FMDatabase.h

 1 #if ! __has_feature(objc_arc)
 2     #define FMDBAutorelease(__v) ([__v autorelease]);
 3     #define FMDBReturnAutoreleased FMDBAutorelease
 4
 5     #define FMDBRetain(__v) ([__v retain]);
 6     #define FMDBReturnRetained FMDBRetain
 7
 8     #define FMDBRelease(__v) ([__v release]);
 9
10     #define FMDBDispatchQueueRelease(__v) (dispatch_release(__v));
11 #else
12     // -fobjc-arc
13     #define FMDBAutorelease(__v)
14     #define FMDBReturnAutoreleased(__v) (__v)
15
16     #define FMDBRetain(__v)
17     #define FMDBReturnRetained(__v) (__v)
18
19     #define FMDBRelease(__v)
20
21 // If OS_OBJECT_USE_OBJC=1, then the dispatch objects will be treated like ObjC objects
22 // and will participate in ARC.
23 // See the section on "Dispatch Queues and Automatic Reference Counting" in "Grand Central Dispatch (GCD) Reference" for details.
24     #if OS_OBJECT_USE_OBJC
25         #define FMDBDispatchQueueRelease(__v)
26     #else
27         #define FMDBDispatchQueueRelease(__v) (dispatch_release(__v));
28     #endif
29 #endif

　　这里的预编译主要是 FMDB 用来兼容 ARC 和 MRC 的。