http://d.puremagic.com/issues/show_bug.cgi?id=5719

           Summary: [patch] std.conv.to should support structs with custom
                    converters in addition to objects
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: patch
          Severity: normal
          Priority: P3
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: sandford@jhu.edu


--- Comment #0 from Rob Jacques <sandford@jhu.edu> 2011-03-07 21:50:57 PST ---
Currently std.conv.to allows classes to define custom conversion routines to
non-class types, but doesn't provide structs with the same functionality. This
functionality can be extended to structs by a minor change in one of the toImpl
template constraints from
is(S : Object) && !is(T : Object)
to
((is(S : Object) && !is(T : Object)) || is(S == struct))

The particular toImpl is currently located at Line 687 in git (line 680 in DMD 2.051) and has been reproduced in full below with the patch, along with an updated unit test. The patch passes the unit tests below and the unit tests in my current fork of std.variant and std.json, but I haven't tested it against the entire phobos test suite.

/**
Object-_to-non-object conversions look for a method "to" of the source
object.

Example:
----
class Date
{
    T to(T)() if(is(T == long))
    {
        return timestamp;
    }
    ...
}

unittest
{
    debug(conv) scope(success) writeln("unittest @", __FILE__, ":", __LINE__, "
succeeded.");
    auto d = new Date;
    auto ts = to!long(d); // same as d.to!long()
}
----
 */
T toImpl(T, S)(S value) if ( ((is(S : Object) && !is(T : Object)) || is(S ==
struct)) && !isSomeString!T
        && is(typeof(S.init.to!(T)()) : T))
{
    return value.to!T();
}

unittest
{
    debug(conv) scope(success) writeln("unittest @", __FILE__, ":", __LINE__, "
succeeded.");
    class B { T to(T)() { return 43; } }
    auto b = new B;
    assert(to!int(b) == 43);
    struct C { T to(T)() { return 43; } }
    C c;
    assert(to!int(c) == 43);
}

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--- Comment #1 from Rob Jacques <sandford@jhu.edu> 2011-07-01 13:04:20 PDT ---
I've noticed that with generic code that

Target toImpl(Target, Source)(Source value)
if (implicitlyConverts!(Source, Target))

can cause an error via multiple template matches with

T toImpl(T, S)(S value) if (is(S : Object) && is(T : Object))

and

T toImpl(T, S)(S value)
    if (((is(S : Object) && !is(T : Object)) || is(S == struct))
    && !isSomeString!T && is(typeof(S.init.to!(T)()) : T))

Both issues can be simply fixed by adding an additional template constraint:
    && !implicitlyConverts!(S,T)

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Jonathan M Davis <jmdavisProg@gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jmdavisProg@gmx.com
           Severity|normal                      |enhancement


--- Comment #2 from Jonathan M Davis <jmdavisProg@gmx.com> 2011-07-01 13:12:48 PDT ---
There are several pull requests affect std.conv.to at the moment which will change the situation and which will likely be merged in soon. Among other things, they get rid of the member function to conversion and make std.conv.to work with overloaded opCast.

https://github.com/D-Programming-Language/phobos/pull/118 https://github.com/D-Programming-Language/phobos/pull/119 https://github.com/D-Programming-Language/phobos/pull/122

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--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-09 03:42:53 PDT ---
(In reply to comment #2)
> https://github.com/D-Programming-Language/phobos/pull/118 https://github.com/D-Programming-Language/phobos/pull/119 https://github.com/D-Programming-Language/phobos/pull/122

The three pull requests are already merged, and now std.conv.to supports both two kinds of custom conversions:

1) object to non-object conversion with opCast
   to!T(s)
   --> s.opCast!T()

2) non-object to object conversion with construction
   to!T(s)
   --> new T(s) // if T is class
       T(s)     // if T is struct

Today, is this a closable issue?

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--- Comment #4 from Rob Jacques <sandford@jhu.edu> 2011-10-09 10:06:45 PDT ---
(In reply to comment #3)
> (In reply to comment #2)
> > https://github.com/D-Programming-Language/phobos/pull/118 https://github.com/D-Programming-Language/phobos/pull/119 https://github.com/D-Programming-Language/phobos/pull/122
> 
> The three pull requests are already merged, and now std.conv.to supports both two kinds of custom conversions:
> 
> 1) object to non-object conversion with opCast
>    to!T(s)
>    --> s.opCast!T()
> 
> 2) non-object to object conversion with construction
>    to!T(s)
>    --> new T(s) // if T is class
>        T(s)     // if T is struct
> 
> Today, is this a closable issue?

By 'object' do you mean both classes and structs? Specifically, these conversions seem to be missing:

3) non-object to non-object conversion with opCast
    to!T(s)
    --> s.opCast!T()

4) non-object to non-object conversion with construction
   to!T(s)
       T(s)     // if T is struct

But this might be a case of semantics, not implementation. (I generally don't refer to structs as 'objects')

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--- Comment #5 from Jonathan M Davis <jmdavisProg@gmx.com> 2011-10-09 14:25:50 PDT ---
Structs are very much objects. Whether something is an object or not has nothing to do with whether it's a reference type or a value type. Take C++ for example. In C++, classes are generally value types rather than reference types, but they're very much objects. The same goes for whether an object is polymorphic or not. It's an object regardless of whether it's polymorphic. The concept of an object in OO programming has nothing to do with the difference between a struct and a class in D.

You're going to confuse people and generally be confused by others if you refer to instances of structs as if they weren't objects. If you want to be clear about the differences between structs and classes, you need to refer to structs and classes and not try and use the term object to differentiate them. Instances of both are objects.

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Brad Roberts <braddr@puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |braddr@puremagic.com


--- Comment #6 from Brad Roberts <braddr@puremagic.com> 2011-10-09 16:34:26 PDT ---
Careful, given that the name of the entity at the root of the hierarchy of classes is called 'Object', which structs have no relationship to, there is an ambiguity in terminology.

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--- Comment #7 from Jonathan M Davis <jmdavisProg@gmx.com> 2011-10-09 16:48:59 PDT ---
True, there's a difference between something which is an instance of Object and something which is an object. But D structs are still very much OO and are definitely objects.

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--- Comment #8 from meh. <meh@paranoici.org> 2012-05-19 04:14:25 PDT ---
Created an attachment (id=1106)
Reduced test case for weird cast behavior.

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meh. <meh@paranoici.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |meh@paranoici.org


--- Comment #9 from meh. <meh@paranoici.org> 2012-05-19 04:17:51 PDT ---
Sorry for that, bugzilla did something funny, I don't know why it attached that to this bug.

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