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.