std::inserter, you can insert an element into a container at the beginning, at the end or an arbitrary position, respectively.

• Stream Iterators

Stream iterator adapters can use streams as a data source or data sink. C++ offers two functions to create istream iterators and two to create ostream iterators. The created istream iterators behave like input iterators, the ostream iterators like insert iterators.

Structure

The following two class diagrams show the structure of the Adapter Pattern, based on classes or on objects. For short, I call them class adapters and object adapters.

Class Adapter

Object Adapter

Client

• Uses the member function methodA() of the Adapter

Adaptor

• Class
• Provides the functionality of methodA() using multiple inheritances
• Is publicly derived from Interface and privately from Implementation
• Object
• Delegates the member function call to its member Adaptee

Adaptee

• Implements the functionality of the client

Now, the class or object-based Adapter Pattern should be straightforward.

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Implementation

Class Adapter

In the following example, the class RectangleAdapter adapts the interface of the LegacyRectangle.

// adapterClass.cpp

#include <iostream>

typedef int Coordinate;
typedef int Dimension;

class Rectangle {
public:
    virtual void draw() = 0;
    virtual ~Rectangle() = default;
};

class LegacyRectangle {
 public:
    LegacyRectangle(Coordinate x1, Coordinate y1, Coordinate x2, Coordinate y2) : x1_(x1), y1_(y1), x2_(x2), y2_(y2){
        std::cout << "LegacyRectangle:  create.  (" << x1_ << "," << y1_ << ") => ("
                  << x2_ << "," << y2_ << ")" << '\n';
    }

    void oldDraw() {
        std::cout << "LegacyRectangle:  oldDraw.  (" << x1_ << "," << y1_ 
                  << ") => (" << x2_ << "," << y2_ << ")" << '\n';
    }

 private:
    Coordinate x1_;
    Coordinate y1_;
    Coordinate x2_;
    Coordinate y2_;
};


class RectangleAdapter : public Rectangle, private LegacyRectangle {
 public:
    RectangleAdapter(Coordinate x, Coordinate y, Dimension w, Dimension h) : LegacyRectangle(x, y, x + w, y + h) {  // (1)
        std::cout << "RectangleAdapter: create.  (" << x << "," << y 
                  << "), width = " << w << ", height = " << h << '\n';
    }

    void draw() override {
        oldDraw();
        std::cout << "RectangleAdapter: draw." << '\n';
    }
};

int main() {

    std::cout << '\n';

    Rectangle* r = new RectangleAdapter(120, 200, 60, 40);
    r->draw();

    delete r;

    std::cout << '\n';
    
}

RectangleAdapter derived the interface from the Rectangle, and the implementation for LegacyRectangle using multiple inheritances. Additionally, RectangleAdapter adapts the size of the LegacyRectangle (line 1).

This implementation of the Adapter Pattern is one of the rare use cases for private inheritance. Let me write a few words about interface inheritance and implementation inheritance.

• Interface inheritance uses public inheritance. It separates users from implementations to allow derived classes to be added and changed without affecting the users of the base class. Derived classes support the interface of the base class.
• Implementation inheritance often uses private inheritance. Typically, the derived class provides its functionality by adapting functionality from the base class. Derived classes don't support the interface of the base class.

Finally, here is the output of the program:

Object Adapter

In the following implementation, the RectangleAdapter delegates its calls to its adaptee LegacyRectangle.

// adapterObject.cpp

#include <iostream>

typedef int Coordinate;
typedef int Dimension;

class LegacyRectangle {
 public:
    LegacyRectangle(Coordinate x1, Coordinate y1, Coordinate x2, Coordinate y2) : x1_(x1), y1_(y1), x2_(x2), y2_(y2){
        std::cout << "LegacyRectangle:  create.  (" << x1_ << "," << y1_ << ") => ("
                  << x2_ << "," << y2_ << ")" << '\n';
    }

    void oldDraw() {
        std::cout << "LegacyRectangle:  oldDraw.  (" << x1_ << "," << y1_ 
                  << ") => (" << x2_ << "," << y2_ << ")" << '\n';
    }

 private:
    Coordinate x1_;
    Coordinate y1_;
    Coordinate x2_;
    Coordinate y2_;
};

class RectangleAdapter {
 public:
    RectangleAdapter(Coordinate x, Coordinate y, Dimension w, Dimension h) : legacyRectangle{LegacyRectangle(x, y, x + w, y + h)} {  // (1)
        std::cout << "RectangleAdapter: create.  (" << x << "," << y 
                  << "), width = " << w << ", height = " << h << '\n';
    }

    void draw() {
        legacyRectangle.oldDraw();
        std::cout << "RectangleAdapter: draw." << '\n';
    }
 private:
     LegacyRectangle legacyRectangle;
};

int main() {

    std::cout << '\n';

    RectangleAdapter r(120, 200, 60, 40);
    r.draw();

    std::cout << '\n';
}

The class RectangleAdapter creates it LegacyRectangle directly in its constructor (line 1). Another option would be to make LegacyRectangle  a constructor parameter of RectangleAdapter:

class RectangleAdapter {
 public:
    RectangleAdapter(const LegacyRectangle& legRec): legacyRectangle{legRec} {}
 ...
};


  

The output of this program is identical to the previous one.

Related Patterns

• The Bridge Pattern is similar to the object adapter but has a different intent. The Bridge Pattern's purpose is to separate the interface from the implementation, but the adapter's purpose is to modify an existing interface.
• The Decorator Pattern extends an object without changing its interface. Decorators are pluggable but not bridges or adapters.
• The Proxy Pattern extends the implementation for the object it stands for but doesn't change its interface.

You may ask yourself: Should I use a class adapter or an object adapter.

Class Adapter versus Object Adapter

Class Adapter

The class adapter applies classes and their subclasses. It uses the separation of interface and implementation and runtime dispatch with virtual function calls. Its functionality is hard-coded and available at compile time. The class adapter provides less flexibility and dynamic behavior, such as the object adapter.

Object Adapter

The object adapter uses the relationship of objects.

You build your abstraction by composing objects and delegating their work. This composition can be done at runtime. Consequentially, an object adapter is more flexible and allows it to exchange the delegated object at run time.

What's Next?

The Bridge Pattern helps to separate the interface from its implementation. Let me introduce it in my next post.