How to use extern to link to a function in other cpp file?

Question

Here is my main.cpp:

#include <iostream>
#include "function.cpp"

using namespace std;

extern int giveMain();

int main() {
    int x = 4;
    x = giveMain(x);
    cout << x << endl;
}

And here is my function.cpp:

#include <iostream>

using namespace std;

int giveMain(int a) {
    a = 3 + a;
    return a;
}

But when I compile, it says that "Linker command failed". Can anyone helps me to solve this problem.

Answer 1

You declared the function int giveMain() in main.cpp but the function in function.cpp takes an int . Declare the correct function and it should work. Also extern is the default for functions so you don't need to include the keyword.

EDIT: Just noticed that you #include <function.cpp> in main.cpp. Never include .cpp files. The issue you were having was multiple definitions for int giveMain(int) because the contents of functions.cpp was being compiled twice.