Struct constructor, opCall mess.

[Remo]

On Tuesday, 25 February 2014 at 07:59:33 UTC, Maxim Fomin wrote:
> On Monday, 24 February 2014 at 14:14:43 UTC, Tobias Pankrath 
> wrote:
>> On Monday, 24 February 2014 at 13:56:01 UTC, Remo wrote:
>>> Hi,
>>>
>>> right now I am truing to figure out how the constructors 
>>> behave in D2.
>>>
>>> Question 1: why it is not possible to create custom ctor for 
>>> struct?
>>
>> The design of D relies on the fact that every type has a 
>> T.init property that is known to the compiler and used when in 
>> C++ the default ctor would get called. In constructors you can 
>> rely on (this == T.init), for example.
>>
>> You need to pick one T.init or default constructors and D 
>> picked T.init.
>
> The design of D relies on Andrei opinion. He is indeed 
> convinced that default constructors are impossible.
>
> However, you can write "default constructor" right now like:
>
> struct S
> {
>    Type t;
>    this(int)
>    {
>       t = whather_is_callable_in_CTFE();
>    }
> }
>
> enum E : S
> {
>    A = S(0)
> }
>
> void main()
> {
>   E e;
>   assert (e.t == whather_is_callable_in_CTFE());
> }
>
> Since compiler does this, it can also accept straight syntax 
> and semantic:
>
> struct S
> {
>    Type t;
>    this() // proxibited default constructor now
>    {
>       t = whather_is_callable_in_CTFE();
>    }
> }
>
> What D really lacks is ability to call function in runtime 
> after struct instance creation like:
>
> struct S
> {
>    @runtime this() {}
>    ~this(){}
> }
>
> lowering to:
> S s;
> s.__runtime_ctor();
>
> However, since compiler does this all over the place 
> (postblits, copy constructors, destructors, etc.) There is no 
> conceptual difference between having:
>
> S s
> s.__runtime_ctor();
>
> and
>
> S s;
> s.__dtor();
>
> In other words, there is no objective necessity not to have 
> compile time and runtime default struct constructors since 
> compiler already heavily does conceptually and technically 
> similar things.

Thanks for all this replies.
Now I have better idea how it supposed to work.

Yes something like @runtime this() {} would be really great!

https://d.puremagic.com/issues/show_bug.cgi?id=3438
> I think at some point we'll need to support default 
> constructors that execute
code.
This was posted 2009-10-23 and now years later it is still not 
possible. :(


Unfortunately this problems and workarounds makes porting C++ to 
D2 more complicated.



Here are is also a small experiment that also show that 'ref' is 
also necessary in D2 just like in C++.
There seems to be no optimization for this so using 'in' or 
nothing at all is slower and may have side effects.

http://melpon.org/wandbox/permlink/FyaksIPW4u1dNpwh

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