C++ What should I do instead of global variables? [duplicate] - c++

Question

This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
C++: Easiest way to access main variable from function?
I need to get my variable "input" from my main function in a.cpp to another function named check in b.cpp. I looked into it on Google and this forum/thingy, and I found you could do it with global variables using extern, but that's it's also bad to use those and I couldn't find an answer to what an alternative is? How should I transfer the data in the variable to the other function without using globals?
Code of how I got arguments to work.
(What I'm trying to do here is a console "manager" for solutions of project Euler which I can call to solve/view via input, I started working on the code 40 mins ago.)
main.cpp
#include <iostream>
#include <windows.h>
#include "prob.h"
using namespace std;
int check(string x);
int main()
{
string input = "empty";
clear();
cout << "Welcome to the Apeture Labs Project Euler Console! (ALPEC)" << endl << endl;
cout << "We remind you that ALPEC will never threaten to stab you" << endl;
cout << "and, in fact, cannot speak. In the event that ALPEC does speak, " << endl;
cout << "we urge you to disregard its advice." << endl << endl;
cin >> input;
cin.get();
check(input);
cout << input << endl;
cin.get();
return 0;
}
prob.h
#ifndef PROB_H_INCLUDED
#define PROB_H_INCLUDED
int main();
int clear();
int check();
int back();
int P1();
int P2();
int P3();
int P4();
#endif // PROB_H_INCLUDED
back.cpp
#include <iostream>
#include <windows.h>
#include "prob.h"
using namespace std;
int clear()
{
system( "#echo off" );
system( "color 09" );
system( "cls" );
return 0;
}
int check( string x )
{
if( x == "help" );
if( x == "empty" )
{
cout << "And.... You didn't enter anything..." << endl << endl;
}
else
{
cout << "Do you have any clue what you are doing? " << endl << endl;
}
return 0;
}

Answer 1

By passing the data as an function argument.
For example:
int doSomething(int passedVar)
{
}
int main()
{
int i = 10;
doSomething(i);
return 0;
}
Note that the function definition may reside even in a different cpp file. The main only needs to see the function declaration, and the linker shall link the function definition correctly.
Usually, one would add the function declaration in a header file and include the header file in main, while providing the function definition in another cpp file.
The code you show has number of problems:
You do not need to declare main in the header file.
Your function declaration and definition of check() do not match. Your header file says it takes no argument and you define a the function definition to take one argument. Obviously, they don't match. As they stand now they are two completely different functions.
As the compiler sees it you declared one function who's definition you never provided and you defined another function in the cpp file. Thus the function declared(one with no parameters) was never defined and hence the definition not found error.

Answer 2

Andrei Tita is absolutely correct. If you have a "value" in one module (e.g. "main()" in a.cpp), and you wish to use that value in a function (e.g. "foo()" in b.cpp) ... then just pass that value as a function argument!
As your programs become more sophisticated, you'll probably start using classes (instead of functions) .