Re: Am I using NVI?

IMO both books are right ;-), they address two different situations,
that's all.

Pure virtual interfaces are very good for "design by contract". e.g. a
plug-in-type class: server defines an interface, plug-in does what it
wants within the realm of the interface contract spec.

Nonpublic virtuals are good as extensibility points with
implementation inheritance. E.g. in a class X, method Y does Z. It is
desirable, in some situations, that some processing occur before or
after Z. That is best solved through e.g.

void ZComplete()
{
   PreZ(); // virtual
   Z();
   PostZ(); // virtual
}

I can't see a clear-cut line between the two approaches. Evolution of
code shows best which to choose. E.g. one might start off with
implementation inheritance and PreZ/PostZ. Then, after two years, one
realizes that PreZ and PostZ are more often used than not. In that
case, pure virtual interface and composition is a better solution,
e.g.

void ZComplete()
{
   GetZDecoration().PreZ();
   Z();
   GetZDecoration().PostZ();
}

// obtained through X::GetZDecoration()
struct /*interface*/ ZDecoration
{
   virtual void PreZ() = 0;
   virtual void PostZ() = 0;
}

HTH,
Goran.

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