Access member function of another .cpp within same source file?

You should define your class in a .h file, and implement it in a .cpp file. Then, include your .h file wherever you want to use your class.

For example

file use_me.h

#include <iostream>
class Use_me{

   public: void echo(char c);

};

file use_me.cpp

#include "use_me.h" //use_me.h must be placed in the same directory as use_me.cpp

void Use_me::echo(char c){std::cout<<c<<std::endl;}

main.cpp

#include "use_me.h"//use_me.h must be in the same directory as main.cpp
    int main(){
       char c = 1;

       Use_me use;
       use.echo(c);

       return 0;

    }