ARC 状态时候 标记一些 函数不能够使用

比如 ARC 状态时候， retain release 等消息发送是不能使用的。

看宏定义：

没限定：

- (BOOL)conformsToProtocol:(Protocol *)aProtocol;
- (BOOL)respondsToSelector:(SEL)aSelector;

有限定：
- (id)retain NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
- (oneway void)release NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
- (id)autorelease NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
- (NSUInteger)retainCount NS_AUTOMATED_REFCOUNT_UNAVAILABLE;

然后是 NS_AUTOMATED_REFCOUNT_UNAVAILABLE 的宏：

// Marks methods and functions which cannot be used when compiling in automatic reference counting mode.
#if __has_feature(objc_arc)
#define NS_AUTOMATED_REFCOUNT_UNAVAILABLE __attribute__((unavailable("not available in automatic reference counting mode")))
#else
#define NS_AUTOMATED_REFCOUNT_UNAVAILABLE
#endif

这里还有一个 __has_feature( )   简述如下：

_has_feature and __has_extension
These function-like macros take a single identifier argument that is the name of a feature. __has_feature evaluates to 1 if the feature is both supported by Clang and standardized in the current language standard or 0 if not (but see below), while __has_extension evaluates to 1 if the feature is supported by Clang in the current language (either as a language extension or a standard language feature) or 0 if not. They can be used like this:

#ifndef __has_feature         // Optional of course.
  #define __has_feature(x) 0  // Compatibility with non-clang compilers.
#endif
#ifndef __has_extension
  #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
#endif

...
#if __has_feature(cxx_rvalue_references)
// This code will only be compiled with the -std=c++11 and -std=gnu++11
// options, because rvalue references are only standardized in C++11.
#endif

#if __has_extension(cxx_rvalue_references)
// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
// and -std=gnu++98 options, because rvalue references are supported as a
// language extension in C++98.
#endif
For backwards compatibility reasons, __has_feature can also be used to test for support for non-standardized features, i.e. features not prefixed c_, cxx_ or objc_.

Another use of __has_feature is to check for compiler features not related to the language standard, such as e.g. AddressSanitizer.

If the -pedantic-errors option is given, __has_extension is equivalent to __has_feature.

The feature tag is described along with the language feature below.

The feature name or extension name can also be specified with a preceding and following __ (double underscore) to avoid interference from a macro with the same name. For instance, __cxx_rvalue_references__ can be used instead of cxx_rvalue_references.