为什么我们在类中声明静态变量并在类外部声明定义？

Question

我们static在类中声明一个变量并在类外部初始化变量,但我们在函数中使用变量.

有人告诉我原因吗？提前致谢

Answer 1

我不确定,但我的猜测是,因为在一个类成员变量只是声明.它们通过构造函数或其他成员函数初始化.

在实例化对象时会发生这种情况.但是对于静态成员,不需要实例化对象.因此,他们需要在课外进行一次初始化.

编辑:
实际上没有必要初始化静态变量,但有必要在类外定义它们以为它们分配内存.只有在定义之后,才能将它们初始化然后在程序中使用.

Answer 2

因为静态变量需要在某处分配一些存储空间。

让我们以这个为例：

struct Example
{
    static int counter;
    Example() { counter++; }
};

You can create as many Example instances as you want, but there will only ever be one Example::counter variable. It's basically a global variable in disguise. But where should it live? Back in the early days of C++, Stroustrup decided the solution to this was for you to explicitly choose one translation unit (i.e. .cpp file) and declare it there, just as you would a "real" global variable. So you need to do something like

// Only in one .cpp file
int Example::counter = 0;

(Of course, later on templates were invented, and weak symbols to go with them, which could have solved this awkward mess, but it was too late by then.)