On 8/31/2011 3:07 PM, Ian Lance Taylor wrote:
Edward Diener<eldlistmailingz@xxxxxxxxxxxxxx> writes:Attempting to compile this small example program with gcc, with SOME_MACRO defined on the command line: #if !defined(SOME_MACRO) #define MACRO_1() 1 #else #define MACRO_1() #endif #if defined(SOME_MACRO) int main() { return 0; } #elif MACRO_1() == 1 int main() { return 1; } #endif gives the error: eliftest.cpp:9:17: error: operator '==' has no left operand "g++" -ftemplate-depth-128 -O0 -fno-inline -Wall -pedantic -g -Wno-variadic-macros -DSOME_MACRO -c "eliftest.cpp" If SOME_MACRO is defined, it seems to me that gcc should not be expanding the expression 'MACRO_1() == 1' and reporting an error.gcc's behaviour matches my reading of the C99 standard. The standard says that each #if/#elif condition is checked. It doesn't say that the compiler should stop checking conditions after it finds one that is true. Instead, it says that the first group which is true is processed.
You may be correct but then clearly #if - #elif - #endif works differently than #if - #else - #if - #endif - #endif, and that seems odd. Also I would have thought that an #if branch taken would have precluded the evaluation of an #elif statement and branch just as it precludes the evaluation of an #else branch.
