Re: When might it make sense to use inheritance when templates
(compile-time polymorphism) is enough?
On Saturday, 17 August 2013 22:37:17 UTC+1, K. Frank wrote:
Prior to templates, if you wanted to reuse code across
different types, you would use inheritance / polymorphism
where your different types would derive from a common
base type that offered the capabilities needed by your
reusable code.
Now (with templates) if you know at compile time which
objects are of which concrete types you can use templates.
(If the concrete type of an object is only determined at
run time, you still need inheritance.)
This is kind of a soft question, but I'm wondering whether
there are situations where inheritance would still be
preferable, even when concrete types are known at compile
time, and a template solution could have been used.
Are there really that many cases where both are appropriate.
Most of the time, only one will do the job, and in the few
remaining cases, one is usually significantly better than the
other. They do different things. Radically different, in
fact. Inheritance allows different implementations of the same
interface; templates allow the same implementation for different
interfaces.
Assume for this question that the code is special purpose,
so we're not trying to write some general, open-ended
library. That is, the code will be reused across a
smallish number of different types that are all being
designed together, and will not be extended to new types
in the future.
Also assume that efficiency isn't a concern, so that we
don't care about the cost of vtables or the cost (size)
of duplicated template code.
To illustrate my question, below is a simple, do-nothing
example that uses both inheritance and templates for
compile-time polymorphism. printPrintable is an
inheritance-based polymorphic function, while
printHasPrintMe is a generic template function.
If you're requiring a printMe function in any class you call it
on, you might as well go with inheritance, for the improved
flexibility, and the (almost certainly) better error messages if
you try to call it on a class which doesn't support it. The
template is just extra baggage.
--
James