Question

I see many texts on the STL (e.g. http://www.cplusplus.com/reference/std/iterator/) that imply that Forward iterators (and by extension Bidi and Random iterators) must satisfy both Input and Output iterator requirements. This seems odd to me, since const iterators are clearly iterators, but are just as clearly not Output iterators.

What is the truth of this understanding? Do I not understand the concept of Output iterator, or do Output iterators not fall into the expanding requirements order of iterators? (Does this idea have a better/official name? I mean the idea that Random iterators are also Bidi iterators which are also Forward…)

Answer 1

C++03 says (24.1/4):

Besides its category, a forward, bidirectional, or random access
iterator can also be mutable or constant depending on whether the
result of the expression *i behaves as a reference or as a reference
to a constant. Constant iterators do not satisfy the requirements for
output iterators, and the result of the expression *i (for constant
iterator i) cannot be used in an expression where an lvalue is
required.

cplusplus.com chose not to mention that. The wording in the standard is confusing, since it states “Forward iterators satisfy all the requirements of the input and output iterators”, and contradicts that in the next paragraph to say that some forward iterators do not satisfy the requirements of output iterators.

C++11 simplifies a little (24.2.1/3-4):

Forward iterators satisfy all the requirements of input iterators and
can be used whenever an input iterator is specified; Bidirectional
iterators also satisfy all the requirements of forward iterators and
can be used when- ever a forward iterator is specified; Random access
iterators also satisfy all the requirements of bidirectional iterators
and can be used whenever a bidirectional iterator is specified.

Iterators that further satisfy the requirements of output iterators
are called mutable iterators. Nonmutable iterators are referred to as
constant iterators.