在iPhone OS上进行结构对齐的确切规则是什么？

I'm using the Objective-C NSMethodSignature / @encode facilities to do some cross-language data type translation, which means that I need to be able to programmatically copy values into a structure described in @encode() format. e.g., I may have 4 floats and need to insert them into a CGRect, which is a structure containing 2 structures, each of which contains 2 floats each. In @encode terminology, the type is this:I'm using the Objective-C NSMethodSignature / @