Re: function pointer to constructor
* James Kanze:
(For some strange reason, the original posting didn't show up at
Google, and I can't see it, although it appears in the list of
the thread. It seems to be happening with a lot of postings at
the moment.)
And vice versa -- SuperNews fails to provide a lot of postings that
show up in Google's archive, and that includes about 50% (it feels like)
of comp.lang.c++.moderated postings...
I'm wondering whether there is some too active spam-filter somewhere.
In case it helps, the original posting:
* Morten Lemvigh:
Is it possible to pass a pointer to a constructor or a class
definition as argument to a function? Maybe in a way similar
to passing function pointers...?
The function should construct a number of objects using the given
constructor. The objects should all inherit from a base class.
It's not possible to pass actual objects, since it's not given on
beforehand, how many should be created.
And as pointed out else-thread,
* Tom1s # hilidhe:
Instead of a getting a pointer directly to the contructor, you can
get the address of a very small function which calls the constructor:
void Func(T *const p)
{
::new(p) T(whatever parameters);
}
I don't like having that storage argument typed as T*, but then, as I
recall the standard's default allocator class does that dirty thing too.
A more direct design is to try to translate the OP's requirements to C++.
"It's not given on beforehand, how many should be created" -> dynamic
allocation.
"The objects [to be created] should all inherit from a base class" -> a
function returning pointer to Base:
std::auto_ptr<Base> newDerived();
Also it seems it should have some arguments, perhaps just one (doesn't
matter):
std::auto_ptr<Base> newDerived( Arg const& );
Now all that's needed is to define such a function for a given
constructor of a given subclass of Base, and pass that function as
argument. If there are many such potential subclasses, the function can
be templatized in order to do this once and for all:
template< typename Derived >
std::auto_ptr<Base> newDerived( Arg const& arg )
{
return std::auto_ptr<Derived>( new Derived( arg ) );
}
So all the client code needs to do is
foo( &newDerived<MyDerivedClass> );
Cheers,
- Alf
PS: Just mentioning it, yet again, re terminology, source level
constructor "call" -> needed for standard's definition of default
constructor, + use of term "explicit constructor call" in standard and
in articles by Bjarne Stroustrup and Andrew Koenig, +, of course,
keyword "explicit"... Do you remember discussing this before? <g>
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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