objective c - With ARC, what's better: alloc or autorelease initializers? -

is better (faster & more efficient) use alloc or autorelease initializers. e.g.:

- (nsstring *)hello:(nsstring *)name {     return [[nsstring alloc] initwithformat:@"hello, %@", name]; } 

or

- (nsstring *)hello:(nsstring *)name {     return [nsstring stringwithformat:@"hello, %@", name]; //    return [@"hello, " stringbyappendingstring:name]; // simpler } 

i know in cases, performance here shouldn't matter. but, i'd still in habit of doing better way.

if same thing, prefer latter option because it's shorter type , more readable.

in xcode 4.2, there way see arc compiles to, i.e., puts retain, release, autorelease, etc? feature useful while switching on arc. know shouldn't have think stuff, it'd me figure out answer questions these.

the difference subtle, should opt autorelease versions. firstly, code more readable. secondly, on inspection of optimized assembly output, autorelease version more optimal.

the autorelease version,

- (nsstring *)hello:(nsstring *)name {     return [nsstring stringwithformat:@"hello, %@", name]; } 

translates to

"-[sgcappdelegate hello:]":     push    {r7, lr}     movw    r1, :lower16:(l_objc_selector_references_-(lpc0_0+4))     mov r3, r2     movt    r1, :upper16:(l_objc_selector_references_-(lpc0_0+4))     movw    r0, :lower16:(l_objc_classlist_references_$_-(lpc0_1+4))     movt    r0, :upper16:(l_objc_classlist_references_$_-(lpc0_1+4))     add r1, pc     add r0, pc     mov r7, sp     ldr r1, [r1]     ldr r0, [r0]     movw    r2, :lower16:(l__unnamed_cfstring_-(lpc0_2+4))     movt    r2, :upper16:(l__unnamed_cfstring_-(lpc0_2+4))     add r2, pc     blx _objc_msgsend    ; stringwithformat:     pop {r7, pc} 

whereas [[alloc] init] version looks following:

"-[sgcappdelegate hello:]":     push    {r4, r5, r6, r7, lr}     movw    r1, :lower16:(l_objc_selector_references_2-(lpc1_0+4))     add r7, sp, #12     movt    r1, :upper16:(l_objc_selector_references_2-(lpc1_0+4))     movw    r0, :lower16:(l_objc_classlist_references_$_-(lpc1_1+4))     movt    r0, :upper16:(l_objc_classlist_references_$_-(lpc1_1+4))     add r1, pc     add r0, pc     ldr r5, [r1]     ldr r6, [r0]     mov r0, r2     blx _objc_retain    ; arc retains name string temporarily     mov r1, r5     mov r4, r0     mov r0, r6     blx _objc_msgsend   ; call alloc     movw    r1, :lower16:(l_objc_selector_references_4-(lpc1_2+4))     mov r3, r4     movt    r1, :upper16:(l_objc_selector_references_4-(lpc1_2+4))     add r1, pc     ldr r1, [r1]     movw    r2, :lower16:(l__unnamed_cfstring_-(lpc1_3+4))     movt    r2, :upper16:(l__unnamed_cfstring_-(lpc1_3+4))     add r2, pc     blx _objc_msgsend   ; call initwithformat:     mov r5, r0     mov r0, r4     blx _objc_release   ; arc releases name string     mov r0, r5     pop.w   {r4, r5, r6, r7, lr}     b.w _objc_autorelease 

as expected, little longer, because calling alloc , initwithformat: methods. particularly interesting arc generating sub-optimal code here, retains name string (noted call _objc_retain) , later released after call initwithformat:.

if add __unsafe_unretained ownership qualifier, in following example, code rendered optimally. __unsafe_unretained indicates compiler use primitive (copy pointer) assignment semantics.

- (nsstring *)hello:(__unsafe_unretained nsstring *)name {     return [[nsstring alloc] initwithformat:@"hello, %@", name]; } 

as follows:

"-[sgcappdelegate hello:]":     push    {r4, r7, lr}     movw    r1, :lower16:(l_objc_selector_references_2-(lpc1_0+4))     add r7, sp, #4     movt    r1, :upper16:(l_objc_selector_references_2-(lpc1_0+4))     movw    r0, :lower16:(l_objc_classlist_references_$_-(lpc1_1+4))     movt    r0, :upper16:(l_objc_classlist_references_$_-(lpc1_1+4))     add r1, pc     add r0, pc     mov r4, r2     ldr r1, [r1]     ldr r0, [r0]     blx _objc_msgsend     movw    r1, :lower16:(l_objc_selector_references_4-(lpc1_2+4))     mov r3, r4     movt    r1, :upper16:(l_objc_selector_references_4-(lpc1_2+4))     add r1, pc     ldr r1, [r1]     movw    r2, :lower16:(l__unnamed_cfstring_-(lpc1_3+4))     movt    r2, :upper16:(l__unnamed_cfstring_-(lpc1_3+4))     add r2, pc     blx _objc_msgsend     .loc    1 31 1     pop.w   {r4, r7, lr}     b.w _objc_autorelease