在类模板中正确初始化静态constexpr数组？ [英] Proper initialization of static constexpr array in class template?

问题描述

由于标准的困扰，C ++中的静态类成员对我造成了一些困惑：

Static class members in C++ have caused a little confusion for me due to the standard's verbiage:

9.4.2静态数据成员 [class.static.data]

在其类定义中声明静态数据成员不是定义...

The declaration of a static data member in its class definition is not a definition...

但是在其声明时（例如，在 constexpr 中需要初始化 constexpr ）

However a constexpr is required to be initialized (AFAIK, couldn't find a quote from the standard) at its declaration (e.g., in the class definition).

由于对constexpr的限制，我实际上已经忘记了在类外定义静态成员的必要条件，直到尝试访问静态constexpr数组为止。 此相关问题提供了定义数组成员的正确方法，但是我我对类模板中此定义的含义感兴趣。

Because of the restrictions on constexpr I had actually forgotten about the requisite for static members to be defined outside of the class, until I tried accessing a static constexpr array. This related question provides the correct way of defining the array member, but I'm interested as to the implications on this definition in a class template.

这就是我最终得到的结果：

This is what I ended up with:

template<typename T>
class MyClass
{
private:
  static constexpr std::size_t _lut[256] = { /* ... */ };
  T _data;

public:
  static constexpr std::size_t GetValue(std::size_t n) noexcept
  {
    return _lut[n & 255];
  }

  // ...
};

template<typename T>
constexpr std::size_t MyClass<T>::_lut[256];

这是正确的语法吗？特别是在定义中使用模板感觉很尴尬，但是GCC似乎在适当地链接所有内容。

Is this the right syntax? Particularly the use of template in the definition feels awkward, but GCC seems to be linking everything appropriately.

作为后续问题，是否应该类似地定义非数组静态constexpr成员（使用类之外的模板定义）？

As a follow-up question, should non-array static constexpr members be similarly defined (with template definition outside of class)?

推荐答案

我想您想要9.4.2p3：

I think you want 9.4.2p3:

如果非易失性 const static 数据成员为整数或枚举类型，则其在类定义中的声明可以指定 brace-or-equal-initializer ，其中每个作为 assignment-expression 的 initializer-clause 都是一个常数表达式（5.19）。可以使用 constexpr 说明符在类定义中声明文字类型的静态数据成员；如果是这样，则其声明应指定一个
brace-or-equal-initializer ，其中每个 assignment-expression 的 initializer-clause 常数表达式。 [...]如果在程序中使用了成员（3.2），则该成员仍应在命名空间范围内定义，并且该命名空间范围定义不应包含 initializer 。

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [...] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

template 静态数据成员的定义是 template-declaration （14p1）。在14.5.1.3p1中给出的示例是：

The definition of a template static data member is a template-declaration (14p1). The example given in 14.5.1.3p1 is:

template<class T> class X {
  static T s;
};
template<class T> T X<T>::s = 0;

但是，与上面的 constexpr static 或在类声明中指定 initializer 的 const static 成员，其名称空间范围定义中不应包含 initializer 。语法变为：

However, as above a constexpr static or const static member whose in-class declaration specifies an initializer should not have an initializer in its namespace scope definition, so the syntax becomes:

template<class T> class X {
  const static T s = 0;
};
template<class T> T X<T>::s;

与非数组（即整数或枚举）静态constexpr数据成员的区别在于其用途在左值到右值转换中，不能使用odr；您只需定义其地址或对其构成const引用即可对其进行定义。

The difference with the non-array (i.e. integral or enumeration) static constexpr data member is that its use in lvalue-to-rvalue conversion is not odr-use; you would only need to define it if taking its address or forming a const reference to it.

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