Question

I’m learning C++, and find no answer to the following:

I have a formula that calculates a result of two numbers from user input​​.

• How should the header file (“.h”) look, which contains this formula?
• How do I get the formula from the source code (“.cpp”)?
• How do I pass the result to the source code (“.cpp”)?

For you old-timers this is not a topic. Where can I find a good tutorial on C + +?

Greetings, Dino

Answer 1

The best tutorials can be found in books. There’s numerous topics on SO with good books recommendations.

In C++, free-standing functions should be either grouped in namespaces or as static class members (but only if they are logically connected to a class).

For your case, I’d go with the namespace. Even so, there 3 possibilities, out of which 2 are prefferable:

1) declare the function in a header and define it in a source file:

//formulas.h
#ifndef FORMULAS_H
#define FORMULAS_H
namespace Formulas
{
   int myFormula(int,int);
};
#endif

The preprocessor directives are include guards (you can google that). After this, you define it in a source file:

//formulas.cpp

#include "formulas.h"
namespace Formulas
{
    int myFormula(int, int)
    {
       //....
       return 42;
    }
}

2) Both declare and define the function in the header as inline (this will prevent multiple definitions of the functions). Usually done with relatively small functions that are called frequently, to prevent the call overhead and allow better optmizations:

//formulas.h
#ifndef FORMULAS_H
#define FORMULAS_H
namespace Formulas
{
   inline int myFormula(int,int)
   {
       //...
       return 42;
   }
};
#endif

3) Declare and define the function in the header, marking it static. This is not to be preffered, as it will create a copy of the function for each translation unit that includes the header.

To use the function, you need only include the header where it is declared and call it.