Why should virtual functions not be used excessively?

Question

I just read that we should not use virtual function excessively. People felt that less virtual functions tends to have fewer bugs and reduces maintenance.

What kind of b

Answer 1

Virtual functions are slightly slower than regular functions. But that difference is so small as to not make a difference in all but the most extreme circumstances.

I think the best reason to eschew virtual functions is to protect against interface misuse.

It's a good idea to write classes to be open for extension, but there's such a thing as too open. By carefully planning which functions are virtual, you can control (and protect) how a class can be extended.

The bugs and maintenance problems appear when a class is extended such that it breaks the contract of the base class. Here's an example:

class Widget
{
    private WidgetThing _thing;

    public virtual void Initialize()
    {
        _thing = new WidgetThing();
    }
}

class DoubleWidget : Widget
{
    private WidgetThing _double;

    public override void Initialize()
    {
        // Whoops! Forgot to call base.Initalize()
        _double = new WidgetThing();
    }
}

Here, DoubleWidget broke the parent class because Widget._thing is null. There's a fairly standard way to fix this:

class Widget
{
    private WidgetThing _thing;

    public void Initialize()
    {
        _thing = new WidgetThing();
        OnInitialize();
    }

    protected virtual void OnInitialize() { }
}

class DoubleWidget : Widget
{
    private WidgetThing _double;

    protected override void OnInitialize()
    {
        _double = new WidgetThing();
    }
}

Now Widget won't run into a NullReferenceException later.