struct: default construction or lazy initialization.

[kinke via Digitalmars-d-learn]

On Wednesday, 1 February 2017 at 23:32:12 UTC, bitwise wrote:
> On Wednesday, 1 February 2017 at 23:24:27 UTC, kinke wrote:
>> It's not that bad. D just doesn't support a default ctor for 
>> structs at all and simply initializes each instance with 
>> T.init. Your `s2` initialization is most likely seen as 
>> explicit default initialization (again with T.init). 
>> Destructing both instances is exactly what should happen.
>
>
> I was going to add a point about this.
>
> 1| S s1;
> 2| S s2 = S();
>
> The effect of line 1 and 2 are exactly the same - which is that 
> the lhs ends up with S.init. S.this() should either be called 
> at line 2, or the syntax of line 2 should be forbidden on the 
> grounds that default struct ctors cannot be declared.

Yep, they are the same. Initialization with S.init is a trivial 
blit from a compile-time constant, so there's no call to a 
default ctor. This in turn makes construction of static arrays 
and structs containing a field of type S trivial, as no default 
ctor has to be generated if a field has one, there are no 
potential exceptions etc.

Ideally, S.this() would be called in both cases (just like C++). 
The explicit syntax in line 2 is useful for stuff like `const s = 
S();`.