策略模式

例如：商场促销手段有打折、返利、积分，都是一些算法。
使用工厂来生成算法对象没有问题，但是这些算法是随时可以互相替换的，这是变化点：

#include<iostream>
#include<memory>
/* 抽象算法类 */
class Strategy {
public:
    virtual void AlgorithmInterface() = 0;
    virtual ~Strategy() = default;
};
/* 具体算法A */
class ConcreteStrategyA final : public Strategy {
public:
    ConcreteStrategyA() = default;
    void AlgorithmInterface() override {
        std::cout << "algo A implement\n";
    }
};
/* 具体算法B */
class ConcreteStrategyB final : public Strategy {
public:
    ConcreteStrategyB() = default;
    void AlgorithmInterface() override {
        std::cout << "algo B implement\n";
    }
};
/* 具体算法C */
class ConcreteStrategyC final : public Strategy {
public:
    ConcreteStrategyC() = default;
    void AlgorithmInterface() override {
        std::cout << "algo C implement\n";
    }
};
/* 上下文 */
class Context final {
public:
    Context() = default;
    explicit Context(Strategy* strategy) : strategy_(strategy) {}
    Context& operator=(Context&& context) {
        strategy_ = std::move(context.strategy_);
        return *this;
    }
    void ContextInterface() {
        strategy_->AlgorithmInterface();
    }
private:
    std::unique_ptr<Strategy> strategy_;
};
auto main(void) -> int {
    Context context(new ConcreteStrategyA);
    context.ContextInterface();
    context = Context(new ConcreteStrategyB);
    context.ContextInterface();
    context = Context(new ConcreteStrategyC);
    context.ContextInterface();
}

策略模式的 Strategy 层次为 Context 定义了一系列可供重用的算法或行为，继承有助于析取出这些算法中的公共部分。