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Re: apple blocks extension

On Sep 15, 2009, at 9:04 AM, Richard Henderson wrote:

On 09/15/2009 08:28 AM, Vincent R. wrote:
I just was curious to know if closures in apple gcc(called blocks from
what I read) is
also in mainline.
What is the status about this extension ?

It is unlikely that this will ever be brought into GCC, since it appears to be largely identical to the C++0x Lambda feature.

There are major differences between lambdas and blocks, though they serve a superficially similar purpose. If you are interested, there is a ton of discussion online. Here are the two biggest differences:

The first difference is that every instance of a lambda gives you a value of a new (anonymous) type, which makes them mostly only useful with templates. Blocks literals give values of a predictable type, which means that you can pass them around to non-templates. This is also why they "work" in C. Yes, I know that there are various adaptor classes that can be used with C++ lambdas to help work around this, but this is in inherent part of blocks.

The second major feature of Blocks vs c++ lambdas is that they can be "copied onto the heap". This allows things like "Grand Central Dispatch" to work: you can write code that executes blocks asynchronously or on other threads/work queues (after the function containing the block has returned). A simple example is:

void print_on_different_thread(int X) {
      printf("Hi %d\n", X);

With lambdas, the closure would be go out of scope when print_on_different_thread returns, but blocks allows "run_asynch" to extend the lifetime of the block.

Blocks and Lambdas have intentionally different syntax, allowing them to coexist peacefully in the same compiler / language. They are also superficially similar to nested functions, but have other sets of benefits. For example, blocks don't require executable stacks etc.


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