Using multiple .cpp files in c++ program?

You must use a tool called a “header”. In a header you declare the function that you want to use. Then you include it in both files. A header is a separate file included using the #include directive. Then you may call the other function.

other.h

void MyFunc();

main.cpp

#include "other.h"
int main() {
    MyFunc();
}

other.cpp

#include "other.h"
#include <iostream>
void MyFunc() {
    std::cout << "Ohai from another .cpp file!";
    std::cin.get();
}