C ++-包含库中的extern变量

Question

Even after quite a bit of googling, I have a problem in C++ where I made one project with a header and a source file and tried to make another project that included the first project. I found this question but in that the asker was able to generate a .a file which I cannot seem to do.

Header.hpp

extern int num;

Source.cpp

#include "Header.hpp"

int num = 2;

(By the way, this is all in Code::Blocks)

I made another project that would include the first project.

Main.cpp

#include <iostream> 

#include "Header.hpp"

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

To include the first project, I:

• Added the root directory of the first project to the "Search Directories" -> "Compiler" list of the second one

• Added that same root directory of the first project to the "Search Directories" -> "Linker" list of the second one

When I try to compile this code with MinGW, I get an error:

undefined reference to 'num'

Some googling suggested that I add the .lib/.a file of the first project to the "Linker Settings" -> "Link Libraries" list. However, my first project does not have a .lib/.a file and I am not quite sure how to create one.

What more is required to ensure that my "library" project can be included by my second project?

Answer 1

Thanks to the comment by Sami Kuhmonen, I realized that the problem was that I had set the build target to an executable instead of a static library, which was why I was getting a .exe file instead of a .a file. All I had to do was change the build target, add the .a file to the "Linker Settings" -> "Link Libraries" list and change the "Search Directories" -> "Compiler"/"Linker" lists to search the include and src directories respectively and it worked.