How to retrieve memory size of NSArray instance?

Question

Possible Duplicate:
Checking the size of an object in Objective-C

How to compute the memory size of one live object-c instance, take NSArray for example

NSArray *myArray;

For C, sizeof(item)*count could be one solution, how about Object-C ?

To make more clear, I want to know the memory size of Object-C instance, for example we have UIImage *image

I want to figure out how much memory size of "image", ( do not compute by width*height*4 etc )

Same with NSArray etc. malloc_size can not figure out the real instance memory size actually.