转发声明constexpr变量模板 [英] Forward declare a constexpr variable template

问题描述

我试图像这样向前声明一个constexpr变量模板:

I tried to forward-declare a constexpr variable template like this:

template<typename>
constexpr std::size_t iterator_category_value;

目标是记录每个专业化名称应为constexpr，但我不得不承认我从未检查过它是否合法，而g ++对此感到满意.但是，当我尝试使用clang ++编译此Spinnet时，出现以下错误:

The goal was to document that every specialization should be constexpr but I have to admit that I never checked whether it was legal or not and g++ was happy with it. However, when I tried to compile this spinnet with clang++ instead, I got the following error:

error: default initialization of an object of const type 'const std::size_t' (aka 'const unsigned long')
    constexpr std::size_t iterator_category_value;
                          ^
                                                  = 0

该错误是有道理的，删除constexpr会使该错误消失，所以这不是真正的问题.但是，我现在很好奇:该标准是否允许对变量模板使用这样的constexpr前向声明，或者它是非法的? g ++和clang ++似乎不同意，我想知道如果需要的话我应该在哪里提交错误报告.

The error makes sense, and removing constexpr makes it disappear, so that's not a real problem. However, I am curious now: does the standard allow such a constexpr forward declaration for a variable template or is it illegal? g++ and clang++ seem to disagree and I would like to know where I should submit a bug report if needed.

他们两个都抱怨一个前向声明的constepxr变量，它不是变量模板，因此变量模板上下文似乎是导致编译器不同意的原因.

Both of them complain for a forward-declared constepxr variable which is not a variable template, so the variable template context seems to be what makes the compilers disagree.

推荐答案

在C ++ 14标准中，很明显需要初始化.根据第7.5.1节第9段，

In the C++14 standard, it seems pretty clear that initialization is required. From section 7.5.1 paragraph 9,

对象声明中使用的constexpr说明符 将该对象声明为const.此类物体应具有 文字类型，应进行初始化.

A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized.

关于对象声明"的确切含义，第7条第7款规定:

As for the exact meaning of "object declaration", Section 7 paragraph 7 states:

如果decl-specifier-seq不包含typedef说明符， 该声明称为函数声明，如果 与名称关联的类型是函数类型，并且 否则是对象声明.

If the decl-specifier-seq contains no typedef specifier, the declaration is called a function declaration if the type associated with the name is a function type and an object declaration otherwise.

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