在 .cpp 文件中定义静态类成员时的未定义引用错误

Question

This question may seem similar to other "undefined reference error when accessing static class member" questions. I have explored them and what I understood is I need to define the static class member separately in a source file, so that an object is created for the static class member that holds data.

My problem is that I am following definition rules but still get the undefined reference errors.

problem can be reproduced using this code:

main.cpp

#include <iostream>

#include "src/a.hpp"

int main() {
    std::cout << a::x;
    return 0;
}

src/a.hpp

class a {
public:
    static int x;
};

src/a.cpp

#include "a.hpp"

int a::x = 20;

I compile main.cpp using g++ main.cpp -o main . I have a test directory that has main.cpp and a sub directory src , src contains a.hpp and a.cpp .

The error resolves if i define the static variable within the header file, but other posts suggest it should lead to linker errors.

Answer 1

The problem was in the compile command I used, g++ main.cpp -o main does not compile src/a.cpp. Compile it with g++ main.cpp src/a.cpp -o main and it works fine.