Conversion of objective-c and other C pointers

First, look at the typical nsstring and Cfstringref conversions.

http://www.tuicool.com/articles/MJRr226

Cfstringref to nsstring *nsstring *yourfriendlynsstring = (__bridge NSString *) yourfriendlycfstring; //NSString * to Cfstringrefcfstringref yourfriendlycfstring = (__bridge cfstringref) yourfriendlynsstring;

Cfstringref to NSString * NSString * yourfriendlynsstring = (__bridge NSString *) yourfriendlycfstring; NSString * to Cfstringref Cfstringref yourfriendlycfstring = (__bridge cfstringref) yourfriendlynsstring;

A keyword __bridge appears above, which is the key to the entire transformation. Apple's official explanation for __bridge is as follows:

**__bridge** transfers A pointer between Objective-c and Core Foundation with no transfer of ownership.

**__bridge_retained** or **cfbridgingretain** casts an objective-c pointer to a Core Foundation pointer and also transfers Ownership to you. You is responsible for calling cfrelease or a related function to relinquish ownership of the object.

**__bridge_transfer** or **cfbridgingrelease** moves a non-objective-c pointer to objective-c and also transfers ownership To ARC. ARC is responsible for relinquishing ownership of the object.

These words are summed up as:

• __bridge is used for conversions between OBJECTIVE-C and core foundation pointers, which do not replace ownership of objects (ownership).
• The __bridge_retained or cfbridgeretain is used for pointer conversions from OBJECTIVE-C to Core Foundation, and the ownership (ownership) of the object is transferred, So you need to call the Cfrelease method to dereference when you are no longer using the object.
• __bridge_transfer or cfbridgerelease is used to convert a non-objective-c pointer to a objective-c pointer, and the object's ownership (ownership) is given to arc, so you don't have to worry about when the object is released. Just give it to the arc.

Why do I need to call Cfrelease to release an object when I use __bridge_retained for the conversion, in fact the reason is simple: the object of Core Foundation is within the jurisdiction of arc.

Here is the sample code:

Don ' t transfer ownership. You won ' t has toPager' cfrelease ' cfstringref str = (__bridge cfstringref) string;//Transfer ownership (i.e. get ARC out of the). The object is now yours and you mustPager ' cfrelease ' when you ' re do with itcfstringref str = (__bridge_retained cfstringref) string; You'll have the to call ' cfrelease '//Don ' t transfer ownership. ARC stays out of the, and you must call  ' cfrelease ' on  ' str ' if appropriate (depending on how the  ' cfstring ' was created) nsstring*string = (__bridge nsstring*) str;//Transfer Ownership to ARC. ARC kicks in and it's now in charge of releasing the string object. You won ' t has to explicitly call  ' cfrelease ' on  ' str ' nsstring*string = (__bridge_transfer nsstring*) str;    

 

//Don ' t transfer ownership. You won ' t has to call ' cfrelease ' cfstringref str =  ( __bridge CFStringRef )  string ; //Transfer ownership (i.e. get ARC out of the). The object is now yours, and you must call ' cfrelease ' if you ' re do with it cfstringref str =  (&NB sp;__bridge_retained cfstringref )  string ; //you'll have the to call ' Cfrelease ' // Don ' t transfer ownership. ARC stays out of the the-the-must call ' cfrelease ' on ' str ' if appropriate (depending on how the ' cfstring ' was creat ed) nsstring* string =  ( __bridge NSString* )  str ; //Transfer ownership to ARC. ARC kicks in and it's now in charge of releasing the string object. You won ' t has to explicitly call ' cfrelease ' on ' str ' nsstring* string =  ( __bridge_ transfer nsstring* )  str ;

Objective-c and other C-pointers conversions