Re: Last element in a container

"Daniel T." <>
Sun, 23 Dec 2007 14:26:40 CST
<> wrote:

I want to get an iterator pointing to the last element of a container,
where the container is any of the standard containers (vector, list,
set, map, etc). For example:

  typedef std::set<int> Container;
  typedef Container::iterator Iter;

  Iter last_element(Container & c)
    if (c.empty())
      return c.end();

    Iter it = c.end();
    return --it;

Is this guaranteed to work? Is it legal to decrement the end iterator
of a container (provided the container is non-empty), and will that
always give the last element?

It is not legal to decrement input or forward iterators, but otherwise
yes. Keep in mind though, you still have to check the returned value to
see if it points to the end. You would be better off, simply asserting

Look at the code from the caller's perspective:

int main() {
   Container c;
   // do something that may, or may not put something in 'c'
   Iter i = last_element( c );
   // i may equal c.end() have to check
   if ( i != c.end() )
      // can work with 'i' now.

What happens if you rewrite your function so that it assumes the
container is not empty?

Iter last_element( Container& c ) {
   assert( !c.empty() );
   return --c.end();

int main() {
   Container c;
   // do something that may, or may not put something in 'c'
   // we can't pass 'c' in if it is empty...
   if ( !c.empty() )
      Iter i = last_element( c );
      // can work with 'i' here.

You see, in either case, the calling code must perform some sort of
check, so having the function *also* make an if check is redundant and
increases the over-all conditional complexity of the program.

One last thing... If you simply want to work with the last element.
c.back() returns a reference to it...

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