double 类型的静态 class 成员的常量表达式初始值设定项
Constant expression initializer for static class member of type double
在 C++11 和 C++14 中，为什么在以下代码段中需要 constexpr：
class Foo {
static constexpr double X = 0.75;
};
而这个会产生编译器错误：
class Foo {
static const double X = 0.75;
};
并且（更令人惊讶的是）这个编译没有错误？
class Foo {
static const double X;
};
const double Foo::X = 0.75;
在C++03中我们只允许为枚举类型的const整型静态成员变量提供一个in class初始化器，在C++11中我们可以在in中初始化一个字面量类型的静态成员class 使用 constexpr。此限制在 C++11 中保留用于 const 变量，主要是为了与 C++03 兼容。我们可以从 closed issue 1826: const floating-point in constant expressions 中看到：
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 最终关闭了这个请求，认为它不是缺陷 (NAD)，基本上是说：
that programmers desiring floating point values to participate in constant expressions should use constexpr instead of const.
供参考 N1804 最接近 C++03 的标准草案在 9.4.2 [class.static.data] 部分公开提供说：
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 C++11 标准草案 9.4.2 [class.static.data] 说：
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 C++14 标准草案中几乎相同。
In-class static const "definitions" 其实就是声明。定义变量时，编译器会为该变量分配内存，但这里不是这种情况，即获取这些 static-const-in-class 的地址格式不正确，NDR。
这些东西应该写到代码中，但是对于浮点类型来说不太容易做到，因此是不允许的。
通过在 class 之外定义静态 const 变量，您向编译器发出信号，这是真正的定义 - 具有内存位置的真实实例。
在 C++11 和 C++14 中，为什么在以下代码段中需要 constexpr：
class Foo {
static constexpr double X = 0.75;
};
而这个会产生编译器错误：
class Foo {
static const double X = 0.75;
};
并且（更令人惊讶的是）这个编译没有错误？
class Foo {
static const double X;
};
const double Foo::X = 0.75;
在C++03中我们只允许为枚举类型的const整型静态成员变量提供一个in class初始化器，在C++11中我们可以在in中初始化一个字面量类型的静态成员class 使用 constexpr。此限制在 C++11 中保留用于 const 变量，主要是为了与 C++03 兼容。我们可以从 closed issue 1826: const floating-point in constant expressions 中看到：
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 最终关闭了这个请求，认为它不是缺陷 (NAD)，基本上是说：
that programmers desiring floating point values to participate in constant expressions should use constexpr instead of const.
供参考 N1804 最接近 C++03 的标准草案在 9.4.2 [class.static.data] 部分公开提供说：
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 C++11 标准草案 9.4.2 [class.static.data] 说：
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 C++14 标准草案中几乎相同。
In-class static const "definitions" 其实就是声明。定义变量时，编译器会为该变量分配内存，但这里不是这种情况，即获取这些 static-const-in-class 的地址格式不正确，NDR。
这些东西应该写到代码中，但是对于浮点类型来说不太容易做到，因此是不允许的。
通过在 class 之外定义静态 const 变量，您向编译器发出信号，这是真正的定义 - 具有内存位置的真实实例。