模板相关的假 [英] Template dependent false

问题描述

我有一个不能直接使用的类模板，只允许专业化.而且我想使用static_assert来显示有意义的错误消息. 我不能只键入static_assert(false, "error");，因为false与值无关，即使从不使用模板，编译器也可能会显示错误消息.

I have a class template which can't be used directly, only specializations are allowed. And I want to use static_assert to show meaningful error message. I can't just type static_assert(false, "error"); since false isn't value dependent and compiler may show error message even if the template is never used.

我的解决方案:

template<class>
struct AlwaysFalse : std::false_type{};

#define DEPENDENT_FALSE(arg) AlwaysFalse<decltype(sizeof(arg))>::value

template<class T>
struct Foo{
    static_assert(DEPENDENT_FALSE(T), "You must use specialization!");
};

template<int i>
struct Bar{
    static_assert(DEPENDENT_FALSE(i), "You must use specialization!");
};

但是我不确定实现DEPENDENT_FALSE.因为MSVC 不会将sizeof(arg)视为模板相关的表达式(很好.

But I'm not sure about realization DEPENDENT_FALSE. Because MSVC doesn't treat sizeof(arg) as template dependent expression(unlike GCC), but decltype(sizeof(arg)) is fine.

有人可以用标准来解释这种行为吗?便携吗?

Can somebody explain this behavior in terms of standard? Is it portable?

推荐答案

此:

#define DEPENDENT_FALSE(arg) AlwaysFalse<decltype(sizeof(arg))>::value

实际与无关. decltype(sizeof(arg))始终是size_t，它实际上不以任何方式依赖arg(更广泛地说，这是

fails to actually be dependent. decltype(sizeof(arg)) is always size_t, it doesn't actually depend on arg in any way (more broadly, here is a list of expressions that are never type-dependent). Since it's not dependent, a compiler is perfectly able to see that DEPENDENT_FALSE(T) is false and just trigger that static_assert.

您想要的只是:

#define DEPENDENT_FALSE(arg) AlwaysFalse<decltype(arg)>::value

也就是说，放下sizeof.现在，这是依赖.

That is, drop the sizeof. This now is dependent.

这将直接对int不起作用，因为这又不再是依赖项(decltype(i)只是int，我们现在需要依赖于值的东西).为此，您可以将其包装在一个整数常量中:

This won't work for the int directly, since that again won't be dependent (decltype(i) is just int, and we need something value-dependent now). For that, you can just wrap it in an integral constant:

template<class T>
struct Foo{
    static_assert(AlwaysFalse<T>::value, "You must use specialization!");
};

template<int i>
struct Bar{
    static_assert(AlwaysFalse<std::integral_constant<int, i>>::value, "You must use specialization!");
};

这篇关于模板相关的假的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！