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c++cincludec-preprocessor

String concatenation for include path

Is there a way to concatenate 2 strings literals to form an include path?

Code stub:

#define INCLUDE_DIR "/include"
#include INCLUDE_DIR "/dummy.h"

Looking at this question, the answers point in a different direction (compiler command line). It is mentioned here that it is seemingly not possible, but I wonder if the topic has been dug enough.

(I do have an use case in which this is relevant, please focus your answers/comments on this question only.)

Solution

  • It really seems this is not possible. I will report here the relevant section from Eric Postpischil's answer (he doesn't seem to be active anymore).

    The compiler will do macro replacement on an #include line (per C 2011 [N1570] 6.10.2 4), but the semantics are not fully defined and cannot be used to concatenate file path components without additional assistance from the C implementation. So about all this allows you to do is some simple substitution that provides a complete path, such as:

    #define MyPath "../../path/to/my/file.h"
#include MyPath

    Link to documentation. In particular this section doesn't leave much hope for portable solutions:

    The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.

    For completeness, maybe something can be tried using https://stackoverflow.com/a/27830271/2436175. I'll investigate that when I have a moment...