Create class on stack

[FoxyBrown via Digitalmars-d-learn]

On Sunday, 6 August 2017 at 02:10:31 UTC, Moritz Maxeiner wrote:
> On Sunday, 6 August 2017 at 01:18:50 UTC, Johnson Jones wrote:
>> On Saturday, 5 August 2017 at 23:09:09 UTC, Moritz Maxeiner 
>> wrote:
>>> On Saturday, 5 August 2017 at 17:08:32 UTC, Johnson Jones 
>>> wrote:
>>>> using gtk, it has a type called value. One has to use it to 
>>>> get the value of stuff but it is a class. Once it is used, 
>>>> one doesn't need it.
>>>>
>>>> Ideally I'd like to treat it as a struct since I'm using it 
>>>> in a delegate I would like to minimize unnecessary 
>>>> allocations. Is there any way to get D to allocate a class 
>>>> on the stack like a local struct?
>>>
>>> The easy way is through std.typecons.scoped [1].
>>> Here be dragons, though, because classes are reference types.
>>>
>>> [1] https://dlang.org/phobos/std_typecons.html#.scoped
>>
>> Thanks, I didn't think it created on the stack but it makes 
>> sense to do so.
>
> See the source [1] as to why: typeof(scoped!T) is a 
> (non-copyable) struct that holds the memory for the T object 
> inside it.
>
>> The only issue is that it escaping the reference?
>
> Yes, don't escape references, that's the reason for my comment:
>>> Here be dragons, though, because classes are reference types.
>
> [1] 
> https://github.com/dlang/phobos/blob/v2.075.0/std/typecons.d#L6613

I don't think you understand what I'm saying.

If I use this method to create a "reference" type on the stack 
rather than the heap, is the only issue worrying about not having 
that variable be used outside that scope(i.e., have it "escape")?

Obviously since it's on the stack it will be invalid after the 
function call, but I'm talking about other pitfalls. I don't see 
any but I want to be sure. Also, does it do the allocation at 
compile time(reserve space on the stack for the variable along 
with all the others or does it "allocate" space on the stack at 
runtime?... which is slightly slower).