C++ link error.. is invalid? - c++

Question

I keep getting this Visual C++ 2010 LNK2005 linker error saying I have multiple definitions of two functions that are contained in "Error.h". (a header I made for error display)
My project is like this:
BaseImage.h
BaseImage.cpp --> includes BaseImage.h , Error.h
PNGImage.h --> includes BaseImage.h
PNGImage.cpp --> includes PNGImage.h , Error.h
main.cpp --> includes PNGImage.h
And, of course, Error.h:
/*
Optional macros:
AE_EXIT_AT_ERROR
*/
#pragma once
#include <stdexcept>
void aeError(const char *str, int code=1)
{
throw std::runtime_error(str);
#ifdef AE_EXIT_AT_ERROR
std::exit(code);
#endif
}
void aeAssert(bool b, const char *failStr = "assertion failed")
{
if(!b)
aeError(failStr);
}
I have #pragma once in every header file, and I tried adding include guards to Error.h as well.
Here's the compile output:
1>PNGImage.obj : error LNK2005: "void __cdecl aeError(char const *,int)" (?aeError##YAXPBDH#Z) already defined in BaseImage.obj
1>PNGImage.obj : error LNK2005: "void __cdecl aeAssert(bool,char const *)" (?aeAssert##YAX_NPBD#Z) already defined in BaseImage.obj
1>C:\...\Project.exe : fatal error LNK1169: one or more multiply defined symbols found
Could this be a bug?

Answer 1

When you define functions in a .h file, make them inline. Otherwise, the function definitions are part of the object code of all the .cpp files that #include it, with external linkage.
inline void aeError(const char *str, int code=1)
{
throw std::runtime_error(str);
#ifdef AE_EXIT_AT_ERROR
std::exit(code);
#endif
}
inline void aeAssert(bool b, const char *failStr = "assertion failed")
{
if(!b)
aeError(failStr);
}
The other option for you is to declare the functions in the .h file and define them in exactly one .cpp file.
The .h file:
extern void aeError(const char *str, int code=1);
extern void aeAssert(bool b, const char *failStr = "assertion failed");
The .cpp file:
// Don't use inline and don't include the default argument values.
void aeError(const char *str, int code)
{
throw std::runtime_error(str);
#ifdef AE_EXIT_AT_ERROR
std::exit(code);
#endif
}
void aeAssert(bool b, const char *failStr)
{
if(!b)
aeError(failStr);
}