Delegate Memory Usage & Optimization

[Orvid King]

On Wed, 11 Dec 2013 22:16:08 -0600, Manu <turkeyman at gmail.com> wrote:

> On 12 December 2013 14:06, Manu <turkeyman at gmail.com> wrote:
>> On 12 December 2013 13:45, Maxime Chevalier-Boisvert  
>> <maximechevalierb at gmail.com> wrote:
>>>> It only allocates a closure if (1) it thinks the delegate may escape  
>>>> the scope and (2) uplevel references are used in the delegate.
>>>
>>> The delegate will escape the enclosing scope. I'm wondering if there  
>>> will still be some kind of scope object allocated to represent  
>>> escaping values in >>>the englobing stack frame, even when there are  
>>> no escaping values.
>>>
>>> I don't mind paying the small cost of an extraneous null context  
>>> pointer, but having a whole unnecessary context object allocated seems  
>>> wasteful.
>>
>> If you have no use for the context pointer, then it sounds like what  
>> you want is a function pointer, not a delegate.
>>
>> A delegate is just a function-ptr+context-ptr pair, it is a trivial  
>> struct that is passed by value, the same as a dynamic array. The  
>> context pointer may be >>to a stack frame (in case of a closure), or a  
>> class object (in the case of a class method pointer).
>> If you intend a null context pointer (ie, function doesn't reference  
>> any state), than what you really have is a function pointer, not a  
>> delegate. You >>should use a function pointer instead.
>
> Oh sorry, I see. you just mean in some cases functions that don't access  
> state will be bound to your delegate.
> It seems unusual to me for a function that doesn't access any state to  
> produce a delegate. Wouldn't it be static, or a free function in that  
> case?
>
> There are tricks to bind a free function to a delegate using a little  
> call-through stub. It sets the delegate function to a callthrough-stub  
> which internally >casts 'this' to a 'function' and calls it with the  
> same arguments.
> Don does it to bind static functions to delegates in his C++  
> FastDelegate library. I wonder if there's a helper in phobos?
> You can be sure when assigning functions to delegates in this way that  
> there will never be any associated state.
>
>> Can you show your usage?
>>
>>>> I strongly suggest trying out a couple examples, and disassembling  
>>>> the result to confirm.
>>>
>>> I'll look into that.
>>
>

Actually, you could simply use function pointers rather than delegates,  
and when you need to pass them as a delegate, pass them through  
std.functional.toDelegate, which, for a function pointer at least,  
utilizes a union to set the delegate's context pointer to null, and sets  
the function pointer to the one you've passed in.
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