Adapter is a structural design pattern, which allows incompatible objects to collaborate.
The Adapter acts as a wrapper between two objects. It catches calls for one object and transforms them to format and interface recognizable by the second object.
Usage examples: The Adapter pattern is pretty common in C++ code. It’s very often used in systems based on some legacy code. In such cases, Adapters make legacy code work with modern classes.
Identification: Adapter is recognizable by a constructor which takes an instance of a different abstract/interface type. When the adapter receives a call to any of its methods, it translates parameters to the appropriate format and then directs the call to one or several methods of the wrapped object.
Conceptual Example
This example illustrates the structure of the Adapter design pattern. It focuses on answering these questions:
What classes does it consist of?
What roles do these classes play?
In what way the elements of the pattern are related?
main.cc: Conceptual example
/**
* The Target defines the domain-specific interface used by the client code.
*/
class Target {
public:
virtual ~Target() = default;
virtual std::string Request() const {
return "Target: The default target's behavior.";
}
};
/**
* The Adaptee contains some useful behavior, but its interface is incompatible
* with the existing client code. The Adaptee needs some adaptation before the
* client code can use it.
*/
class Adaptee {
public:
std::string SpecificRequest() const {
return ".eetpadA eht fo roivaheb laicepS";
}
};
/**
* The Adapter makes the Adaptee's interface compatible with the Target's
* interface.
*/
class Adapter : public Target {
private:
Adaptee *adaptee_;
public:
Adapter(Adaptee *adaptee) : adaptee_(adaptee) {}
std::string Request() const override {
std::string to_reverse = this->adaptee_->SpecificRequest();
std::reverse(to_reverse.begin(), to_reverse.end());
return "Adapter: (TRANSLATED) " + to_reverse;
}
};
/**
* The client code supports all classes that follow the Target interface.
*/
void ClientCode(const Target *target) {
std::cout << target->Request();
}
int main() {
std::cout << "Client: I can work just fine with the Target objects:\n";
Target *target = new Target;
ClientCode(target);
std::cout << "\n\n";
Adaptee *adaptee = new Adaptee;
std::cout << "Client: The Adaptee class has a weird interface. See, I don't understand it:\n";
std::cout << "Adaptee: " << adaptee->SpecificRequest();
std::cout << "\n\n";
std::cout << "Client: But I can work with it via the Adapter:\n";
Adapter *adapter = new Adapter(adaptee);
ClientCode(adapter);
std::cout << "\n";
delete target;
delete adaptee;
delete adapter;
return 0;
}
Output.txt: Execution result
Client: I can work just fine with the Target objects:
Target: The default target's behavior.
Client: The Adaptee class has a weird interface. See, I don't understand it:
Adaptee: .eetpadA eht fo roivaheb laicepS
Client: But I can work with it via the Adapter:
Adapter: (TRANSLATED) Special behavior of the Adaptee.
Multiple Inheritance
In C++ the Adapter pattern can be implemented using multiple inheritance.
main.cc: Multiple Inheritance
/**
* The Target defines the domain-specific interface used by the client code.
*/
class Target {
public:
virtual ~Target() = default;
virtual std::string Request() const {
return "Target: The default target's behavior.";
}
};
/**
* The Adaptee contains some useful behavior, but its interface is incompatible
* with the existing client code. The Adaptee needs some adaptation before the
* client code can use it.
*/
class Adaptee {
public:
std::string SpecificRequest() const {
return ".eetpadA eht fo roivaheb laicepS";
}
};
/**
* The Adapter makes the Adaptee's interface compatible with the Target's
* interface using multiple inheritance.
*/
class Adapter : public Target, public Adaptee {
public:
Adapter() {}
std::string Request() const override {
std::string to_reverse = SpecificRequest();
std::reverse(to_reverse.begin(), to_reverse.end());
return "Adapter: (TRANSLATED) " + to_reverse;
}
};
/**
* The client code supports all classes that follow the Target interface.
*/
void ClientCode(const Target *target) {
std::cout << target->Request();
}
int main() {
std::cout << "Client: I can work just fine with the Target objects:\n";
Target *target = new Target;
ClientCode(target);
std::cout << "\n\n";
Adaptee *adaptee = new Adaptee;
std::cout << "Client: The Adaptee class has a weird interface. See, I don't understand it:\n";
std::cout << "Adaptee: " << adaptee->SpecificRequest();
std::cout << "\n\n";
std::cout << "Client: But I can work with it via the Adapter:\n";
Adapter *adapter = new Adapter;
ClientCode(adapter);
std::cout << "\n";
delete target;
delete adaptee;
delete adapter;
return 0;
}
Output.txt: Execution result
Client: I can work just fine with the Target objects:
Target: The default target's behavior.
Client: The Adaptee class has a weird interface. See, I don't understand it:
Adaptee: .eetpadA eht fo roivaheb laicepS
Client: But I can work with it via the Adapter:
Adapter: (TRANSLATED) Special behavior of the Adaptee.
WINTER SALE IS ON!
