C preprocessor macros - definition vs. value portability & practice

Question

For compiler-specific code, it's common to see cpp directives such as:

#if defined (__GNUC__) && (__GNUC__ >= 4)

which is the preprocessor test I typically use - not exclusively for __GNUC__, but it's a common example. Alternatively,

#if (__GNUC__ >= 4)

appears to satisfy the same requirements. Are there potential problems with the latter? Not only with gcc, but any standards-conforming preprocessor. Can the LHS be evaluated as a certain value, even if it's not defined? Are there any pitfalls to the second approach that any language lawyers are aware of?

Answer 1

The preprocessor assumes undefined macros to have the value 0 in comparisons, so your simplification is ok in this case. If you want to check against a lower version than 4 in gcc, you may get into trouble though since it would evaluate as true with a < even if it's not gcc.

I think the reason for using both is also a question of understandability, if you check

#if defined(__GNUC__) && (__GNUC>=4)

it's rather obvious you're not already in a block with code that only is for GCC, while the simplification

#if (__GNUC__ >= 4)

does not make that obvious and can be read as a version check only when you already know it's gcc.