在iOS開發(fā)中總能看見__has_feature宏，最常見的例如__has_feature(objc_arc)，表示支持自動(dòng)引用計(jì)數(shù)機(jī)制（ARC），類似的還有objc_arc_weak、objc_arc_fields等
__has_feature在Clang文檔中的定義是這樣的
__has_featureand__has_extension
These function-like macros take a single identifier argument that is the name of a feature.__has_featureevaluates 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_extensionevaluates 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:
大致的意思是通過給定的值房维，判斷編譯器是否支持該特性
類似的特性檢測宏還有__has_builtin、__has_attribute等抬纸，他們都屬于Feature Checking Macros
類似的還有Include File Checking Macros大類的宏定義咙俩，用于檢測是否包含文件，常見的有 __has_include等
__has_feature支持哪些參數(shù)湿故，可以看一下Clang源碼PPMacroExpansion.cpp文件中的HasFeature方法阿趁，在文件的855行到1005行，方法再結(jié)合文檔坛猪，基本可以熟練的使用這個(gè)宏了
