检查类是否具有给定签名的成员函数 [英] Check if a class has a member function of a given signature

问题描述

我要求一个模板技巧来检测类是否具有给定签名的特定成员函数。

I'm asking for a template trick to detect if a class has a specific member function of a given signature.

问题与此处引用的问题类似
http://www.gotw.ca/gotw/071.htm
，但不一样：在Sutter的书中，他回答了一个问题：类C必须提供一个带有特定签名的成员函数，否则程序不会编译。在我的问题，我需要做一些事情，如果一个类有该函数，否则做别的东西。

The problem is similar to the one cited here http://www.gotw.ca/gotw/071.htm but not the same: in the item of Sutter's book he answered to the question that a class C MUST PROVIDE a member function with a particular signature, else the program won't compile. In my problem I need to do something if a class has that function, else do "something else".

boost :: serialization遇到类似的问题，但我不喜欢他们采用的解决方案：一个模板函数，默认调用一个自由函数定义）特定的签名，除非你定义一个特定的成员函数（在他们的情况下，serialize有一个给定的类型的2个参数），否则会发生编译错误。这就是实现侵入和非侵入序列化。

A similar problem was faced by boost::serialization but I don't like the solution they adopted: a template function that invokes by default a free function (that you have to define) with a particular signature unless you define a particular member function (in their case "serialize" that takes 2 parameters of a given type) with a particular signature, else a compile error will happens. That is to implement both intrusive and non-intrusive serialization.

我不喜欢这个解决方案有两个原因：

I don't like that solution for two reasons:

1. 非侵入性的，你必须重写boost :: serialization命名空间中的全局serialize函数，所以你有你的客户代码打开命名空间boost和命名空间序列化！


2. 
   mess是10到12个函数调用。

我需要为没有该成员的类定义一个自定义行为函数，我的实体位于不同的命名空间中（当我在另一个命名空间中时，我不想覆盖一个命名空间中定义的全局函数）

I need to define a custom behavior for classes that has not that member function, and my entities are inside different namespaces (and I don't want to override a global function defined in one namespace while I'm in another one)

给我一个提示来解决这个难题？

Can you give me an hint to solve this puzzle?

推荐答案

我不知道我是否理解你正确，但你可能利用SFINAE以在编译时检测功能存在。示例从我的代码（测试类是否有成员函数size_t used_memory（）const）。

I'm not sure if I understand you correctly, but you may exploit SFINAE to detect function presence at compile-time. Example from my code (tests if class has member function size_t used_memory() const).

template<typename T>
struct HasUsedMemoryMethod
{
    template<typename U, size_t (U::*)() const> struct SFINAE {};
    template<typename U> static char Test(SFINAE<U, &U::used_memory>*);
    template<typename U> static int Test(...);
    static const bool Has = sizeof(Test<T>(0)) == sizeof(char);
};

template<typename TMap>
void ReportMemUsage(const TMap& m, std::true_type)
{
        // We may call used_memory() on m here.
}
template<typename TMap>
void ReportMemUsage(const TMap&, std::false_type)
{
}
template<typename TMap>
void ReportMemUsage(const TMap& m)
{
    ReportMemUsage(m, 
        std::integral_constant<bool, HasUsedMemoryMethod<TMap>::Has>());
}

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