CODEWITHSUNDEEP
c++
Manh Nguyen
Manh Nguyen

Reputation: 413

Why is header guard ineffective when including stb_image?

I want to use stb_image library, which the author has put into a very large header file. To use the library, the documentation tells me to add these 2 lines to my code. 

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

I did just that, putting it into a header file; I even made sure that it has include guard. 

#ifndef STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#endif

The linker complained however.

I look up and find the solution. https://github.com/nothings/stb/issues/3

Basically, I have to add the two lines above to the cpp file and not header file. It works like a charm. But why is header guard not effective in this way? I have been reading about translation units and statics yet I could not understand why my method would not work.

Upvotes: 0

Views: 882

Answers (1)

Miles Budnek
Miles Budnek

Reputation: 30579

stb_image.h has its own include guards. That's not what defining STB_IMAGE_IMPLEMENTATION is for. Defining STB_IMAGE_IMPLEMENTATION tells stb_image.h to include not only declarations, but also definitions for its functions and variables into that translation unit. If stb_image.h is included into multiple translation units with STB_IMAGE_IMPLEMENTATION defined, then all of those translation units will have definitions for stb_image's functions and variables, and the One Definition Rule is violated.

Upvotes: 3

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