Is there a way to use class wtihout header file?

Hi
For example i have .h and .cpp file that define my class like that:

Code:

//a.h
class CExample {
  public:
    int a,b,c;
    CExample (int n, int m);
    void multiply()  ;
  };

and

Code:

//a.cpp
#include "a.h"
CExample::CExample (int n, int m) { a=n; b=m; };
void CExample::multiply () { c=a*b; };

I want to use Cexample class object in another .cpp file without include a.h in the same project
Like that:

Code:

//main.cpp
#include "stdafx.h"
int _tmain(int argc, _TCHAR* argv[])
{
	extern CExample c(5,4); //gives error.
	return 0;
}

It gives error.
Is there a way to use an object in another .cpp file (in the same project) without include its header file in c++?
I tried extern CExample c(5,4);
but doesnt work
Thanks

What is wrong with including the header file? I hope your actual header file has inclusion guards.

you could write the class definition again in the source file, but then you'd have two things to change if you change your class.

why don't you want to include the header file?

Thank you
nothing wrong.
I only want to learn if there is a way.
I tried with extern keyword. But i think we cant use class with extern keyword.
Am i wrong?

extern is for instances, not types.

I think you could do this if you are not going to access any of the members:

Code:

class MYclass;
void func(Myclass *point)
{
  point = point;
  return;
}

edit: yeah like laserlight said.

You could use a forward declaration (e.g., class CExample; ) but not in this case since the class declaration is needed to instantiate an object.

Originally Posted by robwhit

extern is for instances, not types.

Thank you but i dont understand what you mean.
Could you explain this sentence.

Originally Posted by sawer

Thank you but i dont understand what you mean.
Could you explain this sentence.

an instance is when you make a variable. A type is what kind of variable it is.

int a;
a is an instance, and int is its type.

Yes
so what is wrong with explicit CExample c(5,4)

In MSDN says:
"Objects and variables declared as extern declare an object that is defined in another translation unit or in an enclosing scope as having external linkage."

CExample is a type c is a variable.
What is wrong here?
Thanks for all help

I think the confusion here is about "what can be made extern and what cant".

Types MUST be known by the compiler when their content is being used. Since a declaration of a class (or struct) in C++ involves calling the constructor for that class (and the constructor(s) of it's parent class(es)), the compiler must know what those constructors are and where these constructors are, and to know that, the compiler must know the entire content of the type, meaning that you have to include the header-file that contains that type (or some other way give the compiler the contents of the type - if you want to repeat the declaration of the class multiple times, that's fine too - but has the drawback of needing to be modified in several places if there's ever a need to modify the class).

A variable (instance) can be declared extern, such as "extern CExample b;" - however, that tells the compiler that there is a declaration of this variable somewhere else. You can't initialize (give it values) here, that must be done at the "main declaration", which also where the constructor is being called.

If you ever need to use the CONTENT of b as declared above, it will still need to be known to the compiler, so there's really no way around having it either included in the source code, or from a header-file.

--
Mats

Have you tried to declare an instance (in main) simply as

Code:

CExample name(2, 3);

?

explicit is a keyword to apply additional restrictions on constructors. extern is used when you have globals.