C/C++编程：模板中的命名参数

╰+哭是因爲堅強的太久メ 2023-01-16 02:51 133阅读 0赞

有些高级脚本语言，如Perl、PL/SQL等，他们的函数参数在调用时都支持命名参数，既在调用时可以不按照顺序传递参数，而是p可以按照参数的名字传递。模板中命名参数的实现如下

#include <iostream>
    #include <cstring>
    
    
    // PolicySelector 的任务是利用typedef将各个模板实参合并到一个单一的类型，即Discriminator；该类型能够根据指定的非缺省类型，改写缺省定义的typedef成员
    // PolicySelector <A, B, C, D>生成A，B，C，D作为基类
    // Discriminator<>使Policy Seletor可以多次继承自相同的基类
    
    // 中间模板Discriminator的引入是为了一致的处理各个Setter类型(不能直接从多个相同类型的基类继承，但可以借助中间类间接继承)
    template<typename Base, int D>
    class Discriminator: public Base{
        
    
    };
    
    template<typename Setter1,
            typename Setter2,
            typename  Setter3,
            typename Setter4>
    class PolicySelector : public Discriminator<Setter1, 1>,
                           public Discriminator<Setter2, 2>,
                           public Discriminator<Setter3, 3>,
                           public Discriminator<Setter4, 4>{
        
    
    };
    
    //先定义出不同的策略类。
    class DefaultPolicy1 {
        };
    class DefaultPolicy2 {
        };
    class DefaultPolicy3 {
        
    public:
        static void doPrint(){
        
            printf("DefaultPolicy3::doPrint()\n");
        }
    };
    class DefaultPolicy4 {
        };
    
    // 将缺省值集中到
    //该类将会是所有Policy Class的基类。他提供了缺省的四个Policy的类型重定义。
    //因此在缺省情况下，这四个Policy将会是BreadSlicer的四个Policy。
    class DefaultPolicies {
        
    public:
        typedef DefaultPolicy1 P1;
        typedef DefaultPolicy2 P2;
        typedef DefaultPolicy3 P3;
        typedef DefaultPolicy4 P4;
    };
    
    //这里之所以给出中间类DefaultPolicyArgs，同时又让该类以虚拟继承的方式继承
    //DefaultPolicies，一是为了避免后面在多重继承同一基类时而导致的二义性，同时
    //也是为了方便后面其他类的继承。
    class DefaultPolicyArgs : virtual public DefaultPolicies {
        
    };
    
    template <typename Policy>
    class Policy1_is : virtual public DefaultPolicies {
        
    public:
        typedef Policy P1;  // overriding typedef
    };
    
    template <typename Policy>
    class Policy2_is : virtual public DefaultPolicies {
        
    public:
        typedef Policy P2;  // overriding typedef
    };
    
    template <typename Policy>
    class Policy3_is : virtual public DefaultPolicies {
        
    public:
        typedef Policy P3;  // overriding typedef
    };
    
    template <typename Policy>
    class Policy4_is : virtual public DefaultPolicies {
        
    public:
        typedef Policy P4;  // overriding typedef
    };
    
    template <typename PolicySetter1 = DefaultPolicyArgs,
            typename PolicySetter2 = DefaultPolicyArgs,
            typename PolicySetter3 = DefaultPolicyArgs,
            typename PolicySetter4 = DefaultPolicyArgs>
    class BreadSlicer {
        
        typedef PolicySelector<PolicySetter1, PolicySetter2,
                PolicySetter3, PolicySetter4>
                Policies;
        // use Policies::P1, Policies::P2, //... to refer to the various policies.
    public:
        void print () {
        
            Policies::P3::doPrint();
        }
        //...
    };
    
    
    // define a custom policy
    class CustomPolicy {
        
    public:
        static void doPrint() {
        
            std::cout << "CustomPolicy::doPrint()\n";
        }
    };
    
    int main()
    {
        
        BreadSlicer<> bc1;
        bc1.print();
    
        BreadSlicer<Policy3_is<CustomPolicy> > bc2;
        bc2.print();
    }

待研究博客：[在现代C ++中引入命名参数][C]

[C]: https://marcoarena.wordpress.com/2014/12/16/bring-named-parameters-in-modern-cpp/