How do I know whether the compiler has ARC support

问题:

I need to write a lib in my iOS app.

The statement should be pre-process define as :

myObject ...

#if ARC
   // do nothing
#else
   [myObject release]
#endif

or run-time process as:

if (ARC) {
   // do nothing
} else {
   [myObject release];
}

How can I do?

Please help me! Thank you.

回答1:

You can use __has_feature, like so:

#if __has_feature(objc_arc)
// ARC is On
#else
// ARC is Off
#endif

If you want to also build with GCC (Apple's GCC does not support ARC), you may also need the following to determine the compiler:

#if defined(__clang)
// It's Clang
#else
// It's GCC
#endif

Update

Combined, they would take the general form:

 #if defined(__clang)     

 #if !defined(__has_feature)
 // idk when clang introduced this
 #error This version of clang does not support __has_feature
 #endif

 #define MON_IS_ARC_ENABLED_IN_THIS_TRANSLATION __has_feature(objc_arc)

 #else
 // for every compiler other than clang:

 #if defined(__has_feature)
 #error Another compiler supports __has_feature
 #endif

 #define MON_IS_ARC_ENABLED_IN_THIS_TRANSLATION 0

 #endif

Then just use MON_IS_ARC_ENABLED_IN_THIS_TRANSLATION in your sources or for further #defines.

If a compiler you use adds support, you would have to add a case for that (and compiler errors would likely catch the error in this case, since it would likely forbid use of ref count ops).

Note that this has extra checks to demonstrate how one can (and should) avoid defining reserved identifiers (based on a conversation in the comments). It's not exhaustive, but a demonstration. If you find yourself writing conditional __has_feature checks often, you may want to define a new macro for that to reduce and simplify definitions.

回答2:

You can do it using macros:

#if !defined(__clang__) || __clang_major__ < 3
    #ifndef __bridge
        #define __bridge
    #endif

    #ifndef __bridge_retain
        #define __bridge_retain
    #endif

    #ifndef __bridge_retained
        #define __bridge_retained
    #endif

    #ifndef __autoreleasing
        #define __autoreleasing
    #endif

    #ifndef __strong
        #define __strong
    #endif

    #ifndef __unsafe_unretained
        #define __unsafe_unretained
    #endif

    #ifndef __weak
        #define __weak
    #endif
#endif

#if __has_feature(objc_arc)
    #define SAFE_ARC_PROP_RETAIN strong
    #define SAFE_ARC_RETAIN(x) (x)
    #define SAFE_ARC_RELEASE(x)
    #define SAFE_ARC_AUTORELEASE(x) (x)
    #define SAFE_ARC_BLOCK_COPY(x) (x)
    #define SAFE_ARC_BLOCK_RELEASE(x)
    #define SAFE_ARC_SUPER_DEALLOC()
    #define SAFE_ARC_AUTORELEASE_POOL_START() @autoreleasepool {
    #define SAFE_ARC_AUTORELEASE_POOL_END() }
#else
    #define SAFE_ARC_PROP_RETAIN retain
    #define SAFE_ARC_RETAIN(x) ([(x) retain])
    #define SAFE_ARC_RELEASE(x) ([(x) release])
    #define SAFE_ARC_AUTORELEASE(x) ([(x) autorelease])
    #define SAFE_ARC_BLOCK_COPY(x) (Block_copy(x))
    #define SAFE_ARC_BLOCK_RELEASE(x) (Block_release(x))
    #define SAFE_ARC_SUPER_DEALLOC() ([super dealloc])
    #define SAFE_ARC_AUTORELEASE_POOL_START() NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    #define SAFE_ARC_AUTORELEASE_POOL_END() [pool release];
#endif

The above came from the site: http://raptureinvenice.com/arc-support-without-branches/; but I've pasted it to ensure it's not lost.

回答3:

You generally do not want to do things like this:

#if ARC
   // do nothing
#else
   [myObject release]
#endif

Because it’s a recipe for disaster, there are many subtle bugs lurking in such code. But if you do have a sane use case for that, you would perhaps be better off with a macro (I didn’t know __has_feature, thanks Justin!):

#if __has_feature(objc_arc)
    #define MY_RELEASE(x) while (0) {}
#else
    #define MY_RELEASE(x) [x release]
#endif

But I would be quite nervous to use even this, the pain potential is huge :)