struct constructor co nfusion

[Atila Neves via Digitalmars-d-learn]

On Friday, 6 November 2015 at 17:34:29 UTC, Spacen Jasset wrote:
> Hello,
>
> I have read various things about struct constructors, 
> specifically 0 argument constructors, and using opCall and 
> @disable this(); which no longer seems to work.
>
> What I am after I think is the behavior of C++'s structs on the 
> stack, namely for some or all of these uses at a given time:
>
>
> 1. Allocation on the stack
> 2. Value type semantics
> 3. RAII (combined with (1) often)

This is common in D as well. The difference to C++ is 0-argument 
struct constructors to do extra work to satisfy invariants.

> Is it the case that a struct should now be used with a factory 
> method? Does this also mean that the struct destructor must be

It's the easiest way to emulate C++'s 0-argument struct 
constructors.

> made to work when .init is called instead of the factory method?

If the factory method isn't called, then yes, the destructor 
shouldn't blow up just because all the struct members are T.init.

> This idiom is inconsistent with struct constructors that do 
> have one or more arguments, and I think that this question is 
> likely to arise time immemorial from others who are not 
> expecting this particular inconstancy.


How is it inconsistent? Nobody stops me from doing this:

struct Struct {
     void* ptr = cast(void*)5;
     this(int size) {
         ptr = malloc(size);
     }

     ~this() {
         free(ptr);
     }

}

void main() {
     auto ok = Struct(10);
     //auto oops = Struct.init;
}

Atila