在C ++ 11中，非 - 静态数据成员， static constexpr 数据成员，并且整数或枚​​举类型的 static const 数据成员可以在类声明中初始化。例如

In C++11, non-static data members, static constexpr data members, and static const data members of integral or enumeration type may be initialized in the class declaration. e.g.

struct X {
    int i=5;
    const float f=3.12f;
    static const int j=42;
    static constexpr float g=9.5f;
};

在这种情况下， i 编译器生成的构造函数将 X 的所有实例初始化为 5 ， f 成员初始化为 3.12 。 static const 数据成员 j 初始化为 42 并将 static constexpr 数据成员 g 初始化为 9.5

In this case, the i member of all instances of class X is initialized to 5 by the compiler-generated constructor, and the f member is initialized to 3.12. The static const data member j is initialized to 42, and the static constexpr data member g is initialized to 9.5.

由于 float 和 double 或枚举类型，这些成员必须是 constexpr 或非 - static

Since float and double are not of integral or enumeration type, such members must either be constexpr, or non-static in order for the initializer in the class definition to be permitted.

在C ++ 11之前，只有 static const 整数或枚举类型的数据成员才可以在类定义中有初始值。

Prior to C++11, only static const data members of integral or enumeration type could have initializers in the class definition.