Re: Undefined reference to...

From:

Andrea Crotti <andrea.crotti.0@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 11 Nov 2010 11:58:11 +0100

Message-ID:

<m1k4kkcekc.fsf@ip1-201.halifax.rwth-aachen.de>

James Kanze <james.kanze@gmail.com> writes:

Just for the record, that's *not* a Meyers' singleton, or any
other type of singleton. (The first condition to be a singleton
is that there is no way of getting more than one instance. In
a Meyers' singleton, this is done by making the object
noncopyable, and the constructor private. Which, of course,
excludes derivation, unless you make the derived class
a friend.)

However, what you've just shown *is* the closest working
approximation of what he seems to be trying to do. Unless he
actually wants more than one instance---it's not really clear.
For more than one instance, he'd need a factory function, e.g.

    class Base
    {
    public:
        virtual void printOut() = 0;
        static std::auto_ptr<Base> getLower();
    };

    class Extended: public Base
    {
    public:
        void printOut() { cout << "hello"; }
    };

    std::auto_ptr<Base> Base::getLower()
    {
        return std::auto_ptr<Base>( new Extended );
    }

--
James Kanze

Ah nice I like this solution, so I can do something like

auto_ptr<Base> Base::getLower() {
    auto_ptr<Base> val(new Extended);
    return val;
}

int main() {
    auto_ptr<Base> b = Base::getLower();
    b->printOut();
    delete b.release();
    return 0;
}

right?

The delete if I got it it's not normally needed since when the auto_ptr
goes out of scope the object pointed is delete automatically, right?
Thanks