LNK2005: Object Already Defined. Help?

So, I'm trying to build this program that has two source files and one include file. Both source files include the single include file, I know that's why I'm getting the errors. I also know that the supposed solution is to do something like:

Code:

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#ifndef _SOMETHING_
#define _SOMETHING_

//All sorts of whatever code here
#endif

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My problem is this. I did that, and I still get the errors, I have that kind of block around the entire include file.

What I really want to know, is why this is needed, because I only get these kinds of errors in certain include files. Does it have to do with having functions defined (not just declared) in the include file? Does it have to do with having static variables or functions? If it's one of those two reasons, should I only have the #ifndef, #define, #endif block around the declared functions and/or static variables? Or should I have it around the entire file?

Thanks for the help.

Quote:

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Originally Posted by maxhavoc

Does it have to do with having functions defined (not just declared) in the include file?

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Yup, most likely. Define functions and data in a separate module if each is being separately compiled (with an exception for template stuff); have only declarations in a header. The header guards avoid compile errors, but you are seeing linker errors.

I don't understand though, I have another program where I have a class fully declared (including its member fuctions) and a few other inline functions fully declared within a header file that is used by multiple source files and I get no error there, even without the #ifndef, #define, #endif stuff.

What causes that linker error?

Do you #inlcude the header in more than one source file? If so, each file will contain its own copy of the code. And the linker will find multiple instances of the same thing and complain.

Include guards don't help against linker errors. You simply must not define variables in headers. Same goes for functions ( unless declared inline ).
If you want to declare variables in headers use the extern keyword and define them in a single source file.
The problem here is that these variables wil be created multiple times in different tranlation units. The linker then sees multiple versions of the same symbol and gives up.
Kurt

EDIT: I'm a bit slow today

Yes I do, and in the other program I get no errors.

I've discovered something. If I declare the offending variables and functions static, then I don't get the linker error for those objects.

Quote:

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Originally Posted by maxhavoc

Yes I do, and in the other program I get no errors.

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Does the other program consist of only a single source file? Well then, 1 instance is nothing to complain about.

Please take our advice and learn to do it right.

Quote:

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Originally Posted by ZuK

Include guards don't help against linker errors. You simply must not define variables in headers. Same goes for functions ( unless declared inline ).
If you want to declare variables in headers use the extern keyword and define them in a single source file.
The problem here is that these variables wil be created multiple times in different tranlation units. The linker then sees multiple versions of the same symbol and gives up.
Kurt

EDIT: I'm a bit slow today

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Interesting, can you possibly explain why declaring them inline and/or static fixes the error?

Well, for static I can understand, that makes it so the object remains in memory once it's declared, and so a second declaration has no meaning right? But why does inline work?

Quote:

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Originally Posted by Dave_Sinkula

Does the other program consist of only a single source file? Well then, 1 instance is nothing to complain about.

Please take our advice and learn to do it right.

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I'm not trying to argue with you here, I want to learn why the error happens.

Quote:

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Originally Posted by maxhavoc

Yes I do, and in the other program I get no errors.

I've discovered something. If I declare the offending variables and functions static, then I don't get the linker error for those objects.

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Yes thats right. If you declare them static they are defined at file scope. But then they are different variables, If you change one the other does not. Will give you hard to find bugs.
Kurt

An #include is like copy-and-paste. So if you have multiple modules with the same functions defined, the linker may see something like this (definitely pseudocode).

Code:

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// header
int foo(int x)
{
  return x * x;
}

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After each module is compiled...

Code:

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int foo(int x) // module #1
{
  return x * x;
}
int foo(int x) // module #2
{
  return x * x;
}
int foo(int x) // module #3
{
  return x * x;
}

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One for each module in which the header was #included.

Ok, thank you both for the help. I appreciate it.