C++策略模式实战：优雅切换算法

策略模式的定义与核心思想

策略模式（Strategy Pattern）是一种行为型设计模式，允许在运行时选择算法的行为。该模式将算法封装在独立的类中，使得它们可以相互替换而不影响客户端代码。核心思想是通过组合而非继承来实现算法的动态切换，提高代码的灵活性和可维护性。

在C++中，策略模式通常通过抽象基类或接口定义算法族，具体策略类实现这些接口，上下文类持有策略对象的引用并通过委托调用具体算法。

策略模式的结构与实现

策略模式包含三个主要角色：

Context（上下文）：维护对策略对象的引用，通过接口调用具体策略。
Strategy（策略接口）：定义所有支持的算法的公共接口。
ConcreteStrategy（具体策略）：实现策略接口的具体算法。

以下是一个C++实现示例：

// 策略接口
class Strategy {
public:
    virtual void execute() const = 0;
    virtual ~Strategy() = default;
};

// 具体策略A
class ConcreteStrategyA : public Strategy {
public:
    void execute() const override {
        std::cout << "Executing Strategy A" << std::endl;
    }
};

// 具体策略B
class ConcreteStrategyB : public Strategy {
public:
    void execute() const override {
        std::cout << "Executing Strategy B" << std::endl;
    }
};

// 上下文类
class Context {
private:
    Strategy* strategy;
public:
    explicit Context(Strategy* s) : strategy(s) {}
    void setStrategy(Strategy* s) {
        strategy = s;
    }
    void executeStrategy() const {
        strategy->execute();
    }
};

// 使用示例
int main() {
    ConcreteStrategyA strategyA;
    ConcreteStrategyB strategyB;

    Context context(&strategyA);
    context.executeStrategy(); // 输出: Executing Strategy A

    context.setStrategy(&strategyB);
    context.executeStrategy(); // 输出: Executing Strategy B
}

策略模式的优势与应用场景

优势：

开闭原则：无需修改上下文即可扩展新策略。
消除条件语句：避免复杂的条件分支逻辑。
运行时灵活性：动态切换算法行为。

典型应用场景：

需要多种算法变体的场景（如排序、压缩、加密等）。
算法需要独立于使用它的客户端。
替换大量条件分支逻辑时。

策略模式与工厂模式的结合

策略模式常与工厂模式结合，通过工厂类创建具体策略对象，进一步解耦客户端代码。例如：

class StrategyFactory {
public:
    static Strategy* createStrategy(const std::string& type) {
        if (type == "A") return new ConcreteStrategyA();
        if (type == "B") return new ConcreteStrategyB();
        return nullptr;
    }
};

// 使用示例
Context context(StrategyFactory::createStrategy("A"));
context.executeStrategy();

实际案例：支付系统设计

假设需要实现一个支持多种支付方式的系统（信用卡、支付宝、微信支付），策略模式的实现如下：

// 支付策略接口
class PaymentStrategy {
public:
    virtual void pay(double amount) const = 0;
    virtual ~PaymentStrategy() = default;
};

// 具体支付策略
class CreditCardPayment : public PaymentStrategy {
public:
    void pay(double amount) const override {
        std::cout << "Paid " << amount << " via Credit Card" << std::endl;
    }
};

class AlipayPayment : public PaymentStrategy {
public:
    void pay(double amount) const override {
        std::cout << "Paid " << amount << " via Alipay" << std::endl;
    }
};

// 支付上下文
class PaymentContext {
private:
    PaymentStrategy* strategy;
public:
    explicit PaymentContext(PaymentStrategy* s) : strategy(s) {}
    void executePayment(double amount) const {
        strategy->pay(amount);
    }
};

// 使用示例
PaymentContext ctx(new AlipayPayment());
ctx.executePayment(100.0); // 输出: Paid 100 via Alipay

注意事项与常见问题

策略对象的生命周期管理：建议使用智能指针（如std::unique_ptr）避免内存泄漏。
性能开销：频繁创建和销毁策略对象可能影响性能，可考虑对象池优化。
过度设计：简单场景直接使用函数指针或Lambda可能更简洁。

通过合理应用策略模式，可以显著提升代码的可扩展性和可测试性，尤其在需要频繁变更或扩展算法的系统中效果显著。

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