Hello everyone, i believe this is my first post on this site, i looked through previous posts but i couldn't find anything explain this to me...

i created a project with 5 files:
main.cpp
class1.h
class1.cpp
class2.h
class2.cpp

in every file but main.cpp i wrapped it with:

#ifndef SOME_DEF_OR_OTHER
#define SOME_DEF_OR_OTHER
...code....
#endif

although it was a different define for each file...

and in both class1.cpp and class2.cpp i have:
#include class1/2.h

what is confusing me is to be able to create objects from these classes in main.cpp i had to:
#include "class1.cpp"
but
#include "class2.h"

if i change these lines of code at all, i either get linker errors complaining of multiple definitions or compile errors telling me that they can't find the object i'm trying to construct...

what is also confusing me, is that nowhere in my code did i write:
#include "class2.cpp"
yet the code inside that file is still used and compiled...

is there someone or somewhere i can get the information on what is actually going on behind the scenes here? Any clarification would be greatly appreciated...

thankyou for your time.

Header files (*.h) contain declarations, like

class a { public int do_it(); ... }
or
void some_func( int d );

but never definitions like

int a::do_it() { do_something(); }
or
void some_func( int d ) { do_something_else(); }
or
char some_variable;

(it's a different story with inline functions, but that's not important now). Further you never include source files (*.cpp) (except for some very rare cases), only header files.

It seems you forgot some declarations in class1.h, and instead wrote them in class1.cpp. Is that the case?

If you find yourself having to include cpp files then typically one of the following is true:

1. Your missing a declaration in your header file.
2. Your doing some obscure embedded programming with a poor compiler.
3. Your not linking your program properly.
4. You have a major design problem.

I guess the question I would have its.. are the errors your seeing linker errors or compiler errors?

i really appreciate your help, heres a quick example i have put together so you might be able to spot my mistake:

//main.cpp
#include <iostream>
#include "test.h"

int main()
{
CLAS clas;

clas.sety(4);
std::cout << clas.gety() << endl;

return 0;
}

//test.h
#ifndef __CLAS_H__
#define __CLAS_H__

class CLAS
{
protected:
int y;

public:
int gety();
bool sety(int newy);
};

#endif

//test.cpp
#ifndef __CLAS__
#define __CLAS__

#include "test.h"

int CLAS::gety()
{
return y;
}

bool CLAS::sety(int newy)
{
y = newy;
return true;
}

#endif

test.cpp compiles ok, except a no main error
when i try to compile main.cpp i get:

[Linker error] undefined reference to `CLAS::sety(int)'
[Linker error] undefined reference to `CLAS::gety()'

thankyou again for your time.

Ok, I'm not sure what compiler your using here, but this is how I built this on Mac OSX (works just the same in Linux).

g++ -Wall -c -o test.o test.cpp
g++ -Wall -o main main.cpp test.o

Output:
This would also work:
g++ -Wall -o main main.cpp test.cpp


In my first method I build a non-linked object file (the -c option does this) that contains the binary code for the test.cpp class. This binary must be included in the 2nd compile for linking purposes. The 2nd method gets rid of the middle step... but is less ideal if you are building a large project with lots and lots of files.


EDIT:
BTW, to compile that example I did have to change endl to std::endl in the cout statement. It would have been equally correct to just set the namespace to standard by putting in a "use namespace std;" line in main.cpp.

ok, i think you've spotted my mistake... i didn't realise you had to point the compiler to test.o, hence my confusion of how the code in test.cpp was being accessed...

I'm using KDevelop to compile and possibly it's project capabilities are interfering and confusing me, so i'll switch to gcc to compile in future.

Thankyou very much for your time.

Not a problem, glad you got your stuff working.

I have seen this only once, in glibc, but there it is used very, very often. Entering
grep -r '#include.*\.c[">]' glibc-2.3.5 | wc -l
gives me 1607 lines. I don't know why it is done there, certainly not for point 1., 2., or 3., and probably not for 4.; it seems that sometimes there are good reasons to do so.

I guess I should expand reason 2 or add a reason 5...

5. Lack of runtime support, base libraries, other misc. low level system things.

The point is, if your doing it in application level programming your probably doing something wrong.