C++ 外观模式讲解和代码示例
外观是一种结构型设计模式， 能为复杂系统、 程序库或框架提供一个简单 （但有限） 的接口。
尽管外观模式降低了程序的整体复杂度， 但它同时也有助于将不需要的依赖移动到同一个位置。
复杂度：
流行度：
使用示例： 使用 C++ 开发的程序中会经常使用外观模式。 它在与复杂程序库和 API 协作时特别有用。
识别方法： 外观可以通过使用简单接口， 但将绝大部分工作委派给其他类的类来识别。 通常情况下， 外观管理其所使用的对象的完整生命周期。
概念示例
本例说明了外观设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.cc: 概念示例
/**
* The Subsystem can accept requests either from the facade or client directly.
* In any case, to the Subsystem, the Facade is yet another client, and it's not
* a part of the Subsystem.
*/
class Subsystem1 {
public:
std::string Operation1() const {
return "Subsystem1: Ready!\n";
}
// ...
std::string OperationN() const {
return "Subsystem1: Go!\n";
}
};
/**
* Some facades can work with multiple subsystems at the same time.
*/
class Subsystem2 {
public:
std::string Operation1() const {
return "Subsystem2: Get ready!\n";
}
// ...
std::string OperationZ() const {
return "Subsystem2: Fire!\n";
}
};
/**
* The Facade class provides a simple interface to the complex logic of one or
* several subsystems. The Facade delegates the client requests to the
* appropriate objects within the subsystem. The Facade is also responsible for
* managing their lifecycle. All of this shields the client from the undesired
* complexity of the subsystem.
*/
class Facade {
protected:
Subsystem1 *subsystem1_;
Subsystem2 *subsystem2_;
/**
* Depending on your application's needs, you can provide the Facade with
* existing subsystem objects or force the Facade to create them on its own.
*/
public:
/**
* In this case we will delegate the memory ownership to Facade Class
*/
Facade(
Subsystem1 *subsystem1 = nullptr,
Subsystem2 *subsystem2 = nullptr) {
this->subsystem1_ = subsystem1 ?: new Subsystem1;
this->subsystem2_ = subsystem2 ?: new Subsystem2;
}
~Facade() {
delete subsystem1_;
delete subsystem2_;
}
/**
* The Facade's methods are convenient shortcuts to the sophisticated
* functionality of the subsystems. However, clients get only to a fraction of
* a subsystem's capabilities.
*/
std::string Operation() {
std::string result = "Facade initializes subsystems:\n";
result += this->subsystem1_->Operation1();
result += this->subsystem2_->Operation1();
result += "Facade orders subsystems to perform the action:\n";
result += this->subsystem1_->OperationN();
result += this->subsystem2_->OperationZ();
return result;
}
};
/**
* The client code works with complex subsystems through a simple interface
* provided by the Facade. When a facade manages the lifecycle of the subsystem,
* the client might not even know about the existence of the subsystem. This
* approach lets you keep the complexity under control.
*/
void ClientCode(Facade *facade) {
// ...
std::cout << facade->Operation();
// ...
}
/**
* The client code may have some of the subsystem's objects already created. In
* this case, it might be worthwhile to initialize the Facade with these objects
* instead of letting the Facade create new instances.
*/
int main() {
Subsystem1 *subsystem1 = new Subsystem1;
Subsystem2 *subsystem2 = new Subsystem2;
Facade *facade = new Facade(subsystem1, subsystem2);
ClientCode(facade);
delete facade;
return 0;
}
Output.txt: 执行结果
Facade initializes subsystems:
Subsystem1: Ready!
Subsystem2: Get ready!
Facade orders subsystems to perform the action:
Subsystem1: Go!
Subsystem2: Fire!
