Re: Private method has been invocated as interface

"Alf P. Steinbach" <>
Sat, 06 May 2006 07:54:02 +0200
* Phlip:

Alex Vinokur wrote:

It seems that for the same reason a private method can be invocated as
in a derived class.

class Base
 virtual void foo() = 0;

class Derived : public Base
 void foo() { cout << "foo(): I am private" << endl; }

int main ()
 Base* p = new Derived();
 return 0;

Right. Access is checked at compile time, and virtual methods

Now why does /C++ Coding Standards/, by Alexandrescu & Herb Sutter,
recommend (IIRC) "Prefer to make virtual functions private"? Does that imply
both the Base and Derived methods should be private?

I can't speak for the authors, but one possible reason is that it forces
derived classes -- and client code -- to call virtual functions via
non-virtual ones that can e.g. detect contract violations and serve as
scaffolding for overrides in derived classes. E.g., you might want to
log all calls to some function, and centralize that in the base class.
But if virtual functions are called directly you can't, because the
calls might go directly to an override in a derived class (this is the
"any CS problem can be solved by an additional layer of indirection").

IMO that's not much of an advantage, though.

With private virtual functions one runs the risk of a derived class
overriding some undocumented private virtual function in a base class,
or else, the private virtuals must be documented, which is somewhat at
odds with the notion of using private for implementation details (but
then, also a private base class can affect derived class code).

Perhaps the most that nearly everyone can agree with is that the C++
concepts of public, protected and private do not correspond to the
notions of data hiding & encapsulation that we commonly apply at design
level thinking -- where "private" is really private, hidden detail.

And with such conceptual mismatch, what's reasonable and unreasonable
use becomes very subjective, a matter of resolving the conflict in a
subjectively least painful way.

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