Objectivec 使用 ARC，什么更好：alloc 或 autorelease 初始化程序

差别很小，但你应该 Select autorelease个版本.首先，你的代码更具可读性.第二，在判断优化的装配输出时，autorelease版稍微更为优化.

autorelease版，

- (NSString *)hello:(NSString *)name {
    return [NSString stringWithFormat:@"Hello, %@", name];
}

翻译成

"-[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}

而[[alloc]init]版本如下所示:

"-[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 the name string temporarily
    mov r1, r5
    mov r4, r0
    mov r0, r6
    blx _objc_msgSend   ; call to 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 to initWithFormat:
    mov r5, r0
    mov r0, r4
    blx _objc_release   ; ARC releases the name string
    mov r0, r5
    pop.w   {r4, r5, r6, r7, lr}
    b.w _objc_autorelease

正如预期的那样，它会稍微长一点，因为它正在调用alloc和initWithFormat:方法.特别有趣的是，ARC在这里生成了次优代码，因为它保留了name字符串(通过调用_objc_retain来说明)，然后在调用initWithFormat:之后释放.

如果我们添加__unsafe_unretained所有权限定符，如下面的示例所示，代码将以最佳方式呈现.__unsafe_unretained指示编译器使用原语(复制指针)assignment semantics.

- (NSString *)hello:(__unsafe_unretained NSString *)name {
    return [[NSString alloc] initWithFormat:@"Hello, %@", name];
}

详情如下:

"-[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