How does back_inserter work?

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:
  开发者_如何学运维  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

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;

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;
}

back_inserter() returns a back_insert_iterator, which has to function like an output iterator. Specifically, it has to support operations such as pre- and post increment, and dereference assignment.

If it didn't support those operations, it couldn't be used where output iterators are required.