具有不同接口的策略类 [英] Policy classes with differing interfaces

问题描述

假设有一个策略 FooPolicy 的算法。实现此策略的策略类具有静态成员函数 foo ，但是对于其中一些， foo code> int 参数，而对于其他人则不行。我试图通过 constexpr 静态数据成员使用具有不同接口的这些策略类的使用：

Suppose an algorithm that has a policy FooPolicy. Policy classes that implement this policy feature a static member function foo, but, for some of them, foo takes an int argument, while for others it does not. I am trying to enable the use of these policy classes with differing interfaces by means of constexpr static data members:

struct SimpleFoo {
    static constexpr bool paramFlag = false;
    static void foo() {
        std::cout << "In SimpleFoo" << std::endl;
    }
};

struct ParamFoo {
    static constexpr bool paramFlag = true;
    static void foo(int param) {
        std::cout << "In ParamFoo " << param << std::endl;
    }
};

template <typename FooPolicy>
struct Alg {
    void foo() {
        if (FooPolicy::paramFlag) FooPolicy::foo(5);
        else FooPolicy::foo();
    }
};

int main() {
    Alg<ParamFoo> alg;
    alg.foo();
    return 0;
}

此代码不编译。 gcc 4.8.2 提供错误：

This code does not compile. gcc 4.8.2 gives the error:

ParamFoo :: foo（）'

no matching function for call to ‘ParamFoo::foo()’

else FooPolicy :: foo（）;

else FooPolicy::foo();

else 子句被编译，尽管事实上在编译时已知 FooPolicy :: paramFlag true 。是否有办法让它工作？

The else clause gets compiled despite the fact that it is known at compile time that FooPolicy::paramFlag is true. Is there a way to make it work?

推荐答案

Is there a way to make it work?

一个解决方案是使用标记调度：

One solution is to use tag-dispatching:

#include <type_traits>

template <typename FooPolicy>
struct Alg {
    void foo() {
        foo(std::integral_constant<bool, FooPolicy::paramFlag>{});
    }    
private:
    void foo(std::true_type) {
        FooPolicy::foo(5);
    }
    void foo(std::false_type) {
        FooPolicy::foo();
    }
};

DEMO

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