TITLE: Question on constrained genericity


PROBLEM: beauge@loria.fr (Lionel Beauge'), 11 Oct 94

	Can we use constrained genericity in C++ ?

[ There is a detailed discussion of constrained genericity in
  "Object Oriented Software Construction" by Bertrand Meyer. -adc ]

Imagine this inheritance graph :


			 A		parent
		       /   \		  |
		     A1      A2		heir
		    / \     / \
		 A11  A12  A21 A22
			    \  /
			     A23	
		       ../..

Now, we want to define a template class B :


Class B is a template class which is declared with formal generic
parameter that conforms to certain type of class. (A1 conforms to
another, A, if class A1 is a descendant of A.)

for example :
-------------
template <class A2> class B ...
{ 
	...

	A2* a;

	...

	a->f(..);   /* f is a virtual public fonction of class A2 */

	...
}

we can define : 
-------------
B<A2> b;	ok !
B<A21> b;	ok !
B<A23> b;	ok;

B<A1> b;	Error : A1 doesn't conform to A2 !


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 12 Oct 94

There is no direct language mechanism to express such constraints, but
you can express them indirectly. Example:

	template <class T> class B {
		/* inline */ void constraint(T* t) { A* a = t; }
		...
	};
Function 'constraint' will not compile unless there is an implicit
conversion from T* to A*; this is true if T is publicly derived from A.

Some compilers validate inline template functions whether they are used
or not; in that case, this is all you need. Others validate the
functions only if called, so you would then have to call 'constraint'
from the template constructors of B. (It would probably be sufficient
to call it from the destructor of B, but protection is then less certain.)

The reasons why there is no direct language mechanism (although it has
been proposed several times) to express constrained genericity are
explained in detail in Stroustrup's "Design and Evolution of C++", in
section 15.4.