Re: Magic type return
On Tuesday, 17 July 2012 at 13:56:29 UTC, Andrea Fontana wrote:
class Known
{
void* data; // external data by c api
int type; // 0 for int, 1 for string, etc. ..
}
How can I implement a method like this?
Known known; // <-- suppose known.type == 1;
string s = known.value(); // <-- automatic
I just know how to do this:
string s = know.value!string();
You can't. You could do
string s;
known.value(s);
where
void value(T)(ref T t);
Re: Magic type return
Andrea Fontana:
class Known
{
void* data; // external data by c api
int type; // 0 for int, 1 for string, etc. ..
}
How can I implement a method like this?
Known known; // <-- suppose known.type == 1;
string s = known.value(); // <-- automatic
To do this Known.value() needs to return different types
according to the run-time value of Known.type. This is not
possible in a statically typed language... You need to find other
solutions.
Bye,
bearophile
Re: Magic type return
Better than nothing :)
Hope in better template deduction...
Il giorno mar, 17/07/2012 alle 16.22 +0200, Tobias Pankrath ha scritto:
> On Tuesday, 17 July 2012 at 13:56:29 UTC, Andrea Fontana wrote:
> > class Known
> > {
> > void* data; // external data by c api
> > int type; // 0 for int, 1 for string, etc. ..
> > }
> >
> > How can I implement a method like this?
> >
> > Known known; // <-- suppose known.type == 1;
> > string s = known.value(); // <-- automatic
> >
> > I just know how to do this:
> >
> > string s = know.value!string();
>
> You can't. You could do
>
> string s;
> known.value(s);
>
> where
>
> void value(T)(ref T t);
Re: Magic type return
i'm not completely sure i understand your problem but i think you are looking for something like this: http://pocoproject.org/docs/Poco.DynamicAny.html maybe the c++ source code could be of some inspiration. this should be possible in d, too.
Re: Magic type return
> class Known
> {
> void* data; // external data by c api
> int type; // 0 for int, 1 for string, etc. ..
> }
>
> How can I implement a method like this?
>
> Known known; // <-- suppose known.type == 1;
> string s = known.value(); // <-- automatic
>
> I just know how to do this:
>
> string s = know.value!string();
As bearophile said, you cannot change a value's type (which is a
compile-time construct) with a runtime value, as is Known.type.
Second point, in D, the rhs is fully evaluated before being assigned to the
lhs, I think. So, known.value() must evaluate to *something*, without
knowing it will be assigned to a string.
In your example, what happens if known.type != 1?
You can use Phobos Variant, (or Algebraic if the range of types you plan to
use is known beforehand). Then, you should test typeid before using it.
Re: Magic type return
Yes I did it using Variant and it works fine
Il giorno mer, 18/07/2012 alle 16.42 +0200, Philippe Sigaud ha scritto:
> > class Known
> > {
> > void* data; // external data by c api
> > int type; // 0 for int, 1 for string, etc. ..
> > }
> >
> > How can I implement a method like this?
> >
> > Known known; // <-- suppose known.type == 1;
> > string s = known.value(); // <-- automatic
> >
> > I just know how to do this:
> >
> > string s = know.value!string();
>
> As bearophile said, you cannot change a value's type (which is a
> compile-time construct) with a runtime value, as is Known.type.
>
> Second point, in D, the rhs is fully evaluated before being assigned
> to the lhs, I think. So, known.value() must evaluate to *something*,
> without knowing it will be assigned to a string.
> In your example, what happens if known.type != 1?
>
> You can use Phobos Variant, (or Algebraic if the range of types you
> plan to use is known beforehand). Then, you should test typeid before
> using it.
>
Re: Magic type return
On Tue, 17 Jul 2012 15:23:05 +0100, bearophile
wrote:
Andrea Fontana:
class Known
{
void* data; // external data by c api
int type; // 0 for int, 1 for string, etc. ..
}
How can I implement a method like this?
Known known; // <-- suppose known.type == 1;
string s = known.value(); // <-- automatic
To do this Known.value() needs to return different types according to
the run-time value of Known.type. This is not possible in a statically
typed language... You need to find other solutions.
Unless we had overload based on return type, right?
e.g.
class Known
{
string value()
{
if (type != 1)
throw..;
return cast(string)data;
}
int value()
{
if (type != 0)
throw ..;
return cast(int)data;
}
}
The compiler could produce the correct code/call for the line
string s = known.value();
then, but it's not a feature we're likely to see any time soon.
R
--
Using Opera's revolutionary email client: http://www.opera.com/mail/
Re: Magic type return
Or template inference based on return type like
T hello(T)()
{
static if (is(T ==))
}
string v = hello();
Il giorno mer, 18/07/2012 alle 17.38 +0100, Regan Heath ha scritto:
> On Tue, 17 Jul 2012 15:23:05 +0100, bearophile
> wrote:
>
> > Andrea Fontana:
> >
> >> class Known
> >> {
> >> void* data; // external data by c api
> >> int type; // 0 for int, 1 for string, etc. ..
> >> }
> >>
> >> How can I implement a method like this?
> >>
> >> Known known; // <-- suppose known.type == 1;
> >> string s = known.value(); // <-- automatic
> >
> > To do this Known.value() needs to return different types according to
> > the run-time value of Known.type. This is not possible in a statically
> > typed language... You need to find other solutions.
>
> Unless we had overload based on return type, right?
>
> e.g.
>
> class Known
> {
>string value()
>{
> if (type != 1)
>throw..;
> return cast(string)data;
>}
>
>int value()
>{
> if (type != 0)
>throw ..;
> return cast(int)data;
>}
> }
>
> The compiler could produce the correct code/call for the line
>
> string s = known.value();
>
> then, but it's not a feature we're likely to see any time soon.
>
> R
>
