Writing good header files

mercury · #1 15-May-2005, 03:18

Hello Guys,
I am not a really got in writing header files. My problem is that no matter how I write the header file, I get the "Multiple definition of..." errors. Here is an example of my header:

CPP / C++ / C Code:

#ifndef _h_ftdi_
#define _h_ftdi_

CPP / C++ / C Code:

#include "windows.h"
#include ...

CPP / C++ / C Code:

// declaration of a function
int LoadDll(void);

// pointer to the dll
HINSTANCE h_module;

// FT_ListDevices
typedef FT_STATUS(*PtrFT_ListDevices)(PVOID,PVOID,DWORD);
PtrFT_ListDevices pFT_ListDevices;

#endif

If I include this header file in my cpp files, I get "Multiple definition of h_module" and "Multiple definition of pFT_ListDevices" errors. Any idea what I'm doing wrong?

Thanks and best regards to you all.
George

cable_guy_67 · #2 15-May-2005, 06:34

Quote:

Originally Posted by mercury

Hello Guys,
I am not a really got in writing header files. My problem is that no matter how I write the header file, I get the "Multiple definition of..." errors.
... header code removed
If I include this header file in my cpp files, I get "Multiple definition of h_module" and "Multiple definition of pFT_ListDevices" errors. Any idea what I'm doing wrong?

Hello and Welcome George. How are you using h_module and pFT_ListDevices elsewhere in you program? When you have the variable declaration in your header like you have you would most likely need to declare it in another file (such as your main.cpp) something like the following so you could use it.

CPP / C++ / C Code:

// pointer to the dll declared in header
extern HINSTANCE h_module;

That way, the compiler knows how to make sense of it all. That is of course, just a guess based on what you have posted. I have run across this quite often using FLTK's UI builder Fluid since it makes heavy use of externs when it generates it's header code. You can only declare a variable once (as long as it really is the same variable or function) and using extern says, "I need to let the compiler know what this actually is, I need to use it in this section of code, go find it elsewhere." Just be sure extern is the ONLY difference in the two lines of code (same type or return value) or you will get an error.

I hope this helps, if not (or if my info is wrong) someone is bound to drop their two cents into the bucket.

Mark

WaltP · #3 15-May-2005, 10:41

Quote:

Originally Posted by cable_guy_67

When you have the variable declaration in your header like you have you would most likely need to declare it in another file (such as your main.cpp) something like the following so you could use it.

That is exactly what is going on. You are creating the variable in the header which means every time you use the header you make another copy of the variable.

The header should contain externs and the variable is defined in a code file as cable_guy said.

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