功能声明与extern和没有它的区别

Question

There is such code:

#include <iostream>

extern void fun();

int main(){
    fun();
    return 0;
}

void fun(){ std::cout << "Hello" << std::endl; }

Is there some difference between declarations:

extern void fun();
void fun();

? Code above behaves the same with extern and without extern keyword.

Answer 1

默认情况下，函数声明确实有外部链接 ，因此将extern关键字添加到函数声明没有任何区别，这是多余的。

Answer 2

The difference between the two statements is this:

extern void fun();

tells the compiler and linker to look in another file when the code in this file refers to fun( ), perhaps by calling fun( ); This production is called a "declaration."

void fun ( ) {
  ...
}

Defines the function fun ( ) and, because it's defined in this file, obviates the need for the linker to look for the function elsewhere.

There's no harm in declaring the function extern: the linker does the right thing.