Are Set Iterators Mutable or Immutable?
By Klaus Kreft, October 01, 2000
The C++ Standard mandates that containers such as std::set provide both mutable and immutable iterators. Unfortunately, mutable iterators turn out to be dangerous when used with sets. If you use one to modify an element of a set, you risk corrupting the underlying data structure. Langer and Kreft explain why this is so, and they discuss some of the less-than-desirable solutions - including non-portable implementations of the standard library. The authors then propose a more sensible, and portable, solution using iterator adapters.
October 2000 C++ Experts Forum/Effective Standard C++ Library/Listing 1
class balanceIterator
{
public:
typedef set<account>::iterator adapted_type;
typedef adapted_type::iterator_category iterator_category;
typedef adapted_type::value_type value_type;
typedef adapted_type::distance_type difference_type;
typedef double* pointer;
typedef double& reference;
balanceIterator() {}
explicit balanceIterator(adapted_type i) :adaptee(i) {}
template <class Iter>
adapted_type base() const { return adaptee; }
reference operator*() const
{return const_cast<reference>(adaptee->balance()); }
pointer operator->() const { return &(operator*()); }
balanceIterator& operator++()
{ ++adaptee;
return (*this);
}
balanceIterator operator++(int)
{ balanceIterator _Tmp = *this;
++adaptee;
return (_Tmp);
}
balanceIterator& operator--()
{ --adaptee;
return (*this);
}
balanceIterator operator--(int)
{ balanceIterator _Tmp = *this;
--adaptee;
return (_Tmp);
}
private:
adapted_type adaptee;
};
inline bool operator==(const balanceIterator& x,
const balanceIterator& y) {
return x.base() == y.base();
}
inline bool operator!=(const balanceIterator& x,
const balanceIterator& y) {
return x.base() != y.base();
}
