How does back_inserter work?

Question

I'm trying to understand how back_inserter work, and this is its implementation that I have from SGI-STL:

template<class C>
class back_insert_iterator {
protected:
    C* container;
public:
    typedef C                   container_type;
    typedef output_iterator_tag iterator_category;
    typedef void                value_type;
    typedef void                difference_type;
    typedef void                pointer;
    typedef void                reference;

    explicit back_insert_iterator( C& __x ) :container( &__x ) { 
    }

    back_insert_iterator<C>& operator=( const typename C::value_type& val ) { 
        container->push_back( val );
        return *this;
    }

    back_insert_iterator<C>& operator*() {  
        return *this;  
    }

    back_insert_iterator<C>& operator++() {  
        return *this;  
    }

    back_insert_iterator<C>& operator++( int ) {  
        return *this;  
    }
};

I understood most parts, except the last three operator *, ++, ++( int ). My guess for their existence is because they need to support operations when placed inside the STL algorithm. Other than that, I don't know what are they used for? Could anyone help me clarify this?

Thanks,
Chan

Answer 1

They exist because STL algorithms work on iterators which must be post and pre incrementable and have a dereference operator.

try to think what this does:

(*back_inserter) = value;
++back_inserter;

Answer 2

Your guess is correct and there is nothing more than that. It's all about OutputIterator concept. back_insert_iterator is an OutputIterator, that means that it should work with any algorithm that expects OutputIterators. OutputIterator must have these operators defined so algorithms like this could work:

template<class InputIterator, class OutputIterator>
OutputIterator copy(
    InputIterator first, InputIterator last, OutputIterator result)
{
    while(first != last)
        // uses operators =, * and post ++ of OutputIterator.
        *result++ = *first++;
    return result;
}