templates and virtual

Thomas Richter <thor@math.tu-berlin.de>
Thu, 21 Jan 2010 04:39:05 CST
Hi folks,

yes, templates and virtual member functions do not fit, I know. However,
I have here a situation where exactly that would be practical:

I have N worker classes that all provide the same interface, say:

class Interface {
    virtual double measure(const int *data1,const int *data2) = 0;

class A1 : public Interface {
    // implements measure...

class A2 : public Interface {
    // implements measure...

A clear application of virtual functions. Now, however, the same
function ("measure") is also required for a limited set of scalar types
that is known in advance. Say, "int", "short", "long" (doesn't matter
here). The implementation of "measure" is in all implementing interfaces
similar enough to share the code and generate it by a template.
Unfortunately, while it is know for which types I will need an
implementation for, the specific type required is not known at the type
an instance of the class is created, i.e. I cannot just template the
class itself by the type. It is known at call-time, but not at creation

That is, I would prefer to write:

class Interface {
    template <typename T>
    virtual double measure(const T *data1,const T* data2) = 0;

with classes implementing the interface requiring to implement the
template for all types T from a set of types specified in advance.

Clearly, the above is not possible in C++ (how would I tell the compiler
which T's are part of the interface specifications.

Which alternatives exist to generate a class hierarchy similar to the
above, i.e. require implementing classes of an interface to build
templates for a given set of types?

For example, I could create a temporary "worker class" templated by the
type at call time that does the job, but that requires a dispatcher
within any implementing class, which duplicates at least the dispatch
code. Any other ideas maybe?


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