Constructors for dynamically allocated CLASS data

[John McFarland <mcfarljm at gmail dot com>]

Quick question about constructors in Fortran 2003/08.  Say I have an 
abstract base class such as Shape, with child classes such as Circle, 
Square, etc.  In C++, you can dynamically allocate these data using 
something like:
Shape *s;
s = new Circle(3);  // Construct a circle with radius=3

I'm wondering if there is a corresponding idiom in Fortran 2003.  I can 
write procedures that accept CLASS(Circle) or CLASS(Square) and act as 
constructors, but it appears that you can't pass a dynamically allocated 
instance of CLASS(Shape) into these procedures.  In other words:
CLASS(Shape), ALLOCATABLE :: s
ALLOCATE(Circle::s)
CALL Circle_ctor(s,3) // Not allowed

The only other approach I can think of is to write the constructor 
procedures such that they accept CLASS(Shape), and then use SELECT TYPE 
to access the child data.  This is not very elegant though, and gets 
cumbersome if I want to use separate procedures for each child 
constructor (and then now you need to do error checking in each 
constructor procedure to make sure that the dynamic type actually is 
what was expected).

Surely there is a better way to do this?

Thanks in advance,
John