c++ - g++ 链接器问题

Question

我试图理解 C++ 中的关键字 extern 并编写了一段简短的代码来概述其含义。不幸的是我做错了什么

bla.h

int bla = 4;

测试.cpp

#include <iostream>

using namespace std;

int main() {
       extern int bla;
       cout << bla << endl;
}

g++ -o test bla.h test.cpp
/tmp/ccED67jz.o: In function `main':
test.cpp:(.text+0xa): undefined reference to `bla'
collect2: ld returned 1 exit status

Answer 1

extern，您描述的用法仅适用于全局变量:

bla.cpp

int bla = 4;

测试.cpp

#include <iostream>

extern int bla; // use the global from bla.cpp as a global in this file

int main(int argc, char *argv[]) {
    std::cout << bla << "\n";
    return 0;
}