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Date: Fri, 10 May 2002 14:57:21 -0400
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Date: 2002-05-10T14:57:21-04:00
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Preben Randhol wrote:
> I don't see what you gain by this?

If the instantiated package which is passed as the generic
formal package is usually but not always instantiated with
a subset of the arguments used to instantiate the generic
itself, it could be useful to allow the generic formal to
default to a new instantiation of the usual case.

In C++, for example, the standard containers all take an
allocator class as a generic parameter, and this allocator
class is usually, but not always, an instantiation of a
standard generic allocator class on the type of the object
the container will hold. It looks something like this -

template< typename ElementType,
	  typename Allocator = DefaultAllocator< ElementType >
	 >
class Vector { /* ... */ };

Most of the time, you just use Vector<mytype> and let the
allocator default, and sometimes you provide your own special
purpose one.