auto return on override

[dlangBugzillaToGithub]

Lodovico Giaretta reported this on 2016-08-16T09:19:02Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=16395

CC List

• Seb
• Lodovico Giaretta

Description

Currently DMD does not allow returning auto when overriding a function.

Consider this case:

===========================================
interface Foo { /* some methods */ }
interface Bar { Foo getFoo(); }

class BarImpl: Bar
{
    override auto getFoo()
    {
        class FooImpl: Foo
        {
             // implementation of Foo ...
             // ... plus range methods
        }
    }
}
===========================================

Currently I have two options:
1) remove auto and return Foo instead; this way the user cannot use the range interface of FooImpl, because it receives a Foo
2) put FooImpl outside the function and return FooImpl; this way all functionality is correct, but it's not optimal as I have to put FooImpl outside the only function that uses it (which looks weird), plus FooImpl cannot implicitly access the getFoo context.

This is not a essential enhancement, I don't know if it is feasible to implement it, but it would make the language features more orthogonal, allowing the use of overriding with return type inference and voldemort types.

[dlangBugzillaToGithub]

greeenify commented on 2016-08-16T09:48:23Z

> Currently I have two options:

3) use structs (they are usually used for ranges anyways). With alias this and mixins you can get a pretty decent inheritance, but of course I don't know your code ;-)

[dlangBugzillaToGithub]

lodovico commented on 2016-08-16T10:05:50Z

(In reply to greenify from comment #1)
> > Currently I have two options:
> 
> 3) use structs (they are usually used for ranges anyways). With alias this
> and mixins you can get a pretty decent inheritance, but of course I don't
> know your code ;-)

True in general, but not really an option for my code (I need a certain amount of runtime polymorphism).

Regardless of my example, which I solved with option 2, the question of this issue is "is this feasible to implement? If it is, why not have it? If it's not, fair enough."

[dlangBugzillaToGithub]

greeenify commented on 2016-08-16T10:15:37Z

> True in general, but not really an option for my code (I need a certain amount of runtime polymorphism).

I thought so :/

> Regardless of my example, which I solved with option 2, the question of this issue is "is this feasible to implement? If it is, why not have it? If it's not, fair enough."

I do agree that it's a bug / missing feature. 
If you want to get" dirty" yourself, I can highly recommend Walter's video from Dconf16