Re: STL non virtual DTOR

From:

"James Kanze" <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 25 Feb 2007 11:51:12 CST

Message-ID:

<1172407578.925763.184510@k78g2000cwa.googlegroups.com>

On Feb 25, 8:56 am, "Jiang" <goo.mai...@yahoo.com> wrote:

Within the standard library, public inheritance like this

   template < typename T, typename A
   struct vector : public __sequence<T,A>
                 , public __basic_vector<T,A> {
     // some more members and constructor
   };

is also an implementation detail. The user cannot declare
functions with signature

   some_type some_func ( __sequence<T,A> const & );

without using reserved identifiers. Similarly, there cannot be a

polymorphic

__sequence<T,A> *

in client code without visibly invoking undefined behavior.

But compared with protected or private inheritance, what is the
benifit of using the above public inheritance?

Just a guess, but maybe the base types contain necessary
typedefs. Even in user code, it's not rare to do something
like:

    class MyIter : public std::iterator< ... >

That doesn't mean that using pointers to an std::iterator to
refer to instances of my class is legitimate.

Inheritance can fulfill many roles in C++, and polymorphism, in
the OO sense, is just one of them (albeit by far the most
frequent). Similarly, a class can be designed to be used as a
base class for many reasons---std::iterator is definitly
designed to be used as a base class, but it makes no sense for
anyone ever to have an std::iterator<>*. (Today, I think we'd
probably give the class a protected destructor, just in case,
but the technique wasn't really known when the class was
designed. And frankly, given its semantics, who's going to
create a pointer to it anyway.)

In the case of std::list, we have a class which was NOT designed
to be a base, regardless of the use of inheritance. And one
could very easily imagine reasonable cases where the user had a
pointer or a reference to std::list. So publicly inheriting
from is not a good idea. Even if not doing so implies a bit of
extra typing.

--
James Kanze (Gabi Software) email: james.kanze@gmail.com
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