how can i solve this problem:

//file1.cpp

class a
{
public:
void do_something_with_b();
};

void a::do_something_with_b()
{
b.do_something();
}

//file2.cpp

class b
{
public:
void do_something_with_a();
};

void b::do_something_with_a()
{
a.do_something();
}

Are you sure you're not confusing class declarations and objects? A class is a template for an object, not the object itself. To use it properly, do like this:

class Foo
{
 public:

 int var;
};

Foo bar;
bar.var = 2;

Just to make sure, though.

perhaps the example i gave is not very well
but i found a solution for this:
just put all header files of the classes in one big header file and include this file in every sourcefile so that each class knows the others

yes with a pointer to a class this may work but when you declare a variable of that class it doesn't
like this:

class Elephant;

class Mouse
{
...
Elephant ScaredCreature;
...
};

class Elephant
{
...
Mouse ScaryCreature;
...
};

so how can i break a dependencie like that:
the class player needs to know the class bullet (and its firing method) and the bullet needs to know about the player (for collision detection)

If you need A to know about B and B to know about A you CANNOT have A contain B and B contain A. That makes no sense and it would go on forever.

You must use pointers or references and you must use forward delcarations as was shown to you earlier. Do not place all classes in one big header or whatever you were considering. In your class header, instead of doing #include "Class.h", replace it with class Class; as much as possible. As long as the class you are defining does not inherit from Class, does not contain a Class object, and does not have any methods taking a copy of a Class object (ie only Class* and Class& are seen), you can and should use forward declarations.

o.k. but i'm not very good in pointers, so when i just declare a pointer to the bullet class in the player class how can i call then the bullet.fireit() method

1. so i can use
"pointer_to_bullet=new bullet;" and
"pointer_to_bullet->fireit();"
in the player.cpp code without having declared
the class bullet before
(just forward declaration like "class
bullet;")

2. and when theres a global object bullet1
i can use
"pointer_to_bullet=&bullet1;" and
"pointer_to_bullet->fireit();"
...

You should call them classes, simple enough.
Once you instantiate it, it's an object. Class is to object what prototype is to function, one could say. I think.

How interesting, then, that there are no such in the code snippet.

Just call them forward declarations like everyone else, all right?

s/class prototype/forward declaration of class x/