TITLE: intro to member templates

[ This material is excerpted with permission from the author
  Glen McCluskey. See the trailer for contact info. -adc ]


NEW C++ FEATURE - MEMBER TEMPLATES

In chapter 14 of the draft ANSI/ISO C++ standard is a mention of
something called member templates.  This feature is new in a way and
not yet widely available, but worth mentioning here.

Member templates are simply a generalization of templates such that a
template can be a class member.  For example:

        #include <stdio.h>

        template <class A, class B> struct Pair {
                A a;
                B b;

                Pair(const A& ax, const B& bx) : a(ax), b(bx) {}

                template <class T, class U> Pair(const Pair<T,U>& p) :
                    a(p.a), b(p.b) {}
        };

        int main()
        {
                Pair<short, float> x(37, 12.34);
                Pair<long, long double> y(x);

                printf("%ld %Lg\n", y.a, y.b);

                return 0;
        }

This is an adaptation of a class found in the Standard Template
Library.  Note that an object of class Pair<long, long double> is
constructed from an object of class Pair<short, float>.  By using a
template constructor it is possible to construct a Pair from any other
Pair, assuming that conversion from T to A and U to B are supported.
Without the availability of template constructors one could only
declare constructors with fixed types like "Pair(int)" or else use the
template arguments to Pair itself, as in "Pair(A, B)".

In a similar way to function template use, it's possible to have usage
like:

        template <class T> struct A {
                template <class U> struct B {/* stuff */};
        };

        A<double>::B<long> ab;

In this example, the type value of T within the nested template
declaration would be "double", while the value of U would be "long".

There are a few restrictions on member templates.  A destructor for a
class cannot be defined as a function template, nor may a function
template member of a class be virtual.


[ This information came from the C++ Newsletter available free
  over the net. 

	Issue #009
	April, 1996

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