Blocks are declared and typed as follows:

‘return type’ (^)(‘parameter types’)

An example of a block that returns an int for two strings would be:

int (^)(NSString, NSString)

An example of a block that returns nothing from no arguments would be:

void (^)(void)

Blocks can also be declared as local variables in Objective-C code like so:

int main(int argc, string[] *argv) { int(^x)(int input) = ^(int input){ return input * input; };

int sixteen = x(4); }

*The original message announcing blocks to the Clang list: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2008-August/002670.html *Blocks on the Clang site: http://clang.llvm.org/docs/LanguageExtensions.html#blocks *MikeAsh’s introduction: http://www.mikeash.com/?page=pyblog/friday-qa-2008-12-26.html *A backport of blocks for Leopard and iPhoneOS: http://www.plausiblelabs.com/blog/?p=8 http://code.google.com/p/plblocks/ *A guide to using blocks in C and Objective-C: http://thirdcog.eu/pwcblocks/

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Ok but is it very different then callback function ? —- Yes, blocks capture their parent function’s scope, and can even access it after the parent function has returned. See http://en.wikipedia.org/wiki/Closure_(computer_science)