C++: How to construct object with two iterators?

Question

I have sample big integer class. It contains dynamic array of digits that comprise the big integer. I would like to construct objects of this class using 2 iterators (begin and end) in order I can pass digits from std::vector or std::list.

Some pseudocode illustrating my idea:

BigInteger(std::iterator begin, std::iterator end);
...

Usage:

std::vector<int> v;
// fill vector with digits
...
BigInteger b(v.begin(), v.end());

The question is: how to declare such constructor correctly? Also even is it possible?

Thanks!

Answer 1

Use a template constructor:

template<class InputIterator>
BigInteger( InputIterator begin, InputIterator end )

This should be used like:

std::vector<int> v; //Fill with values    
BigInteger( v.begin(), v.end() );

Answer 2

You can not use it simply!

If you declare the iterator types as templates, you can have this:

template <typename Itr>
BigInteger(Itr begin, Itr end)
{
}

or

BigInteger(std::vector<int>::iterator begin, std::vector<int>::iterator end)
{
}

But, how about std::iterator. Well, std::iterator is a template class and you should provide it's parameters and you should derive from it

class MyItr : public std::iterator<std::input_iterator_tag, int>
{
...
};

BigInteger(MyItr begin, MyItr end)
{
}

It's a long story! A possible definition of std::iterator is

  template<typename _Category, typename _Tp, typename _Distance = ptrdiff_t,
           typename _Pointer = _Tp*, typename _Reference = _Tp&>
    struct iterator
    {
      typedef _Category  iterator_category;
      typedef _Tp        value_type;
      typedef _Distance  difference_type;
      typedef _Pointer   pointer;
      typedef _Reference reference;
    };

As you can see, it's just an empty class with some typedefs. So, you have to implement operator*(), operator->(), begin(), end(), ... for derived iterator.