objective-c - 如何在编译时测试定义的宏是否为空(Cbjective-C/c)？

Question

是否可以检查我的定义是否为空？ IS_EMPTY_OR_UNDEFINED 是我刚刚想出的虚构宏。

#define constantA 0
#define constantB 1
#define constantC null
#define constantF ""
#define constantH 

#if IS_EMPTY_OR_UNDEFINED(constantA)
# error constantA is defined 0 and the above if should not be true - this line should not run
#endif

#if IS_EMPTY_OR_UNDEFINED(constantB)
# error constantB is defined 1 and the above if should not be true - this line should not run
#endif

#if IS_EMPTY_OR_UNDEFINED(constantC)
# error constantC is defined null and the above if should not be true - this line should not run
#endif

#if IS_EMPTY_OR_UNDEFINED(constantF)
# error constantF is defined "" and the above if should not be true - this line should not run
#endif

#if ! IS_EMPTY_OR_UNDEFINED(constantH)
# error constantH is defined empty and the above if should not be true - this line should not run
#endif

#if defined(undefinedConstant) && ! IS_EMPTY_OR_UNDEFINED(undefinedConstant)
# error undefinedConstant is not defined and the above if should not be true - this line should not run
#endif

Answer 1

可以检查表达式是否为空(取模一些非常奇异的边界情况)，但该技术有些复杂。 P99有一个执行此操作的宏，您可以将其用作

#if !defined(constantA) || P99_IS_EMPTY(constantA)
...
#endif

C 标准不允许将它组合到一个宏中。从 C11 6.10.1 p4 开始，标记 defined 不允许在宏或其他表达式中使用。