Re: how to force overriding

From:

Andre Kostur <nntpspam@kostur.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 1 May 2007 15:22:48 +0000 (UTC)

Message-ID:

<Xns992355AC85DB7nntpspamkosutrnet@209.135.99.21>

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in news:f17hi7$ios$1
@news.datemas.de:

timor.super@gmail.com wrote:

I would like to make a class, that if another class derives from this,
people has to override somes functions, obligatory.

I can do that with pure virtual function, like this :

class Test
{
public:
Test()
{
cout << "Ctor Test" << endl;
}
virtual void fToOverride() = 0;
};

but, what i would like to do, is to force to override, but that my
function do something :

class Test
{
public:
Test()
{
cout << "Ctor Test" << endl;
}
virtual void fToOverride()
{
cout << "i'm here" << endl;
}
};

I would like that people that derives from Test, have to override the
function fToOverride and when calling it, that it does output "i'm
here" and do what they have written in their derived class.

How to do that ? (i hope i've been clear enough)

Declare it pure in the class definition and define it (provide it with
a body) *outside* of the class:

    class HasPureImplemented {
    public:
        void fToOverride() = 0;
    };

    void HasPureImplemented::fToOverrid()
    {
        cout << "i'm here" << endl;
    }

Not quite what the OP is asking for. You'd have to rely on the subclass
implementor to chain back to your implementation.

What you could do is have the base class with a non-virtual function
fToOverride(), and it calls a different pure virtual function to allow
the subclass implementator to do their thing:

class Base
{
public:
  void fToOverride()
  {
    cout << "I'm here" << endl;
    this->f_ToOverride_i();
  }

private:
  virtual void fToOverride_i() = 0;
};