Default Implementation For an Interface

[simendsjo]

On 02/16/2012 04:01 AM, Kevin wrote:
> I was implementing a framework and I found that I wanted two things.
> - A strong set of interfaces so that I can get what I want from a
> variety of sources.
> - Some basic implementations of these interfaces.
>
> For example, say I was writing a database class. I could either name the
> interface Database and call the class DatabaseImplementation or
> something but that is ugly. If I call the interface IDatabase, the
> Database class looks nice but I need to convince users to write
> functions that take IDatabases not Databases.
>
> I was wondering if there was any way to implement a default
> implementation. This way, I could create my Database interface and
> classes could implement that but if you called `new Database()` you
> would still get a basic database.
>
> I thought about doing it in different modules but it just gets messier
> as you have to fully qualify all of the names so both look ugly. I
> though about overloading the new operator for the interface but it would
> have to be final and that would mess everything up. I though about a
> meathod like `Database.newDefault()` but that is messy and has no
> meaning in a derived class.
>
> I couldn't find anything about this so I was wondering what you would
> recommend. Should I just pick a naming scheme?

As a user (read developer), I'd rather code to the generic interface 
when possible. I like that concrete implementations looks rather long 
and ugly :)
I don't think you should be worried that your users is using direct 
implementations rather than the interface - their problem!

Remember that in D, interfaces can contain implementations that only 
uses static methods on the interface:

interface DB {
     @property string name();
     // interfaces can have implementations
     static DB createDefault() { return new GenericDB(); }
}

class GenericDB : DB {
     @property string name() {
         return "generic"; }
}

class MySQLDB : DB {
     @property string name() {
         return "mysql"; }
}

void main() {
     assert(DB.createDefault().name == "generic");
     assert((new MySQLDB()).name == "mysql");
}