外部变量 - 多个文件

Question

当我在另一个不同类型的文件中重新定义 extern 变量时，VS 编译器没有给出错误消息。据我所知，它应该引发错误，因为它在另一个文件中被全局定义为 extern 。这种行为的原因是什么？

source1.cpp

extern int x;

source2.cpp

int x = 5;

test.cpp

#include <iostream>
double x = 455;
    
int main()
{
    std::cout << x; // writes 455
}

Answer 1

One Definition Rule

One and only one definition of every non-inline function or variable that is odr-used (see below) is required to appear in the entire program (including any standard and user-defined libraries). The compiler is not required to diagnose this violation, but the behavior of the program that violates it is undefined.

外部变量 - 多个文件

Extern Variable - Multiple Files

c++

extern