为什么要在 C++ 中初始化静态类变量? [英] Why should I initialize static class variables in C++?

问题描述

在 C 和 C++ 中，所有静态变量默认初始化为零.

In C and C++ all static variables are initialized by default to ZERO.

这不是静态类数据成员的情况.这是为什么?

This is not the case of static class data members. Why is that?

#include <iostream>
using namespace std;

int var;

class MyClass
{
public:
    static int classVar;
};
int MyClass::classVar = 0;  // Why I have to init it here?

int main(void)
{
    cout << ::var << endl;          // this is initalized to ZERO by default
    static int var;
    cout << var << endl;            // and this also is initalized to Zero
    cout << MyClass::classVar << endl;

    return 0;
}

推荐答案

在类范围内，

int MyClass::classVar = 0;  // Why I have to init it here?

是一个定义和

static int classVar;

是一个声明，即.承诺变量将在某处定义:您必须只定义一次您声明的变量.

is a declaration, ie. a promise the variable will be defined somewhere: you must define exactly once the variables you declare.

理由是类声明可能包含在多个源文件中.如果它的一部分是一个定义，它会成倍地发生:这是错误的(例外是内联 [成员] 函数).

The rationale is that the class declaration will likely be included in multiple source files. Would a part of it be a definition, it would take place multiply: this is erroneous (exceptions are inline [member] functions).

注意，根据值初始化规则，可以相处

Note that according to value initialization rules, you can get along with

int MyClass::classVar;  // Zero-initialized !

作为定义.

在命名空间范围内声明的变量也是定义(除非它们是 extern 限定的):

Variables declared at namespace scope are definitions too (unless they are extern qualified):

int var;

是一个声明，也是一个定义:如果你把它放在一个标题中并包含在多个翻译单元中，你就会有一个错误(多重定义的符号"，或者类似的东西).

is a declaration, and a definition: if you put this into a header and include it in multiple translation units, you have an error ("multiply defined symbol", or something along those lines).

[注意在C++中(而不是在C中)，如果上面的var是const，它会自动变成static如果将其放入多包含的标题中，则不会违反单一定义规则.这有点偏离主题，但请随时询问详细信息]

[Note that in C++ (and not in C), if the var above is const, it becomes automatically static and there is no violation of the One Definition Rule should it be put into a multiply included header. This goes slightly off topic, but feel free to ask details]

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