Re: [boost] [fusion] unpack into constructor?

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-06-14 16:20:54

Eric Niebler wrote:
> Hartmut Kaiser wrote:
>>  
>> dan marsden wrote:
>>> I don't think we've got anything that supports this directly 
>>> at the moment.

As long as we're talking about Fusion/functional - no we haven't.

>>> Is the sort of thing you are looking for?
>>>
>>> fused_ctor<T> fc;
>>> T t = fc(sequence);
>>>
>>> Where fused_ctor::operator()(Seq const&) breaks up the 
>>> sequence into args for the T ctor.
> 
> 
> Yes, that's the thing.
> 

OK, but why duplicate the unpacking of the Sequence?

All it takes is yet another adapter encapsulating a type's ctor(s) in a 
function object to just use fusion::(invoke|fused)_function_object. E.g:

    // untested code for illustration only

    template<typename T>
    struct ctor
    {
        typedef T result_type;

        // [...] tricky nullary case with default ctor detection
        // omitted for simplicity

        template<typename T0>
        T operator(T0 & a0) const
        {
            return T(a0);
        }

        template<typename T0, typename T1>
        T operator(T0 & a0, T1 & a1) const
        {
            return T(a0,a1);
        }

        // [...]

        // Note:
        // Sequence elements are unpacked as lvalues, so const
        // qualifiers on the argument types of operator() can be
        // deduced
    };

Client code (at function scope):

    invoke_function_object( ctor<my_class>(), argseq );

Regards,
Tobias