Introduction
Macros are string substitution for code. It happens during the preprocessing phase. It is generally not encouraged in c++ nowadays. However, there are still applications where it could be handy / necessary
Here is the general compilation process
Application 1 - Linkage Specifier
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#ifdef __cplusplus
#define CV_IMPL extern "C"
#else
#define CV_IMPL
#endif
CV_IMPL void cvFindExtrinsicCameraParams2(...){
cvConvertPointsHomogeneous(objectPoints, matM);
cvConvertPointsHomogeneous(imagePoints, _m);
}
This is effectively extern "C" void cvFindExtrinsicCameraParams2(...){} and void cvFindExtrinsicCameraParams2(...){}.
extern "C"is a “linkage specifier”. In C++, function names are mangled so they will be unique in the object code. E.g.,foo()could become_fooid1(). This is also the underlying mechanism for function overloading. However, C does not have function overloading. So, when linking C code in C++, we need to make sure names of function symbols are unique, and exclude name mangling.
Compiler Args
Life cycle of adding a compiler arg:
- “turn on” a preprocessor‐symbol by passing
-D<NAME>to the compiler. - In the top-level CMake:
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add_compile_definitions(PRINT_DEBUG_MSGS)
- In source code:
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#ifdef PRINT_DEBUG_MSGS (or #if defined(PRINT_DEBUG_MSGS))
- If this is not defined, it won’t be compiled
