Are const iterators still evil in C++14

Question

Item 26 from Scott Mayers's "Effective STL" is labeled "Prefer iterator to const_iterator, reverse_iterator and const reverse iterator".

The reasoning is that some forms of insert() and erase() require exactly iterator and converting from the other types is tedious and error-prone. Furthermore, comparing iterator and const_iterator could be problematic, depending on the STL implementation.

The book was released at 2001. Is the advice in Item 26 still valid with the current state of gcc?

Answer 1

The C++14 standard (N3936) guarantees that iterator and const_iterator are freely comparable (§23.2.1 [container.requirements.general]/p7):

In the expressions

i == j
i != j
i < j
i <= j
i >= j
i > j
i - j

where i and j denote objects of a container’s iterator type, either or both may be replaced by an object of the container’s const_iterator type referring to the same element with no change in semantics.

In addition, the container member functions take const_iterator parameters as of C++11 (§C.2.13 [diff.cpp03.containers] - as might be inferred from the tag, this is a change from C++03):

Change: Signature changes: from iterator to const_iterator parameters

Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:

• insert(iter, val) for vector, deque, list, set, multiset, map, multimap
• insert(pos, beg, end) for vector, deque, list, forward_list
• erase(iter) forset,multiset,map,multimap`
• erase(begin, end) forset,multiset,map,multimap`
• all forms of list::splice
• all forms of list::merge

The container requirements have been similarly changed to take const iterators. In addition, it is easy to obtain the underlying iterator from a std::reverse_iterator via its .base() member function. Thus, neither of the concerns noted in the question should be an issue in a conforming compiler.

Answer 2

The advice has been reversed, as can be seen from Item 13 of the upcoming Effective Modern C++ which is titled:

Prefer const_iterators to iterators

The reason is that C++11 and C++14 add several tweaks that make const_iterators a lot more practical:

C++11 adds

• member functions cbegin() and cend() (and their reverse counterparts) for all Standard Library containers
• member functions using iterators to identify positions (e.g. insert(), erase()) now take a const_iterator instead of an iterator

C++14 completes that by adding non-member cbegin() and cend() (and their reverse counterparts)