问题
Xcode has recently added __nonnull, __nullable, etc. attributes. However, they're not supported by older versions of clang and other compilers.
How can I use these attributes in a compatible way?
I hoped something like this would work:
#ifndef NS_ASSUME_NONNULL_BEGIN
#define __nonnull
#endif
but it seems that NS_ASSUME_NONNULL_BEGIN is not a real macro, and it's "not defined" in Xcode7.
And it would make sense for this to work:
#if !defined(__is_identifier) || __is_identifier(__nonnull)
#define __nonnull
#define __nullable
#endif
but Xcode 6 chokes on that with "token is not a valid binary operator in a preprocessor subexpression" error.
回答1:
As explained in the question, all good methods seem not to work. The most blunt one does:
#if !defined(__clang_major__) || __clang_major__ < 7
#define __nonnull
#define __nullable
#endif
来源：https://stackoverflow.com/questions/31105433/how-to-use-objective-c-nonnull-in-a-backwards-compatible-way