Question

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pure virtual function with implementation

I have a base class which has a function called time().
My idea is to have a pure virtual function which all the classes that inherits from it must implement.

However I also want to add the default behavior; Whoever calls time() function needs to print “hello”.

Is that possible?

Answer 1

The easiest way to achieve something like this would be to have something like the following:

  class base {
       virtual void time_impl()=0;
  public:
       void time() {
             //Default behaviour
             std::cout << "Hello\n";
             //Call overridden behaviour
             time_impl();
       }
  };

  class child : public base {
       void time_impl() override {
            //Functionality
       }
  };

This way, when any child’s time function is called, it calls the base classes time function, which does the default behaviour, and then make the virtual call to the overridden behaviour.

EDIT:

As Als says in the comments, this is called the Template method pattern which you can read about here:

http://en.wikipedia.org/wiki/Template_method_pattern

Althought for C++ that is a rather confusing name (as templates are something else, and “methods” don’t mean anything in C++ (rather member function).

EDIT:

Someone commented about the use of override, this is a C++11 feature, if you have it USE IT, it will save many headaches in the long run:

http://en.wikipedia.org/wiki/C%2B%2B11#Explicit_overrides_and_final

And also, the _impl function should be private, UNLESS you want the user to be to bypass the default behaviour, but I would argue that’s a poor design in most use-cases.

Working test case

http://ideone.com/eZBpyX