Templatized Virtual function

Question

We know that C++ doesn\'t allow templated virtual function in a class. Anyone understands why such restriction?

Answer 1

Short answer: Virtual functions are about not knowing who called whom until at run-time, when a function is picked from an already compiled set of candidate functions. Function templates, OTOH, are about creating an arbitrary number of different functions (using types which might not even have been known when the callee was written) at compile-time from the callers' sides. That just doesn't match.

Somewhat longer answer: Virtual functions are implemented using an additional indirection (the Programmer's General All-Purpose Cure), usually implemented as a table of function pointers (the so-called virtual function table, often abbreviated "vtable"). If you're calling a virtual function, the run-time system will pick the right function from the table. If there were virtual function templates, the run-time system would have to find the address of an already compiled template instance with the exact template parameters. Since the class' designer cannot provide an arbitrary number of function template instances created from an unlimited set of possible arguments, this cannot work.