From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-06-17 17:53:23
Dear Boost community,
do we have a function object template to wrap arbitrary constructor
calls / operator new expressions somewhere in Boost, already?
If not, is there any interest in such a thing?
Interface draft:
================
factory<T> // constructs X and returns it by value
factory<T*> // uses operator new, returns a pointer
Where operator() takes a variable number of arguments, forwarded to the
constructor.
Should there be an optional 'Allocator' template parameter?
Some use cases:
===============
o Transform through an explicit constructor, e.g:
std::transform(from,to,out,factory<X>());
or a constructor that takes more than one argument, e.g:
std::transform(from,to,out,
boost::bind(factory<Y>(),12,_1));
o Generate data, e.g:
std::generate_n(out,12,boost::bind(factory<Y>(),123,12));
o Creating homogeneous, polymorphic factories from heterogeneous
constructors, e.g:
void register_factory( std::type_info const &,
boost::function<void*()> const &);
// ... at function scope:
register_factory( typeid(X), factory<X>() );
register_factory( typeid(Y), boost::bind(factory<Y>(),12,1) );
Implementation notes regarding the Forwarding Problem:
======================================================
For now (C++89) it might be good enough to just have operator() take its
arguments by non-const reference. As usually the call will be deferred
and as long as the arguments are L-Values they will be deduced even if
const qualified (this might break the code of the first use case above -
wrapping the factory specialization into a Boost.Bind function object
without actually binding anything will work around the problem, though).
Alternatively we could solve the forwarding problem (within the function
object itself) using a "hammer and crowbar utility" in Fusion
(overloading operator() with all combinations of templatized
const/non-const reference parameters) until we have better means to do
so. I personally prefer the former because it's more lightweight and
might allow us to be more portable than Fusion is, however.
Curiously awaiting your feedback,
Tobias