ARC中的虚假va_list [英] fake va_list in ARC

问题描述

我需要在iOS应用程序中创建一个假的 va_list 来传递给NSString initWithFormat：arguments： function ，这是我的代码：

I need to create in an iOS application a fake va_list to pass to a NSString initWithFormat:arguments: function, this is my code:

NSArray *fixedArguments = [[NSArray alloc] initWithArray:arguments]; 

NSRange range = NSMakeRange(0, [fixedArguments count]);

va_list fakeArgList = (va_list)malloc(sizeof(NSString *) * [fixedArguments count]);

__unsafe_unretained id *ptr = (__unsafe_unretained id *)fakeArgList;

[fixedArguments getObjects:ptr range:range];

content = [[NSString alloc] initWithFormat:outputFormat
                                            arguments:(va_list)fakeArgList];
free(fakeArgList);

编译器在投射线上抱怨此消息：

The compiler complains with this message on the cast line:

error: cast of a non-Objective-C pointer type 'va_list' (aka 'char *') to '__unsafe_unretained id *' is disallowed with ARC

getObjects：range：函数定义如下：

- (void)getObjects:(id __unsafe_unretained [])objects range:(NSRange)range;

我已经尝试了一切，但仍然无法摆脱这个错误......

I've tried everything but still can't get rid of this error...

是否有启用ARC的假冒 va_list 的解决方案？我做错了什么？

Is there a solution for creating a fake va_list with ARC enabled? What am i doing wrong?

推荐答案

编辑：这不再有效。正如在最初的答案中所预见的那样，ABI似乎已经从这个答案中改变了

This no longer works. As foreseen in the initial answer, the ABI appears to have changed out from under this answer

玩了一下并让它起作用 - 双重检查泄漏或废弃的内存并没有看到任何内容。

Played around for a bit and got it to work -- Double checked for leaks or abandoned memory and didn't see any.

    NSArray *fixedArguments = [[NSArray alloc] initWithObjects: @"foo", @"bar", @"baz", nil]; 

    NSRange range = NSMakeRange(0, [fixedArguments count]);

    NSMutableData* data = [NSMutableData dataWithLength: sizeof(id) * [fixedArguments count]];    

    [fixedArguments getObjects: (__unsafe_unretained id *)data.mutableBytes range:range];

    NSString* content = [[NSString alloc] initWithFormat: @"1: %@ 2: %@ 3: %@"  arguments: data.mutableBytes];

    NSLog(@"%@", content);

我喜欢（ab）使用这样的NSMutableData来获取任意块上的保留/释放语义记忆 - 它不一定与手头的问题有关，但它是一个巧妙的小技巧。

I like to (ab)use NSMutableData like this to get retain/release semantics on an arbitrary chunk of memory -- It's not necessarily relevant to the issue at hand, but it's a neat little trick.

作为未来读者的一个注释：假装这样的va_list恰好使用适用于MacOS和iOS的当前ABI，但一般来说它不是便携式的，也不是一个好的方法。

As a note to future readers: Faking up a va_list like this happens to work with the current ABI for MacOS and iOS, but in general it's not portable, and not a good approach.

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