#ifdef SWIG：何时考虑此条件编译？

Question

I have in my header sample.h this :

#include <iostream>
#ifdef SWIG
int a = 0;
#endif

And in my sample.i this :

%module sample
%{
   #include "sample.h"

%}
#include "sample.h"

But, I don't see any difference between with and without the #ifdef SWIG...

Can you please let me know where and how I can find the difference, and what would it be??

Thank you a lot!

Answer 1

Your .i file has an error. The last line should be %include not #include . With this change, building your wrapper (I'm using Python) produces:

sample_wrap.cxx
sample_wrap.cxx(3200) : error C2065: 'a' : undeclared identifier
sample_wrap.cxx(3211) : error C2065: 'a' : undeclared identifier

This is because when compiled SWIG is not defined during #include and a does not exist in the resulting code. When SWIG processes %include SWIG is defined and generates an interface for a which does not exist.

Removing the #if wrapper produces a correctly wrapped global variable:

>>> import sample
>>> sample.cvar.a
0

Answer 2

There is a solution to this: if you compile the code above in CMake, you can set the CXX_COMPILE_FLAGS for your interface file that are propagated to the attached header&c++ files. As such:

SET_SOURCE_FILES_PROPERTIES(sample.i PROPERTIES CPLUSPLUS ON)
if(CMAKE_BUILD_TYPE MATCHES "Release")
    SET_SOURCE_FILES_PROPERTIES(sample.i PROPERTIES COMPILE_FLAGS "${CMAKE_CXX_FLAGS} -DSWIG")
    SET_SOURCE_FILES_PROPERTIES(sample.i PROPERTIES CXX_COMPILE_FLAGS "${CMAKE_CXX_FLAGS} -DSWIG")
else(CMAKE_BUILD_TYPE MATCHES "Release")
    SET_SOURCE_FILES_PROPERTIES(sample.i PROPERTIES COMPILE_FLAGS "${CMAKE_CXX_FLAGS_DEBUG} -DSWIG")
    SET_SOURCE_FILES_PROPERTIES(sample.i PROPERTIES CXX_COMPILE_FLAGS "${CMAKE_CXX_FLAGS_DEBUG} -DSWIG")
endif()
SWIG_ADD_MODULE(sample python sample.i sample.cpp)
SWIG_LINK_LIBRARIES(Vector ${PYTHON_LIBRARIES})

In that case, the compilation is conditional (if you leave out the CXX_COMPILE_FLAGS part, there's no "a"), but if you want it it's there.