apple blocks extension

[Chris Lattner]

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.
>
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1968.pdf

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) {
   run_asynch(^{
       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.

-Chris