this() not executing code on structs

[Denis Koroskin]

On Fri, 23 Oct 2009 19:40:33 +0400, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org> wrote:

> Denis Koroskin wrote:
>> On Fri, 23 Oct 2009 18:46:47 +0400, Andrei Alexandrescu  
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> Don wrote:
>>>> Bartosz Milewski wrote:
>>>>> Andrei Alexandrescu Wrote:
>>>>>
>>>>>>     this() { myCount = count++; }       // ERROR
>>>>>
>>>>> It's worse than that. Try this:
>>>>>
>>>>> struct foo {
>>>>>        this(int dummy = 0) { writeln("Default constructor");}
>>>>> }
>>>>>
>>>>> foo x = foo();
>>>>>
>>>>> Nothing gets printed. If default constructors are disallowed, so  
>>>>> should constructors with all parameters defaulted.
>>>>  Ouch.
>>>> It's because it's interpreting foo() as a struct literal.
>>>> If a struct has any constructors, struct literals should be disabled.
>>>
>>> http://d.puremagic.com/issues/show_bug.cgi?id=3438
>>>
>>> The more I think of it, the more imperious it becomes that we allow  
>>> default constructors that execute code. The main question is what to  
>>> do about .init.
>>>
>>> Andrei
>>  I'd suggest ditching it and enforce explicit member initialization  
>> (unless a variable is nullable). This will also remove a lot of bloat  
>> from executables.
>
> I don't understand. The problem right now is that even of all fields are  
> explicitly intialized, e.g.
>
> struct A {
>      Widget x = null;
>      double d = 0;
>      int i = -1;
> }
>
> there is still no ability to execute code upon initialization, e.g.  
> force A to contain a non-null Widget.
>
>
> Andrei

Yes, I was talking about a different scheme, where default ctors are
allowed. They aren't allowed now but we are talking about things we can
change/improve, right?

I mean there is no _real_ need for T.init, just malloc()'ate some memory  
and call the __ctor on it. Struct members default values would be  
converted into runtime initialization expressions like this:

struct A
{
     this(Args)(Args args) // may be an empty set of arguments
     {
         // the following lines are inserted automatically
         x = null;
         d = 0;
         i = -1;

         // user-defined code follows
         i = 42;
     }

     Widget x = null;
     double d = 0;
     int i = -1;
}

Optimization pass would eliminate double initialization (in the case about  
i would be initialized straight to 42)