rationale for function and delegate

[Robert Jacques]

On Sat, 16 Oct 2010 11:57:36 -0400, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> On Sat, 16 Oct 2010 11:22:43 -0400, Juanjo Alvarez <fake at fakeemail.com>  
> wrote:
>
>> On Sat, 16 Oct 2010 14:42:13 +0000 (UTC), dsimcha <dsimcha at yahoo.com>  
>> wrote:
>>> delegate with minimal overhead.  This mitigates the situation a
>> lot, since if an
>>> API requires a delegate and you have a function pointer, you just
>> do a
>>> toDelegate(someFunctionPointer).
>>
>> Sorry for asking here something that should go to D.learn, but how do  
>> you do the reverse, that is, getting a function from a delegate? I need  
>> that in my project so I can pass it to signal so some Unix signal will  
>> trigger a method of an already instantiated object.
>
> auto dg = &obj.method;
>
> auto fptr = dg.funcptr;
> auto context = dg.ptr;
>
> Note, you cannot call fptr, you will get a runtime error.
>
> Here is the related documentation (search for funcptr):  
> http://www.digitalmars.com/d/2.0/function.html
>
> I believe there may be in phobos a type which wraps a function pointer  
> into a delegate.  Not sure if it was ever added though...
>
> -Steve

Actually, you can use funcptr for simple functions. Here's an example how:

void main(string[] args) {
     auto foo = (int x){ writeln(x); };
     void function(int,void*) bar = foo.funcptr;
     bar(5,foo.ptr);
     foo(5);
     return;
}

Structs larger than 8 bytes require a hidden return pointer, so instead of

LargeStruct function(int,void*) bar;

you'd have

void function(int,LargeStruct*,void*) bar;

However, if Unix code needs to call the function, it needs to do so using  
the C calling conventions, which are platform/compiler specific.