Thread: #include

Description of this problem is little longer, here it is:

I have a console application under Visual C++ 6.0. It consists of main file "main.cpp" which uses a class defined in two files as usual: "someclass.h" and "someclass.cpp". Finally there is "library.h" file, which contains support functions (as an object too) for someclass (the class defined in someclass.x files).

Files contain this code:
library.cpp

Code:

//this is the library needed by someclass; implementation goes right after 
//declaration here, all in one file; I use this (declaration & implementation
//in one file) often and that's is not the problem...or it is?

#pragma once  //doesn't help me

#ifndef _AVOID_MULTIPLE_LINKING_  //even this
#define _AVOID_MULTIPLE_LINKING_

class AnyOtherClass {
  public: void method();
};

//linker reports: 
//error LNK2005: "public: void __thiscall CSomeClass::method(void)" (?method@CSomeClass@@QAEXXZ) already defined in main.obj
void AnyOtherClass::method() {
  {}
}

#endif

someclass.h

Code:

//this is a header file for someclass

#include "library.h"  //this library is needed by someclass

//class is not really declared here
//not even implemented in the following cpp file
//that's just for simplicity
//code for someclass - only declarations
//...
//...

someclass.cpp

Code:

//this is a source file for someclass, so this line must be here:
#include "someclass.h"

//code for some class - implementation
//...
//...

main.cpp

Code:

//finally I'd like to use someclass (which uses AnyOtherClass) in a console application like this
#include "someclass.h"

int main() {
  return 0;
}

//but I can't compile it...

/*
I found out this solution: Join someclass.h and someclass.cpp into one header file, keep this 
file linked to main.cpp and keep the include directive ("library.h") on the top of it too - it works.

But I'd like to know what is wrong and how to make it work with someclass.h and someclass.cpp files
separately.
*/

I use the precompiler directives as you can see from my code, but I thought compiler applies the directive to both .h a .cpp files it is in .h file. In system libraries, there are always ifndef, define, endif in header file. but I never saw implementations of them and maybe directives are there too. Interesting, I'll try it. Thanx.

You either need to inline AnyOtherClass::method() or put the implementation in it's own cpp file.

>#ifndef _AVOID_MULTIPLE_LINKING_ //even this
#define _AVOID_MULTIPLE_LINKING_

will prevent multiple inclusion in the same file. It wont prevent multiple linking if it's included in seperate cpp files.

That's it. My problem was that somebody told me to write simple classes in single header file (with implementation). Now I realized this is not good when putting a large number of such classes in one static library (as usually I do, because I haven't still learnt dynamic linking). Compiler needs to have everything with symbolic addresses in .cpp file, doesn't he? OK, now I always use .h and .cpp file even for small classes.
But there is one problem. Let's say I need a stack which is independent on type. Some simple stack using linked lists but with templates. I've read that when you use templates in a static library, you must declare every template "version" you will use (at the end of implementation file of its class). Like:

Code:

  class CMyStack<int>;
  class CMyStack<char>;
  // and so...
  // I think in MSDN Library they call it forward template declarations or something...

But if I put in my static library and then somebody else will use this library, he can't declare an instance of CMyStack with let's say float type. Is there any way to do this?