Default struct constructors if a struct member is a union

[Basile B.]

On Sunday, 30 June 2024 at 11:23:45 UTC, Nick Treleaven wrote:
> On Saturday, 29 June 2024 at 23:33:41 UTC, solidstate1991 wrote:
>> S foo0 = S(TypeEnum.Integer32, S(20));  //Ugly, but works
>> S foo1 = S(TypeEnum.Integer64, S(20L)); //Error: cannot
>
> Did you mean `U(20)`? The 20 applies to the first field of the 
> union, i32. `U(20L)` also works and (I think) it does the same 
> because of VRP.
>
>> My question is can I initialize structs like these in one line 
>> without relying on a second line? My usecase scenario doesn't 
>> really allow constructors for the struct, since it's a binding 
>> to an external library via C API.
>
> You can use a field initializer:
> ```d
> enum TypeEnum { Integer32, Integer64 }
>
> union U {
>     int i32;
>     long i64;
>     float f32;
>     double f64;
> }
>
> struct S {
>     TypeEnum type;
>     U data;
> }
>
> S foo0 = S(TypeEnum.Integer32, U(20)); // init i32
> S foo1 = S(TypeEnum.Integer64, U(f32: 0.5F));
>
> // struct initializer syntax
> S foo2 = {TypeEnum.Integer32, {20}};
> S foo3 = {TypeEnum.Integer64, {f32: 0.5F}}; // init f32
>
> void main()
> {
>     assert(foo0.data.i32 == 20);
>     assert(foo3.data.f32 == 0.5F);
> }
> ```

That's another solution, that being said, the real problem OP 
encountered is the classic one of "implict construction", which 
is not a D thing.