为什么不能在类中定义变量？ [英] Why can't I define variables in classes?

问题描述

每当我定义一个变量，并在类中同时给它一个值，我得到一个错误。这是什么原因？

Whenever I define a variable and give it a value at the same time inside a class, I get an error. What is the reason for this?

如你所见，这不起作用...

As you can see, this doesn't work...

class myClass {
    private:
        int x = 4; // error
};

但是当我保持变量undefined时：

But when I keep the variable undefined it does:

class myClass {
    private:
        int x;
};

推荐答案

由于没有其他人使用成员初始化，将介绍你：

Since no one else is using member initialization, I'll introduce you:

class myClass {
    private:
        int x;
    public:
        myClass() : x (4){}
};

在构造函数的主体中使用它总是更好，开始，所有用户定义的成员将已经被初始化，无论你是否这样说。更好地做一次，实际上初始化非用户定义的成员，并且是唯一适用于非静态 const 成员和引用成员。

It's always better to use this over assigning in the body of the constructor, since by the time the body begins, all user-defined members will have already been initialized whether you said so or not. Better to do it once and actually initialize the non-user-defined members, and it is the only method that works for both non-static const members, and reference members.

例如，以下操作将不起作用，因为 x 正在分配到：

For example, the following will not work because x isn't being initialized in the body, it's being assigned to:

class myClass {
    private:
        const int x;
    public:
        myClass() {x = 4;}
};

但是，使用成员初始值设定程序将会，因为你正在初始化它。 p>

Using a member initializer, however, will, because you're initializing it off the bat:

class myClass {
    private:
        const int x;
    public:
        myClass() : x (4){}
};

请注意，您的 int x = 4; 语法是在C ++ 11中完全有效，其中包含任何需要的初始化，所以如果你开始使用它，你会受益。

Note also that your int x = 4; syntax is perfectly valid in C++11, where it subs in for any needed initialization, so you'll benefit if you start using it.

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