策略模式是将一些操作封装起来，使得他们不受具体使用他们的对象的影响，简单的理解起来可以是封装了一些算法，类似于stl里面的查找等算法；这些算法功能一样，对外接口一样
以下是个简单的示例：

class IMember {
public:
    virtual double discount() = 0;
};

class normal_member : public IMember {
public:
    double discount() { 
        std::cout << "normal member: 0.9, ";
        return 0.9; }
};

class vip_member : public IMember {
public:
    double discount() { 
        std::cout << "vip member: 0.7, ";
        return 0.7; }
};

class v_vip_member : public IMember {
public:
    double discount() { 
        std::cout << "v_vip member: 0.5, ";
        return 0.5; }
};

class custom {
public:
    custom(std::shared_ptr<IMember> sp) : sp_type_(sp){}
    
    double pay(double price) {
        if (sp_type_) {
            price *= sp_type_->discount();
        }
        std::cout << "will pay " << price << std::endl;
        return price;
    }
private:
    std::shared_ptr<IMember> sp_type_;
};
int main() {
    {
        std::shared_ptr<custom> sp_custom1 = std::make_shared<custom>(std::make_shared<vip_member>());
        std::shared_ptr<custom> sp_custom2 = std::make_shared<custom>(std::make_shared<v_vip_member>());
        std::shared_ptr<custom> sp_custom3 = std::make_shared<custom>(std::make_shared<normal_member>());

        //三位用户都买了价值200元的商品
        sp_custom1->pay(200);
        sp_custom2->pay(200);
        sp_custom3->pay(200);
    }
}