在模板专门化中强制发生编译时错误 [英] Force a compile time error in a template specialization

问题描述

我最近开始学习现代模板元编程，并为自己编写了类型的index_of函数。

I recently started to learn modern template metaprogramming and I wrote myself an index_of function for types.

template<std::size_t Index, class C, class A> struct mp_index_of_impl{};
template<std::size_t Index, class C,template<class...> class A,class ...Ts>
struct mp_index_of_impl<Index,C,A<C,Ts...>>{
    using type = std::integral_constant<std::size_t,Index>;
};
template<std::size_t Index, class C,template<class...> class A,class ...Ts,class T1>
struct mp_index_of_impl<Index,C,A<T1,Ts...>>{
    using type = typename mp_index_of_impl<Index + 1,C,A<Ts...>>::type;
};
template<std::size_t Index, class C,template<class...> class A> struct mp_index_of_impl<Index,C,A<>>{
    //static_assert(false,"Does not contain type");
    using type = std::integral_constant<std::size_t,0>;
};

问题是我的最后一个专业化

The problem is my last specialization

template<std::size_t Index, class C,template<class...> class A> 
struct mp_index_of_impl<Index,C,A<>>{
      //throw a compile error here
};

我试图像这样使用static_assert

I tried to use static_assert like this

template<std::size_t Index, class C,template<class...> class A> struct mp_index_of_impl<Index,C,A<>>{
    static_assert(false,"Does not contain type");
};

但这每次都会引发编译错误，即使不匹配也是如此。

But this will throw a compile error every time, even if it is not matched.

如果此模板特化匹配，我想使用自定义错误消息抛出编译错误。我该怎么办？

I want to throw a compile error with a custom error message if this template specialization is matched. How would I do this?

推荐答案

如果您放入 static_assert（false，...）在模板特化中，则始终不可能从中生成有效代码。这是格式错误，不需要诊断。

If you put a static_assert(false,...) in a template specialization then it is always impossible to generate valid code from it. This is ill-formed, no diagnostic required.

一种解决方法是使您的 static_assert 取决于模板参数：

A workaround to this is to make your static_assert depend on the template parameter:

template<typename T>
struct assert_false : std::false_type
{ };

template <typename T>
inline T getValue(AnObject&)
{
    static_assert( assert_false<T>::value , "Does not contain type");
}

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