如何以向后兼容的方式使用Objective-C __nonnull?

问题描述：

Xcode最近添加了__nonnull，__nullable等属性.但是，较旧版本的clang和其他编译器不支持它们.

Xcode has recently added __nonnull, __nullable, etc. attributes. However, they're not supported by older versions of clang and other compilers.

如何以兼容的方式使用这些属性?

How can I use these attributes in a compatible way?

我希望类似的东西能起作用:

I hoped something like this would work:

#ifndef NS_ASSUME_NONNULL_BEGIN
#define __nonnull 
#endif

，但是NS_ASSUME_NONNULL_BEGIN似乎不是真正的宏，并且在Xcode7中是未定义".

but it seems that NS_ASSUME_NONNULL_BEGIN is not a real macro, and it's "not defined" in Xcode7.

这很有意义:

#if !defined(__is_identifier) || __is_identifier(__nonnull)
#define __nonnull
#define __nullable
#endif

但是Xcode 6却因令牌在预处理器子表达式中不是有效的二进制运算符"错误而窒息.

but Xcode 6 chokes on that with "token is not a valid binary operator in a preprocessor subexpression" error.

答

如问题中所述，所有好的方法似乎都不起作用.最直率的人做到了:

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