NSValue:getValue: strange behavior,why this happens?

问题

I try to pass CGRect to NSInvocation (setArgument:atIndex:). I wrap it by NSValue, push to the NSArry,then get from NSArray and use NSValue (getValue:). Calling of (getValue:) method causes the changing of before declared index NSInteger i. Can anybody say why does this happen?

 NSString className = @"UIButton";
    Class cls = NSClassFromString(className);
    cls pushButton5 = [[cls alloc] init];
    CGRect rect =CGRectMake(20,220,280,30);
    NSMethodSignature *msignature1;
    NSInvocation *anInvocation1;

    msignature1 = [pushButton5 methodSignatureForSelector:@selector(setFrame:)];
    anInvocation1 = [NSInvocation invocationWithMethodSignature:msignature1];

[anInvocation1 setTarget:pushButton5];
[anInvocation1 setSelector:@selector(setFrame:)];
NSValue* rectValue = [NSValue valueWithCGRect:rect];
NSArray *params1;
params1= [NSArray arrayWithObjects:rectValue,nil];
id currentVal = [params1 objectAtIndex:0];
NSInteger i=2;
if ([currentVal isKindOfClass:[NSValue class]]) {
    void *bufferForValue;
    [currentVal getValue:&bufferForValue];
    [anInvocation1 setArgument:&bufferForValue atIndex:i];
}else {
    [anInvocation1 setArgument:&currentVal atIndex:i];
}
[anInvocation1 invoke];

When this(getValue:) method implements value of 'i' changes from 2 to something like : 1130102784, and in (setArgument:atIndex:) I have an SIGABRT because index i is out of bounds.

So why does [NSValue getValue:(*void) buffer] change other variables?

(P.S. I do it in function so I simplified an example and initialize array directly. And if I set directly atIndex:2, It works perfect.But as I've sad I simplified a little, and I need to pass i to atIndex:)

Thanks to Tom Dalling(especially) and sergio problem solved. I delete this:

void *bufferForValue;
[currentVal getValue:&bufferForValue];
[anInvocation1 setArgument:&bufferForValue atIndex:i];

and paste this

NSUInteger bufferSize = 0;
NSGetSizeAndAlignment([currentVal objCType], &bufferSize, NULL);
void* buffer = malloc(bufferSize);
[currentVal getValue:buffer];
[anInvocation1 setArgument:buffer atIndex:i];

Thank you. Stackoverflow.com really helpful site with smart people.

回答1:

You can't fit an NSRect into a void*. The correct code would be:

NSRect buffer;
[currentVal getValue:&buffer];

Or alternatively, if you don't know the contents of the NSValue object:

NSUInteger bufferSize = 0;
NSGetSizeAndAlignment([currentVal objCType], &bufferSize, NULL);
void* buffer = malloc(bufferSize);
[currentVal getValue:buffer]; //notice the lack of '&'
//do something here
free(buffer);

The value for i is changing because your code causes an overflow into other stack-allocated variables.

回答2:

In:

 void *bufferForValue;
 [currentVal getValue:&bufferForValue];

you are copying currentVal value to a buffer which has the size of void* (a pointer); now, see getValue reference:

getValue:

Copies the receiver’s value into a given buffer.

buffer

A buffer into which to copy the receiver's value. buffer must be large enough to hold the value.

Since the value you copying over to the buffer cannot fit in a void*, you are thus overwriting your stack, since you are writing to the address of a local variable; this has likely the effect of overwriting the i local variable, so that explains what is happening.

来源：https://stackoverflow.com/questions/7036665/nsvaluegetvalue-strange-behavior-why-this-happens

标签

objective-c

nsinvocation