Re: display stack

From: Siemel Naran (SiemelNaran_at_REMOVE.att.net)
Date: 03/31/04 

Date: Wed, 31 Mar 2004 06:46:52 GMT

"Alf P. Steinbach" <alfps@start.no> wrote in message
> * "Siemel Naran" <SiemelNaran@REMOVE.att.net> schriebt:

> > Class std::stack<T, Container> contains a protected member c of type
> > Container. Perhaps you could derive your own stack class from
std::stack
> > and write the print() member function? In this form it would mean
deriving
> > publicly from a class with no virtual destructor, though you could use
> > private/protected inheritance with a lot of using declarations.
>
> Amazing. I saw that in the code but it wasn't documented in MSDN Lib.
But
> now when I read your posting I looked it up in the Holy Standard, and YES.
>
> So a more efficient version of the code I posted, which incidentally
> displays the elements in reverse order of the original code, is:
>
>
> #include <iostream>
> #include <stack>
>
>
> typedef std::stack<int> Stack;
>
> struct Hack: public Stack
> {
> typedef Stack::container_type Container;
> typedef Container::const_iterator ConstIterator;
> static Container const& containerOf( Stack const& s )
> {
> return static_cast<Hack const*>(&s)->c;
> }
> };

The containerOf function is unsound as the Stack may not originally be a
Hack.

Stack s;
Hack::containerOf(s); // invokes defined behavior, though in most compilers
nothing bad will happen

> void display( Stack const& aStack )
> {
> Hack::Container const& container = Hack::containerOf(
aStack );
> Hack::ConstIterator const end = container.end();
>
> for( Hack::ConstIterator it = container.begin(); it != end; ++it )
> {
> std::cout << *it << "\n";
> }
> }

> Now, logically this code should have undefined effect, but I'm not sure
> exactly where in the standard it says that.

In that part where static_cast from Base* to Derived* where the object is
not really a Derived* or higher class. It's in the static_cast section.

I think users should create an instance of Hack.

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