[cfe-dev] Blocks vs. C++ lambda's?

[Douglas Gregor]

Remy, Boris,

This is a really interesting discussion to have, but I don't believe  
it belongs on this mailing list because it will not be seen by people  
familiar with C++ lambdas. I suggest bringing your question to  
comp.lang.c++.moderated.

	- Doug

On Apr 30, 2009, at 8:31 AM, Remy Demarest wrote:

> Hi,
>
> The non-serious answer would be: because the C++(0x) syntax sucks. :P
>
> And now the serious answer: from what I can tell, C++(0x) lambdas
> cannot be used outside of their declaration scope at least if it uses
> variables that are contained in the innermost block scope of its
> creation...
>
> Whereas Apple's blocks have a few advantages, first they have a simple
> syntax, their type declaration is very close from function pointers
> (you replace * by ^), second, you don't need to declare which
> variables from the enclosing scope you want to use inside your  
> block...
> All of them can be used inside, in case you want to modify one of the
> captured variables, you need to add the __block type specifier in
> front of the variable declarations. This is the equivalent to the
> [&total]{ total += 5; } syntax that is used in C++'s lambdas, the
> reference & operator doesn't mean the same thing in C, and we need the
> blocks to be compatible with C.
>
> And most importantly, the best reason to prefer Apple's feature is
> that you can fully use the blocks outside of their declaration scope,
> even though the variables (__block or not) from the enclosing scope
> are no more on the current stack frame. This is done by copying the
> block to the heap along with all the variables it uses, and if two
> blocks uses/modifies the same __block variable in the declaration
> scope, both blocks will see the same changes, even when copied on the
> heap.
>
> Other reasons to prefer Apple's feature is that it's fully available
> in C, C++ and ObjC altogether, it supports the garbage collection of
> ObjC with __weak __block variables, it has additions for ObjC and C++
> types, and also, it's a first class object in ObjC and can thus
> receive messages.
>
> Remy Demarest
>
> Le 30 avr. 2009 à 14:36, Boris Dušek a écrit :
>
>> Hi,
>>
>> I learned about the blocks feature Apple is implementing as an
>> extension to C. After looking at it briefly, I concluded it's
>> basically the same thing as C++(0x) lambdas. But there should be a
>> difference since if not, (I guess) it would make sense to just adopt
>> the C++ lambda syntax in C and Obj-C to achieve the goal. So could
>> someone please point out what blocks provide that C++ lambdas do not
>> (and vice versa)?
>>
>> Thanks,
>> Boris Dušek
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>
>
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