DIP 45 - approval discussion

[Timon Gehr]

On 11/12/2013 11:45 PM, Andrei Alexandrescu wrote:
>> - Every class in any imported module will need to show up in the
>> generated code even if it is never used unless global static analysis is
>> carried out on arguments to Object.factory.
>
> Correct. On the other hand, a lot of unused classes

It is not necessary that there are a lot. Anything transitively 
reachable from there will be pulled in.

> in used modules
> seems to indicate a problem with the system's design.
>

Those could be classes that are only used at compile time, like a 
builder for a grammar for a CTFE parser generator, for lack of a better 
example. Furthermore, library modules may be larger than their actively 
used interface. (Phobos?) It would be interesting to see how much 
Object.factory influences the size of hello world, even though Phobos 
does not use too many classes

>> - If I know the fully qualified name of a class, there are better ways
>> to instantiate this class than passing a string containing that name to
>> Object.factory in order to call the default constructor.
>
> What are the better ways?

Call the default constructor. Call a delegate/virtual function that 
calls the default constructor.

> Note that most of the time you don't "know"
> the name of a class - you get it down the wire during some
> deserialization.

You don't want to call the default constructor during deserialization 
and you need actual information about the Object layout, so seems to be 
an instance of the next point.

> So there must be some way to build an object from a
> token representation of the object.
>
>> - The functionality provided by Object.factory is trivially replaced by
>> a solution more specifically tailored to the problem at hand using
>> compile-time reflection.
>
> It's not quite trivial - somewhere there has to be a map and
> registration and lookup and whatnot. I don't see it why it's unbecoming
> for such functionality to be part of the standard library. I would
> agree, however, that it's a judgment call whether it should be wired in
> or provided on demand.

I think that if supported at all, it should be an UDA declared somewhere 
in druntime.