NSValue:getValue: strange behavior,why this happens?

Question

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

Answer 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.

Answer 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.