Preprocessor macro expansion to another preprocessor directive

c++ macros c-preprocessor expansion preprocessor-directive

moala · Aug 11, 2009 · Viewed 9.2k times · Source

Initially I thought I needed this, but I eventually avoided it. However, my curiosity (and appetite for knowledge, hum) make me ask:

Can a preprocessor macro, for instance in

#include "MyClass.h"

INSTANTIATE_FOO_TEMPLATE_CLASS(MyClass)

expand to another include, like in

#include "MyClass.h"

#include "FooTemplate.h"
template class FooTemplate<MyClass>;

Answer

I believe that cannot be done, this is because the pre-processor is single pass. So it cannot emit other preprocessor directives.

Specifically, from the C99 Standard (6.10.3.4 paragraph 3):

3 The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, ...

Interestingly enough, This is why the unary _Pragma operator was added to c99. Because #pragma could not be emited by macros, but _Pragma can.