Factory pattern in D

[biozic via Digitalmars-d-learn]

On Friday, 1 May 2015 at 11:20:32 UTC, Chris wrote:
> On Friday, 1 May 2015 at 11:11:28 UTC, biozic wrote:
>> On Friday, 1 May 2015 at 11:01:29 UTC, Chris wrote:
>
>>>
>>> Thinking about it,
>>>
>>> T factory(T)() {
>>> return T();
>>> }
>>>
>>> is better suited for a factory (with static type checks).
>>
>> But then I don't know what factory!X() provides that X() alone 
>> doesn't.
>
> Just cleaner code with type checks
>
> T factory(T)() {
>   static if (is (T == A)
>               || (is (T == B)))
>     return T();
>   else
>     assert(0, "Type "~T.stringof~" is not supported");
> }
>
> and then you could have
>
> auto getType(string type = "")() {
>   static if (type == "A")
>     return factory!A();
>   else static if (type == "B")
>     return factroy!B();
>   else
>     return factory!A();  // default
> }
>
> in order to separate the logic, i.e. the factory produces the 
> type and performs all the type checks, whereas `getType` is the 
> interface for the user.

A "factory" that produces a *type* could be:
--
template checked(T) {
     static if (is (T == A) || (is (T == B)))
       alias checked = T;
     else
       static assert(0, "Type "~T.stringof~" is not supported");
}
--