TITLE: What does "virtual constructor" mean?


PROBLEM: ehsjony@ehs.ericsson.se (Jonas Nygren), 8 Nov 93

Reading an article on Booch Components I encountered the term 'virtual
constructor' but never managed to quite understand what this was. I
suppose it is not a language construct but rather a coding convention.
Could anybody help me with an explanation - what is a virtual
constructor?


RESPONSE: kanze@us-es.sel.de (James Kanze)

It is not a language construct; in C++ a constructor cannot be
virtual.

What is referred to, I suppose, is a technique of creating an object
whose type is only determined at run-time.  There are several
different ways of doing this; my own preferred way is for each class
to provide a static function 'createNew', which returns a pointer to
the base class, and create a map from a String (with the class name)
to the corresponding 'createNew' function.  Typically, the map will be
initialized automatically; the actual 'createNew' will be a (static)
member of a private, nested class, and actual class will contain a
static member of this class.  The constructor for the private, nested
class will then register the 'createNew' function with the map.  (Most
class libraries will provide a map container class, also sometimes
called an associative array.)

A frequent variation is to make the private nested class a functor,
derived from a common base factory class.  In this case, the class
constructor registers itself, rather than the function, with the map,
and the 'createNew' function is called virtually through the instance
of the functor.

[ There is also a discussion of ways to implement virtual constructors
  in Coplien's book. -adc ]