I tested the code from: http://dlang.org/struct.html
Section: Dynamic Initialization of Structs


struct S
{
    int a;

    static S opCall(int v)
    {
        S s;
        s.a = v;
        return s;
    }

    static S opCall(S v)
    {
        S s;
        s.a = v.a + 1;
        return s;
    }
}

S s = 3; // sets s.a to 3
S t = s; // sets t.a to 3, S.opCall(s) is not called

which does not compile (Error: cannot implicitly convert expression (3) of type int to S).

What is wrong here? Docs or behavior? Tested it on asm.dlang.org where it fails with all compiler versions.

On 07/19/2015 03:26 AM, TC wrote:
> I tested the code from: http://dlang.org/struct.html
> Section: Dynamic Initialization of Structs
>
>
> struct S
> {
>      int a;
>
>      static S opCall(int v)
>      {
>          S s;
>          s.a = v;
>          return s;
>      }
>
>      static S opCall(S v)
>      {
>          S s;
>          s.a = v.a + 1;
>          return s;
>      }
> }
>
> S s = 3; // sets s.a to 3
> S t = s; // sets t.a to 3, S.opCall(s) is not called
>
> which does not compile (Error: cannot implicitly convert expression (3)
> of type int to S).
>
> What is wrong here? Docs or behavior? Tested it on asm.dlang.org where
> it fails with all compiler versions.

The docs are not clear: s and t are not meant to be module-scope variables, which should be initialized in a 'static this()' (or 'shared static this()'). Put them inside a function like main() and it works fine.

Ali

On Sunday, 19 July 2015 at 13:25:41 UTC, Ali Çehreli wrote:
> On 07/19/2015 03:26 AM, TC wrote:
>> I tested the code from: http://dlang.org/struct.html
>> Section: Dynamic Initialization of Structs
>>
>>
>> struct S
>> {
>>      int a;
>>
>>      static S opCall(int v)
>>      {
>>          S s;
>>          s.a = v;
>>          return s;
>>      }
>>
>>      static S opCall(S v)
>>      {
>>          S s;
>>          s.a = v.a + 1;
>>          return s;
>>      }
>> }
>>
>> S s = 3; // sets s.a to 3
>> S t = s; // sets t.a to 3, S.opCall(s) is not called
>>
>> which does not compile (Error: cannot implicitly convert expression (3)
>> of type int to S).
>>
>> What is wrong here? Docs or behavior? Tested it on asm.dlang.org where
>> it fails with all compiler versions.
>
> The docs are not clear: s and t are not meant to be module-scope variables, which should be initialized in a 'static this()' (or 'shared static this()'). Put them inside a function like main() and it works fine.

I think that's not what the OP means. The documentation is indeed saying that a struct with static opCall() can be used to _construct_ the structure, with exactly this syntax, i.e. not with `S a = S(3);`, but with `S a = 3;`. I, too, find this very surprising. opCall() is not a constructor...

On Sunday, 19 July 2015 at 10:26:04 UTC, TC wrote:
> What is wrong here? Docs or behavior? Tested it on asm.dlang.org where it fails with all compiler versions.

Docs are wrong here. `static opCall` was never intended to be used as constructor and I consider it a bad style in general. This is simply a workaround that people unhappy with lack of default struct constructor used but language actively resists such misusage (`auto s = new S(42)` won't work too btw) and it only causes confusion.