Preprocessor macro using caret ^ symbol at the start of an expression

Question

Looking at this page: http://www.mikeash.com/pyblog/friday-qa-2010-12-31-c-macro-tips-and-tricks.html

I found this snippet of code with ^{ ... }() syntax, what are the caret/brackets doing?

#define MAX(x, y) (^{ \
    int my_localx = (x); \
    int my_localy = (y); \
    return my_localx > my_localy ? (my_localx) : (my_localy); \
}())

It looks like its creating an anonymous function or something. What is this concept called? Where can I read about it?

Answer 1

It's a C block. It's quite like an anonymous function (in use, not in structure). You can read more about them on Mike Ash's site and in Apple's documentation.

Answer 2

It's a block. It's not standard C, but it is supported by Apple's LLVM compiler (around about Xcode 3.2 IIRC and later). See here and here for more details.

It's not just for Objective-C, but is part of the C and C++ compilers also.