Inheriting constructors

[Bruno Medeiros]

Sean Kelly wrote:
> Now that the const discussion has died down a bit, I don't suppose 
> anyone else is interested in this feature?  (As an aside, I'm coming to 
> feel that we may need real placement new support after all, along with 
> something like my .isizeof proposal--it's not always feasible to change 
> the type using a wrapper as described below).
> 
> 
> Sean
> 
> Sean Kelly wrote:
>> Until recently I have been against the idea of inheriting 
>> constructors, but I've changed my mind.  Inheriting constructors would 
>> allow for an entire class of generic programming that simply isn't 
>> possible right now.  The basic rule would be simple and much like the 
>> function lookup rules already in place: if a class contains no 
>> constructors then it inherits the constructors of the nearest parent 
>> where some are defined.  Defining even a single constructor, however, 
>> effectively occludes superclass constructors and disables this 
>> behavior (the workaround being an alias, like with function lookup, 
>> except that it's currently impossible to explicitly reference a class 
>> constructor).
>>
>> The original motivation for this idea was a need for some way to 
>> attach "on destruct" behavior to an arbitrary class:
>>
>>     class PrintOnDestruct( T ) :
>>         public T
>>     {
>>         ~this()
>>         {
>>             printf( "dtor: %.*s\n", super.classinfo.name );
>>         }
>>     }
>>
>>     class MyClass
>>     {
>>         this( int a, int b, int c ) {}
>>     }
>>
>>     auto c = new PrintOnDestruct!(MyClass)( a, b, c );
>>
>> As far as I know, the only way this is currently possible is to 
>> explicitly write all relevant constructors in PrintOnDestruct.
>>
>> I believe this feature may also obviate the need for language support 
>> of placement new:
>>
>>     class Placed( T ) :
>>         public T
>>     {
>>         new( void* p, size_t sz )
>>         {
>>             return p;
>>         }
>>     }
>>
>>     class MyClass
>>     {
>>         this( int x ) { val = x; }
>>         private int val;
>>     }
>>
>>     auto p = new(loc) Placed!(MyClass)( 0 );
>>
>> I can think of other examples as well, but these are the two most 
>> relevant to what prompted this post in the first place.
>>
>>
>> On a semi-related note, it would also be nice if there were some way 
>> to obtain the ParameterTypeTuple for class constructors in general.  
>> Since there is currently no way to explicitly reference constructors, 
>> this sort of thing isn't possible:
>>
>>     class MyClass
>>     {
>>         this( int x ) {}
>>     }
>>
>>     alias ParametersOf!(MyClass.this) CtorParams;
>>
>> The most obvious use case here would be object factories.  Let's say I 
>> want to be able to generate objects with the same set of parameters 
>> each time:
>>
>>     struct TupleInst( Types... )
>>     {
>>         Tuple!(Types) data;
>>     }
>>
>>     class Factory!( T )
>>     {
>>         this( ParametersOf!(T.this) args )
>>         {
>>             foreach( i, arg; args )
>>             {
>>                 store.data[i] = arg;
>>             }
>>         }
>>
>>         T generate()
>>         {
>>             return new T( store.data );
>>         }
>>
>>         private TupleInst!(ParametersOf!(T.this)) store;
>>     }
>>
>>     class MyClass
>>     {
>>         this( int x ) {}
>>     }
>>
>>     auto fact = new Factory!(MyClass)( 0 );
>>     auto objA = fact.generate();
>>     auto objB = fact.generate();
>>
>> So something roughly like parameter binding but for constructors.  
>> This latter idea likely has less utility than constructor inheritance, 
>> but I've run into one or two places where it would have been very 
>> useful, and I had to hack a solution.  Both were essentially similar 
>> to the example above, but with a different goal in mind.
>>
>>
>> Sean

Since you asked, I agree (those semantics are more useful, and more 
consistent), but what does it matter...

-- 
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D