Optional parameters referring to previous parameters?

[Tove]

On Thursday, 10 May 2012 at 17:41:23 UTC, dennis luehring wrote:
> Am 10.05.2012 19:07, schrieb Tove:
>> auto my_extended_alloca(size_t size, void* buf=alloca(size))
>> {
>>    return buf;
>> }
>
> and whats the difference to?
>
> auto my_extended_alloca(size_t size, void* buf)
> {
>   return alloca(size);
> }
>
> except that you hide the alloca in the interface
> which can be easily overwritten with malloc or something?
>
> auto x = my_extended_alloca( 10, malloc(100) ); ???

When used in the parameter list, the alloca() is injected into 
the parent scope.

Your version doesn't work at all, as the allocation automatically 
ends with the scope of my_extended_alloca() instead of the scope 
of the caller!