Template class vs Abstract class

Question

So I started looking back at some of my OOP courses and I found this question: What is the difference between a template class and an abstract class? When should one be used and not the other?

I know that template classes are instantiated the same way as the functions. A copy is created with the template parameters replaced with actual data types. Abstract classes have pure virtual functions and cannot be instantiated. Derived classes must define the virtual functions.

Answer 1

They're both tools that C++ offers to change the implementation and/or behaviour of a part of your application (i.e. extendibility and reuse).

The main difference is that abstract classes (run-time polymorphism) are a run-time mechanism, while templates are a compile-time mechanism. This means that by using abstract classes you can possibly change the behaviour at run-time (e.g, by loading a configuration file at run-time, or by means of plugins). With templates, instead, you bind your implementation at compile-time (i.e., it's the developer that decides statically once and for all the behaviour).

Being a run-time mechanism, abstract classes bring run-time overhead due to virtual functions. Templates don't suffer this issue.

On the other hand, currently (C++14 standard) you cannot specify an interface for your class templates (i.e., you don't have a spec of the classes you can bind to your template and if you try to instantiate a template class binding the wrong class you get incomprehensible compiler errors). An abstract class instead is actually a "specification" of the concrete classes you can use.