Question

In an MSVC C++ program I have a part of code which I want to enable or disable depending on a preprocessor definition

// 1.h
#ifdef MYOPTION
//...
#endif

But I find that it is quite dangerous when it is used in a .h file included in more than one compilation unit, as I can easily get inconsistent headers (I don’t want to define MYOPTION globally as it would require a complete recompilation each time I change it):

// 1.cpp
#define MYOPTION
#include "1.h"

// 2.cpp
#include "1.h"

Of course, it is much more complicated than this simplified example due to the chained header inclusion.

Is there a way to avoid such inconsistency, e.g. have a compile-time error without too much effort?

I thought of doing #define MYOPTION 0 or 1, but then I would have to write something like

#if MYOPTION == 1
//...
#elif !defined(MYOPTION)
#error ...
#endif

which looks too complicated… Maybe there is a better option?

Answer 1

How about something like this: have 1.h define a dummy section in the obj with different options. This way, if MYOPTION is ever used inconsistently, the linker will issue a warning.

1.h:

#ifdef MYOPTION
#pragma section("MYOPTION_GUARD",write)
#else
#pragma section("MYOPTION_GUARD",read)
#endif

namespace { __declspec(allocate("MYOPTION_GUARD")) int MYOPTION_guard; }

Compiling with MYOPTION defined in a.cpp but not in b.cpp yields this linker warning (using VC 2008):

b.obj : warning LNK4078: multiple 'MYOPTION_GUARD' sections found with different attributes (40300040)

A consistent definition yields no linker warnings at all.