静态类成员的常量表达式初始值为double类型 [英] Constant expression initializer for static class member of type double

问题描述

在C ++ 11和C ++ 14中，为什么我需要 constexpr ：

In C++11 and C++14, why do I need constexpr in the following snippet:

class Foo {
    static constexpr double X = 0.75;
};

，但这会产生编译器错误：

whereas this one produces a compiler error:

class Foo {
    static const double X = 0.75;
};

（更奇怪的是）这个编译没有错误？

and (more surprisingly) this compiles without errors?

class Foo {
    static const double X;
};

const double Foo::X = 0.75;

推荐答案

在C ++ 03中，在枚举类型的const积分的静态成员变量的类初始化器中，在C ++ 11中，我们可以使用constexpr初始化类中的文字类型的静态成员。这个限制保留在C ++ 11中的const变量主要是为了兼容性，C ++ 03我们可以从 closed issue 1826：常量表达式中的const浮点数，它说：

In C++03 we were only allowed to provide an in class initializer for static member variables of const integral of enumeration types, in C++11 we could initialize a static member of literal type in class using constexpr. This restriction was kept in C++11 for const variables mainly for compatibility will C++03 we can see this from closed issue 1826: const floating-point in constant expressions which says:

用常数初始化的const整数可以在常量表达式中使用，但是用常量初始化的const浮点变量不能。这是故意的，与C ++ 03兼容，同时鼓励一致使用constexpr。然而，有些人发现这种区别是令人惊讶的。

A const integer initialized with a constant can be used in constant expressions, but a const floating point variable initialized with a constant cannot. This was intentional, to be compatible with C++03 while encouraging the consistent use of constexpr. Some people have found this distinction to be surprising, however.

CWG最后将此请求关闭为不是缺陷），基本上说：

CWG ended up closing this request as not a defect(NAD), basically saying:

程序员希望浮点值参与常量表达式应使用constexpr而不是const。 / p>

that programmers desiring floating point values to participate in constant expressions should use constexpr instead of const.

参考 N1804 9.4.2 [class.static.data] 说：

如果静态数据成员是const整数或const枚举类型，它在类定义中的声明可以
指定一个常量初始化器，它应该是一个整数常量表达式（5.19）。在这种情况下，成员可以在整型常数表达式中出现
。如果在程序中使用，则成员仍然在命名空间范围中定义，并且
命名空间范围定义不包含初始化程序。

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.

和草稿C ++ 11标准部分 9.4.2 [class.static.data] 说：

and the draft C++11 standard section 9.4.2 [class.static.data] says:

如果非易失性const静态数据成员是积分或枚举类型，它在
类定义中的声明可以指定一个大括号 - 等式初始化器，其中作为赋值表达式
的每个initializer子句都是常量表达式（5.19）。可以在
类定义中使用constexpr说明符声明文字类型的静态数据成员;如果是，其声明应指定一个括号或初始值为
，其中作为赋值表达式的每个initializer子句都是一个常量表达式。 [...]

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. [...]

这在草案C ++ 14标准中是相同的。

this is pretty much the same in the draft C++14 standard.

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