opEquals the template?

[kenji hara]

2011/10/1 Steven Schveighoffer <schveiguy at yahoo.com>:
> There is a debate in a sub-thread about logical const on whether
> Object.opEquals should be const or mutable.
>
> Right now, it's mutable.  This disallows comparison between const objects
> (except it is technically allowed because the compiler ignores const
> correctness for this).
>
> I think it should be const.
>
> However, making it const disallows opEquals functions which mutate the
> object (for lazy loading or caching purposes).  I question that this is a
> major concern, but I have discovered an easy fix that appeases all.
>
> If we make opEquals a template (not Object.opEquals, but the free function
> opEquals(Object lhs, Object rhs) ), then the template has access to the full
> type information of the derived object.  This means that Object.opEquals can
> be exclusively const, and you can overload this with a mutable version, and
> everything just works.
>
> I tried it out by making my copy of druntime use a template (it's actually
> very few lines change).  Although I needed to use casts to compile some
> parts of phobos (some opEquals calls depend on the compiler ignoring const
> guarantees as mentioned above), when compiling a simple test program, I can
> prove it works without compiler changes.
>
> Along with the ability to have both const and non-const (and even
> technically immutable) opEquals, we have the following drawbacks and
> benefits:
>
> 1. template bloat.  Every combination of two object types being compared
> will generate a new template.
> 2. inlining of opEquals.  Both the free function template, and possibly a
> final-declared opEquals on a dervied object could be inlined.
> 3. possible specialization of opEquals(MyDerivedType obj).  I don't remember
> exactly the overloading rules, so this may be impossible.
> 4. Fixes the issue with interface comparison (see bug
> http://d.puremagic.com/issues/show_bug.cgi?id=4088), not in the best way,
> but it would be better than the current situation.
>
> What do people think?
>
> -Steve
>

I think your suggestion is almost same as mine.

Discussions: https://github.com/D-Programming-Language/phobos/pull/262
Implementations: https://github.com/D-Programming-Language/druntime/pull/72

My patch does not support #4 (interface comparison), but others are same.

Kenji Hara