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digitalmars.D.learn - Replacement for C++ Style Implicit casts?

reply Mike Chaten <mchaten gmail.com> writes:
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
Oct 18 2010
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
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
Oct 18 2010
parent reply Mike Chaten <mchaten gmail.com> writes:
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
Oct 18 2010
parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
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
Oct 18 2010
parent Mike Chaten <mchaten gmail.com> writes:
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
Oct 19 2010
prev sibling next sibling parent "Denis Koroskin" <2korden gmail.com> writes:
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.
Oct 18 2010
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
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
Oct 18 2010