TITLE: what is a surrogate?

(Newsgroups: comp.lang.c++.moderated, 9 Sep 96)

PROBLEM: nfegan@apcc.com (Noel Fegan)

What is a surrogate class?


RESPONSE: kanze@gabi-soft.fr (J. Kanze)

The same thing as a proxy or a helper class.  (Proxy seems to be the
most widespread name for them.  Surrogate is supposedly more precise,
but proxy appeared first, and I can never remember how to spell
surrogate anyway.)  For a detailed description, see "More Effective
C++", by Scott Meyers (item30) and "Design Patterns", by Gamma, Helm,
Johnson and Vlissides.  (Meyers goes into the whys and wherefores of the
most frequent use in some detail, the Gang of Four have a more general
and much more succinct presentation.)

As a quick example, just to give you an idea, suppose you have a class
with an array interface, but with the actual data on disk.  You cannot
have operator[] return a simple reference to the local buffer, since
this would cause problems if you used it several times on the same
object in the same expression.  The solution is to provide a get and put
function, which handle the actual referencing immediately, and have
operator[] return a proxy.  The proxy object simply contains a pointer
to the real object, and the index.  It has a conversion operator to the
contained type (which simply calls get), and an assignment operator
which in fact calls the put function in the container.  Thus, something
like "a[ i ] = a[ j ]" ends up calling "a.put( a.get( j ) , i )".