build.sh

clang++ -c mat.cpp -o mat.o;
clang++ main.cpp mat.o;

main.cpp

#include "mat.hpp"
#include<iostream>


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

mat.cpp

#include"mat.hpp"

int a = 4;

mat.hpp

#pragma once

extern int a;