状态模式建议为对象的所有可能状态新建一个类， 然后将所有状态的对应行为抽取到这些类中。

原始对象被称为上下文 （context）， 它并不会自行实现所有行为， 而是会保存一个指向表示当前状态的状态对象的引用， 且将所有与状态相关的工作委派给该对象。如需将上下文转换为另外一种状态， 则需将当前活动的状态对象替换为另外一个代表新状态的对象。 采用这种方式是有前提的： 所有状态类都必须遵循同样的接口， 而且上下文必须仅通过接口与这些对象进行交互。

+-------------------+
|     Context       |
+-------------------+
| - currentState    |
| + setState()      |
| + userInput()     |
+-------------------+▲│
+--------│---------------------------------------------+
│        ▼                                             │
│  +----------------+                                   │
│  |     State      | <abstract>                        │
│  +----------------+                                   │
│  | + enter()      |                                   │
│  | + handleInput()| <pure virtual>                    │
│  +----------------+                                   │
│                                                       │
│     ▲              ▲               ▲                 ▲ │
│     │              │               │                 │ │
│ +-----------+ +------------+ +-----------+ +----------------+ 
│ | NotLoggedIn | | InputPassword | | LoggingIn | | LoggedIn | ...
│ +-----------+ +------------+ +-----------+ +----------------+ 

#include <iostream>
#include <string>class LoginContext; // 前向声明// 抽象状态类
class LoginState {
public:virtual ~LoginState() = default;virtual void enter(LoginContext* context) = 0; // 进入状态时的行为virtual void handleInput(LoginContext* context, const std::string& input) = 0; // 处理输入
};// 前向声明所有具体状态类
class NotLoggedInState;
class InputtingPasswordState;
class LoggingInState;
class LoggedInState;
class LoginFailedState;// 上下文类
class LoginContext {
private:LoginState* currentState;std::string username;std::string password;public:LoginContext() : currentState(nullptr) {}~LoginContext() {delete currentState; // 释放状态资源}void setState(LoginState* state) {if (currentState != nullptr) {delete currentState; // 释放旧状态资源}currentState = state;if (currentState != nullptr) {currentState->enter(this); // 自动触发进入逻辑}}void userInput(const std::string& input) {if (currentState != nullptr) {currentState->handleInput(this, input);}}const std::string& getUsername() const { return username; }void setUsername(const std::string& name) { username = name; }const std::string& getPassword() const { return password; }void setPassword(const std::string& pwd) { password = pwd; }
};// 具体状态类定义
class NotLoggedInState : public LoginState {
public:void enter(LoginContext* context) override {std::cout << "进入未登录状态，请输入用户名和密码。\n";}void handleInput(LoginContext* context, const std::string& input) override;
};class InputtingPasswordState : public LoginState {
public:void enter(LoginContext* context) override {std::cout << "请输入密码：\n";}void handleInput(LoginContext* context, const std::string& input) override;
};class LoggingInState : public LoginState {
public:void enter(LoginContext* context) override;void handleInput(LoginContext* /*context*/, const std::string& /*input*/) override {}
};class LoggedInState : public LoginState {
public:void enter(LoginContext* context) override {std::cout << "[状态] 登录成功！欢迎回来。" << std::endl;}void handleInput(LoginContext* /*context*/, const std::string& /*input*/) override {}
};class LoginFailedState : public LoginState {
public:void enter(LoginContext* context) override {std::cout << "[状态] 登录失败，请重新输入用户名。" << std::endl;context->setState(new NotLoggedInState());}void handleInput(LoginContext* /*context*/, const std::string& /*input*/) override {}
};// 状态类成员函数实现（解决循环依赖）
void NotLoggedInState::handleInput(LoginContext* context, const std::string& input) {std::cout << "请输入用户名：" << std::endl;context->setUsername(input);context->setState(new InputtingPasswordState());
}void InputtingPasswordState::handleInput(LoginContext* context, const std::string& input) {context->setPassword(input);std::cout << "正在登陆...\n";context->setState(new LoggingInState());
}void LoggingInState::enter(LoginContext* context) {std::cout << "[状态] 当前状态：登录中" << std::endl;// 模拟登录验证const std::string user = context->getUsername();const std::string pass = context->getPassword();if (user == "admin" && pass == "123456") {context->setState(new LoggedInState());}else {context->setState(new LoginFailedState());}
}//int main() {
//    LoginContext context;
//    context.setState(new NotLoggedInState());
//
//    // 模拟用户输入流程
//    context.userInput("admin");     // 输入用户名
//    context.userInput("wrongpass"); // 输入错误密码
//    context.userInput("admin");     // 再次输入用户名
//    context.userInput("123456");    // 输入正确密码
//
//    return 0;
//}
int main() {LoginContext context;context.setState(new NotLoggedInState());std::string input;while (true) {std::cout << "> ";std::getline(std::cin, input);if (input == "exit") break;context.userInput(input);}return 0;
}