C++ Header files says the variable is undefined:

Question

I have this class:

class A 
{
 public :

   int Echo (int param)
   {
      int num = param + 5;
      return num;
   }
}

And then in my Header.h I put the following definition:

#ifndef A_H
#define A_H
  class A 
{
 public :

   int Echo (int param1)
   {
          return num;
    }

#endif

I have these questions:

1- How classes should be defined in the Header.h? Should they be defined exactly like their original *.cpp file, or their signature is enough?

2- Should the definitions in Header file be accompanied with the return statement of the function? Someting like this:

int Echo(int param)
{
   return num;
}

I have asked similar question but with a different problem, and it is not acceptable in StackOverflow to edit questions which result in a totally different problem statement.

You need an introductory C++ book. Any of them will explain how to make a class. — chris, Jul 18 '14 at 19:52
Why not just buy a C++ book - this would be covered in one of the first chapters — Ed Heal, Jul 18 '14 at 19:54
@davidicus _"header files are used for declarations, not definitions"_ That's not entirely true! You can well provide inline definitions with header files! — πάντα ῥεῖ, Jul 18 '14 at 19:59
OK guys I know that. the problem is when I says `public : function () {}` it throws `erro externals`. — Mostafa Talebi, Jul 18 '14 at 20:01
The why when I omit the `return num;` from my definition the compiler errors that num is an `undefined.` ? — Mostafa Talebi, Jul 18 '14 at 20:02
The question you meant to ask or need answered is here http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Brandin, Jul 18 '14 at 20:06

score 2 · Accepted Answer · answered Jul 18 '14 at 20:11

Your header file should be like this:

//something.h
#ifndef A_H
#define A_H

class A{
 public :
   int Echo (int param1);
};
#endif

That is called making a function prototype. You put the actual body of the function in a .cpp file:

//something.cpp

#include "main.h"

int A::Echo (int param)
{
   int num = param + 5;
   return num;
}

from your main file, include the header file and it should work. give the .cpp file to your compiler (if you are using an IDE it will do it for you) Example main file:

#include <iostream>
#include "main.h"

using namespace std;

int main()
{
   cout << "Hello World" << endl; 
   A a;
   cout << a.Echo(10) << endl;
   return 0;
}

C++ Header files says the variable is undefined:

1 Answers1