Search
Implicit conversion by return
Aug 08
Alex
```Given this:

struct Num
{
this(int a) {}
}

Is there any reason why this works:

Num n = 5;

but this doesnt:

Num funk()
{
return 5;
}

I understand that I can construct it explicitely, but that gets annoying quickly, especially with templates.
```
```On Wednesday, 8 August 2018 at 08:15:16 UTC, Hakan Aras wrote:
> Given this:
>
> struct Num
> {
>     this(int a) {}
> }
>
> Is there any reason why this works:
>
> Num n = 5;
>
> but this doesnt:
>
> Num funk()
> {
>     return 5;
> }
>
>
> I understand that I can construct it explicitely, but that gets annoying quickly, especially with templates.

I suppose, this is too slack.

What would work is

Num funk()
{
return typeof(return)(5);
}
```
```On Wednesday, 8 August 2018 at 08:44:03 UTC, Alex wrote:

>     return typeof(return)(5);

Ah thanks, I was wondering if something like that exists. Still though, that's 16 extra characters that dont need to be there.

```
```On Wednesday, August 8, 2018 2:15:16 AM MDT Hakan Aras via Digitalmars-d- learn wrote:
> Given this:
>
> struct Num
> {
>      this(int a) {}
> }
>
> Is there any reason why this works:
>
> Num n = 5;
>
> but this doesnt:
>
> Num funk()
> {
>      return 5;
> }
>
>
> I understand that I can construct it explicitely, but that gets annoying quickly, especially with templates.

Num n = 5;

doesn't actually do an implict conversion. It's the same as doing

Num n = Num(5);

So, I guess that you could call it implicit construction, but regardless, it's just a different syntax for calling the constructor. The only way to create an implicit conversion with a user-defined type in D is to use alias this, and that only provides a way to implicitly convert _from_ a user-defined type, not to a type. So, having

Num funk()
{
return 5;
}

work is impossible in D, just like having something like

auto foo(Num n)
{
...
}

foo(5);

work is impossible. If you want to return an int and have it converted to a Num, then you're going to need to explicitly construct a Num from the int.

- Jonathan M Davis

```