静态类成员的常量表达式初始值为double类型

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，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:

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:

参考 N1804 9.4.2 [class.static.data] 说：

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:

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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