Member function templates cannot be declared virtual - From Addison Wesley: C++ Templates

Question

From Addison Wesley: C++ Templates

Member function templates cannot be declared virtual. This constraint is imposed because the usual implementation of the virtual function call mechanism uses a fixed-size table with one entry per virtual function. However, the number of instantiations of a member function template is not fixed until the entire program has been translated.

Does the above quote mean that templates have static binding and virtual functions have dynamic binding, that's the reason there cannot be virtual function templates? Please see if a explanation in layman's language is possible.

Answer 1

Yes, and no.

The most popular method to resolve virtual function calls is to use a table ("vtable"), where each virtual function maps to an index in the table. This more or less requires that you know the size of the table.

With templates, new functions will be created as needed in different modules. You would then either have to convince the linker to build the table after figuring out the final number of functions, or use some kind of runtime structure to search for available functions at runtime.

On many systems, the linker is part of the OS and knows nothing about C++, so that option is limited. A runtime search would of course affect the performance negatively, perhaps for all virtual functions.

So, in the end, it was decided that it just was not worth the trouble of introducing virtual templates into the language.