Preserving preprocessor definitions [duplicate]

This question already has answers here: 开发者_开发百科 Closed 10 years ago.

Possible Duplicate:

Can I redefine a C++ macro then define it back?

Say I have some code that uses the name BLAH for a variable. Suppose BLAH is a common preprocessor definition in many standard header files (defined as 10), so if my file is included after any of them, the code breaks because BLAH is transformed into 10; therefore, I must #undef BLAH. But also other headers may depend on BLAH, so I must restore BLAH to it's original value after my header is done. Is it possible to do something like this:

#ifdef BLAH
#define BLAH_OLD BLAH
#undef BLAH
#endif

... code ...

// restore BLAH to 10
#ifdef BLAH_OLD
#define BLAH BLAH_OLD
#end

? This doesn't work of course, because BLAH is not expanded to 10. I have tried doing something like

#define EXPAND_AGAIN(x) x
#define EXPAND(x) EXPAND_AGAIN(x)
#define BLAH_OLD EXPAND(BLAH)

but that doesn't work either, since EXPAND is taken literally and not expanded. I am using MSVC 2008/2010, but it would be lovely if the solution would work on most other compilers too.

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Yes, given that your compiler supports the push/pop macro directives (visual c++, gcc, llvm all do):

#define BLAH 10

#pragma push_macro("BLAH")
#undef BLAH

#define BLAH 5

...

#pragma pop_macro("BLAH")

---

Unfortunately the preprocessor doesn't support a stack of definitions.

The Boost preprocessor library makes the preprocessor do things you'd never imagine it could do (like effectively variadic macros in C++98), but is bound by the inherent limitations of the preprocessor -- so, no can do, sorry.

The only known half-way remedy is to reserve ALL_UPPERCASE_IDENTIFIERS for macros, and consistently always use them for macros. It reduces the name collision problem somewhat. Unfortunately the C standard library defines a number of lowercase macros, or allows for their existence, like e.g. assert, but they are just a few.

From a practical point of view the main problem is in Windows programming, where Microsoft's [windows.h] header defines zillions of non-uppercase macros, including, by default, min and max which conflict with the C++ standard library.

So, for Windows C++ programming, always define NOMINMAX before including [windows.h].

Cheers & hth.,

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I used to believe that the very same trick you tried does work, as I used to use it myself. But I eventually learned that it actually does not work at all. The simple answer is NO, you cannot save a define's current value, change it, and then restore the old value. The preprocessor simply does not work that way. Once you define a new value, the old value is gone.

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One trick that works for me is to use an enum in the class.

class foo
{
public:
  enum { blah = 10 } myenum;
}

Then you can just use

foo:blah

when you need '10'.

Since it's part of the class then other uses of 'blah' won't conflict and you save all that def'ing and undef'ing.