[HOWTO] write classes headers

Hello!

I'm a newbye with C++ and I come from Java;
I've more or less found out how to use different cpp files using .h files, then I wrote a class over my main() method.
What I would like to do now is to put the class in an another file, like Class.cpp
and make its header. The problem is when I build the main program: it states "undefined reference to <class>::<method>"
I think I'm making some mistake writing the h file. Which is the correct way?

If necessary, I can post the code, it's very short.
Thanks, bye

Header file:

Code:

---

#ifndef SOME_RANDOM_STRING_IDENTIFIER
#define SOME_RANDOM_STRING_IDENTIFIER

#include <whatever libs you need>

class my_class {
private:
  // ...

public:
  // ...
};

#endif

---

cpp file:

Code:

---

#include "the above header file, same dir"

my_class::my_methods( my_parameters ) {
  my_body;
}

etc

---

main:

Code:

---

#include "first header file"
// ...

int main() {
  my_class my_instance;
  // ...

  return 0;
}

---

I think I made the same thing, but nothing changed :(

Here's the exact error:
.objs\main.o:main.cpp:(.text+0x8a): undefined reference to `MyClasse::Mymethod()'

I omitted this:
"#ifndef SOME_RANDOM_STRING_IDENTIFIER
#define SOME_RANDOM_STRING_IDENTIFIER"
I've seen it around but I believed it's not obligatory... maybe I was wrong? Must I put it in?

Thanks again

No. You don't have to put it in. Its purpose is to protect against multiple files including the same file again and again when it only needs to be included once. Suppose you have another file and x.h is included in that. Then in your main file you include x.h. It'll error, probably, saying there are multiple definitions of whatever's in the file. The #ifndef preprocessor control statements protect against this. The "SOME_RANDOM_STRING_IDENTIFIER" I generally relate to the class I'm using. So it could be: "TWOMERS_MY_CLASS", for the above example.

Post some code. I'm curious now.

CPP FILE:

Code:

---

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "grafiche.h"

class Schermo {
    SDL_Surface *sfondo, *foo;
  public:
    SDL_Surface *getSfondo() { return sfondo;}
    SDL_Surface *getFoo(){ return foo;}
    void initGfx();
    void pulisci(){
        SDL_FreeSurface( foo ); SDL_FreeSurface( sfondo );
    }

  };

void Schermo::initGfx() {
    sfondo = load_image( "prato.png" );
    foo = load_image( "casa.png" );
}

---

HEADER FILE

Code:

---

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "grafiche.h"

class Schermo {
    SDL_Surface *sfondo, *foo;
  public:
    SDL_Surface *getSfondo();
    SDL_Surface *getFoo();
    void initGfx();
    void pulisci();
  };

---

Thanks!

You are re-declaring the class inside the cpp file. Don't do that, instead put the declaration in the header and #include that header. You're allowed to implement those member functions inside the class declarations in the header, so update your header file with the class definition from the cpp file.

Also, you pretty much do have to put in header include guards. Technically there are workarounds that make it not required, and some small programs won't need it, but most projects will fail to compile or link if you don't add them.

There is a FAQ on writing multi-file projects.

Quote:

---

Originally Posted by gibbofresco

CPP FILE:

Code:

---

#include "headerfilenamefrombelow"

void Schermo::initGfx()
{
    sfondo = load_image( "prato.png" );
    foo = load_image( "casa.png" );
}

void Schermo::pulisci()
{
    SDL_FreeSurface( foo );
    SDL_FreeSurface( sfondo );
}

---

HEADER FILE

Code:

---

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "grafiche.h"

class Schermo
{
    SDL_Surface *sfondo, *foo;

    public:
    SDL_Surface *getSfondo();
    SDL_Surface *getFoo();

    void initGfx();
    void pulisci();
};

---

Thanks!

---

I think that's more along the lines of what you want.

>> I think that's more along the lines of what you want.
Almost. The header file should include the inline function definitions that were in the original source file.

And for safety reasons it's always good to include header guards.