return value

[Steven Schveighoffer]

On Tue, 09 Mar 2010 13:35:41 -0500, Ali Çehreli <acehreli at yahoo.com> wrote:

> bearophile wrote:
>> m Wrote:
>>> Can a function return a function as a return value?
>>> as a delegate?
>>  Yes, you can do those things. In D2 you can return a clusure too:
>
> Is closure a separate feature, or are delegates closures?

A closure is a separate feature.

Basically, a closure is a delegate with a stack frame that is allocated on  
the heap.  The advantage is you can pass it around and not worry about the  
context becoming invalid.

In D1, for example, you could do what bearophile did, but it would result  
in memory corruption:

void delegate(int) adder(int x)
{
   void dg(int y)
   {
      return x + y;
   }
   return &dg;
}

The problem with this is that x is valid in the context of foo, not the  
context of the delegate.  Therefore, when foo returns, it's stack-based  
frame can be overwritten, corrupting the value for x.

In D2, the above code makes the D compiler allocate foo's stack frame on  
the heap, so even when foo returns, its stack frame is valid for the  
delegate to refer to.

-Steve