In C++ it is possible to declare a class as follows
class Foo {
Foo(int x) { }
}
You can then use that constructor to implicitly convert int to Foo. E.g
Foo x = 0; //equivalent to Foo(0)

Is there a way in D to do an implicit or explicit conversion from an integral type to a class?

-Mike

On Mon, 18 Oct 2010 15:47:40 -0400, Mike Chaten <mchaten@gmail.com> wrote:

> In C++ it is possible to declare a class as follows
> class Foo {
> Foo(int x) { }
> }
> You can then use that constructor to implicitly convert int to Foo. E.g
> Foo x = 0; //equivalent to Foo(0)
>
> Is there a way in D to do an implicit or explicit conversion from an
> integral type to a class?

explicit cast == opCast
implicit cast == alias this

Look up those two features in the docs.

-Steve

I was under the impression that alias this just was shorthand for
Class Foo {
int x;
alias x this;
}
Foo foo =  new Foo
foo = 9; // foo.x = 9
Foo Foo = 9 // null.x =9;

Also, for opCast, doesnt that only work for going from Foo to int and not the other way around?

-Mike
On Oct 18, 2010 3:55 PM, "Steven Schveighoffer" <schveiguy@yahoo.com> wrote:
> On Mon, 18 Oct 2010 15:47:40 -0400, Mike Chaten <mchaten@gmail.com> wrote:
>
>> In C++ it is possible to declare a class as follows
>> class Foo {
>> Foo(int x) { }
>> }
>> You can then use that constructor to implicitly convert int to Foo. E.g
>> Foo x = 0; //equivalent to Foo(0)
>>
>> Is there a way in D to do an implicit or explicit conversion from an integral type to a class?
>
> explicit cast == opCast
> implicit cast == alias this
>
> Look up those two features in the docs.
>
> -Steve

On Mon, 18 Oct 2010 23:47:40 +0400, Mike Chaten <mchaten@gmail.com> wrote:

> In C++ it is possible to declare a class as follows
> class Foo {
> Foo(int x) { }
> }
> You can then use that constructor to implicitly convert int to Foo. E.g
> Foo x = 0; //equivalent to Foo(0)
>
> Is there a way in D to do an implicit or explicit conversion from an
> integral type to a class?
>
> -Mike

For structs the following works:

struct Foo
{
	this(int x) { ... }
}

Foo x = 0;

Classes are allocated on heap and as such there is no way to achieve the same.

Mike Chaten:

> In C++ it is possible to declare a class as follows
> class Foo {
> Foo(int x) { }
> }
> You can then use that constructor to implicitly convert int to Foo. E.g
> Foo x = 0; //equivalent to Foo(0)
> 
> Is there a way in D to do an implicit or explicit conversion from an integral type to a class?

Do you mean something like this?


class Foo {
    int x;
    static Foo opCall(int x_) {
        auto f = new Foo;
        f.x = x_;
        return f;
    }
}

void main() {
    Foo f = Foo(5);
    assert(f.x == 5);
}

(With structs it's simpler)

Bye,
bearophile

Mike Chaten <mchaten@gmail.com> wrote:

> I was under the impression that alias this just was shorthand for
> Class Foo {
> int x;
> alias x this;
> }
> Foo foo =  new Foo
> foo = 9; // foo.x = 9
> Foo Foo = 9 // null.x =9;

Not just. this would also work:

int n = foo;
// void bar( int n ) {}
bar( foo );


> Also, for opCast, doesnt that only work for going from Foo to int and not
> the other way around?

Indeed. For implicit casts to Foo, I don't know what, if anything, works.

void baz( Foo f ) {}
baz( 3 ); // How?

Likely, there is no such functionality in D, at least for the moment.

-- 
Simen

Thanks for all your help!

-Mike

On Mon, Oct 18, 2010 at 6:21 PM, Simen kjaeraas <simen.kjaras@gmail.com>wrote:

> Mike Chaten <mchaten@gmail.com> wrote:
>
>  I was under the impression that alias this just was shorthand for
>> Class Foo {
>> int x;
>> alias x this;
>> }
>> Foo foo =  new Foo
>> foo = 9; // foo.x = 9
>> Foo Foo = 9 // null.x =9;
>>
>
> Not just. this would also work:
>
> int n = foo;
> // void bar( int n ) {}
> bar( foo );
>
>
>
>  Also, for opCast, doesnt that only work for going from Foo to int and not
>> the other way around?
>>
>
> Indeed. For implicit casts to Foo, I don't know what, if anything, works.
>
> void baz( Foo f ) {}
> baz( 3 ); // How?
>
> Likely, there is no such functionality in D, at least for the moment.
>
> --
> Simen
>