extern 为什么可以这样使用呢?
//test.cppstatic int m = 10;int *p = &m;//main.cpp#include<iostream>extern int m;extern int *p;int main(){ std::cout << m << std::endl; std::cout << *p << std::endl; return 0;}//I:\learn C++\Learn C++\main.cpp|6|undefined reference to `m'|