C ++预处理器决策

Question

Sorry I know this is basic, but perhaps it doesn't exist or I'm not googling the right words. Is there and an if not (is that ifndef ?) an AND and an OR so I could do something like:

if not DEBUG and MACOS

Answer 1

我认为像#if !defined(DEBUG) && defined(MACOS)这样的东西应该这样做。

Answer 2

#ifndef and #if do different things so it depends on what you want. #ifndef is true when there is no defined preprocessor symbol that matches the name following. #if is true when the following preprocessor expression evaluates to non-zero.

You can use the standard && and || operators.

Answer 3

#if !defined(DEBUG) && defined(MACOS)
#error "Ouch!"
#endif

tests, if those macros/values are defined (even set to 0 means defined). Leave out the "defined()" and test again a value, depending on your macros, like

#if DEBUG==0 && MACOS==1
#error "Spam!"
#endif

Answer 4

#if !DEBUG && MACROS

or

#if !DEBUG & !MACROS

depending on what you are looking for. defined() can also help

#if !defined(DEBUG) && defined(MACROS)

Answer 5

#if !(defined(DEBUG) && defined(MACOS))

or

#if !defined(DEBUG) && !defined(MACOS)

depending on what you're trying to evaluate.

Answer 6

#if , #else and #endif are general. Use #define to declare and #undef to undeclare. Use #ifdef to check if is declared and #ifndef to check, if is not declared.

Example:

#ifndef LABEL
#define LABEL some_value // declares LABEL as some_value
#else
#undef LABEL // undeclare previously declared LABEL...
#define LABEL new_value // to declare a new_value
#endif

Answer 7

Check out the Boost preprocessing library. It can accomplish a large number of tasks using the preprocessor.