C++ 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 C++ 代码中很常见。 大部分情况下， 它被用于代替包含行为的参数化 UI 元素的回调函数， 此外还被用于对任务进行排序和记录操作历史记录等。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
概念示例
本例说明了命令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 Command interface declares a method for executing a command. */ class Command { public: virtual ~Command() { } virtual void Execute() const = 0; }; /** * Some commands can implement simple operations on their own. */ class SimpleCommand : public Command { private: std::string pay_load_; public: explicit SimpleCommand(std::string pay_load) : pay_load_(pay_load) { } void Execute() const override { std::cout << "SimpleCommand: See, I can do simple things like printing (" << this->pay_load_ << ")\n"; } }; /** * The Receiver classes contain some important business logic. They know how to * perform all kinds of operations, associated with carrying out a request. In * fact, any class may serve as a Receiver. */ class Receiver { public: void DoSomething(const std::string &a) { std::cout << "Receiver: Working on (" << a << ".)\n"; } void DoSomethingElse(const std::string &b) { std::cout << "Receiver: Also working on (" << b << ".)\n"; } }; /** * However, some commands can delegate more complex operations to other objects, * called "receivers." */ class ComplexCommand : public Command { /** * @var Receiver */ private: Receiver *receiver_; /** * Context data, required for launching the receiver's methods. */ std::string a_; std::string b_; /** * Complex commands can accept one or several receiver objects along with any * context data via the constructor. */ public: ComplexCommand(Receiver *receiver, std::string a, std::string b) : receiver_(receiver), a_(a), b_(b) { } /** * Commands can delegate to any methods of a receiver. */ void Execute() const override { std::cout << "ComplexCommand: Complex stuff should be done by a receiver object.\n"; this->receiver_->DoSomething(this->a_); this->receiver_->DoSomethingElse(this->b_); } }; /** * The Invoker is associated with one or several commands. It sends a request to * the command. */ class Invoker { /** * @var Command */ private: Command *on_start_; /** * @var Command */ Command *on_finish_; /** * Initialize commands. */ public: ~Invoker() { delete on_start_; delete on_finish_; } void SetOnStart(Command *command) { this->on_start_ = command; } void SetOnFinish(Command *command) { this->on_finish_ = command; } /** * The Invoker does not depend on concrete command or receiver classes. The * Invoker passes a request to a receiver indirectly, by executing a command. */ void DoSomethingImportant() { std::cout << "Invoker: Does anybody want something done before I begin?\n"; if (this->on_start_) { this->on_start_->Execute(); } std::cout << "Invoker: ...doing something really important...\n"; std::cout << "Invoker: Does anybody want something done after I finish?\n"; if (this->on_finish_) { this->on_finish_->Execute(); } } }; /** * The client code can parameterize an invoker with any commands. */ int main() { Invoker *invoker = new Invoker; invoker->SetOnStart(new SimpleCommand("Say Hi!")); Receiver *receiver = new Receiver; invoker->SetOnFinish(new ComplexCommand(receiver, "Send email", "Save report")); invoker->DoSomethingImportant(); delete invoker; delete receiver; return 0; } Output.txt: 执行结果
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object. Receiver: Working on (Send email.) Receiver: Also working on (Save report.) 
