在C ++中的类声明中初始化const成员 [英] Initializing const member within class declaration in C++

问题描述

在PHP和C＃中，常量可以在声明时初始化：

In PHP and C# the constants can be initialized as they are declared:

class Calendar3
{
    const int value1 = 12;
    const double value2 = 0.001;
}

我有一个函数的C ++声明，比较两个数学向量：

I have the following C++ declaration of a functor which is used with another class to compare two math vectors:

struct equal_vec
{
    bool operator() (const Vector3D& a, const Vector3D& b) const
    {
        Vector3D dist = b - a;
        return ( dist.length2() <= tolerance );
    }

    static const float tolerance = 0.001;
};

这段代码编译时没有遇到g ++问题。现在在C ++ 0x模式（-std = c ++ 0x）中，g ++编译器输出一条错误信息：

This code compiled without problems with g++. Now in C++0x mode (-std=c++0x) the g++ compiler outputs an error message:

'

error: ‘constexpr’ needed for in-class initialization of static data member ‘tolerance’ of non-integral type

我知道我可以定义和初始化这个 static const 成员在类定义之外。另外，非静态常量数据成员可以在构造函数的初始化器列表中初始化。

I know I can define and initialize this static const member outside of the class definition. Also, a non-static constant data member can be initialized in the initializer list of a constructor.

但是有什么方法可以在类声明中初始化一个常量

But is there any way to initialize a constant within class declaration just like it is possible in PHP or C#?

我使用了 static 关键字，因为它可以在g ++中的类声明中初始化这样的常量。我只需要一个方法来初始化一个常量在类声明中，无论是否声明为 static 或不。

I used static keyword just because it was possible to initialize such constants within the class declaration in g++. I just need a way to initialize a constant in a class declaration no matter if it declared as static or not.

推荐答案

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.

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