前向迭代器iterator_traits :: reference的要求

Question

I'm trying to figure out exactly what requirements are made on forward_iterators' reference types. In the obvious cases you'll have value_type = T; and reference = T&; . Reading the cppreference page on forward iterator requirements, I saw

Expression  Return      Equivalent expression
*i++        reference   value_type& temp=*i; ++i; return temp;

std::vector<bool> shows that the "equivalent expression" isn't always valid since it returns a proxy object:

std::vector<bool> v(10);
auto i = v.begin();
std::vector<bool>::iterator::value_type& temp = *i; // error
// can't bind bool& to std::_Bit_reference

The equivalent expression isn't mentioned in the standard that I saw. The proxy object allows assignment though, which might be the key to conformance.

Outside of just all around trying to nail down the requirements, my specific question concerns knowing whether or not having value_type == reference where neither is a reference or supports assignment, would work with the standard libraries.

Would some Container<int> with an iterator tagged as forward_iterator_tag and reference == int be valid?

Answer 1

The requirements are enumerated in [forward.iterators]:

A class or pointer type X satisfies the requirements of a forward iterator if

• X satisfies the requirements of an input iterator (24.2.3),
• X satisfies the DefaultConstructible requirements (17.6.3.1),
• if X is a mutable iterator, reference is a reference to T ; if X is a const iterator, reference is a reference to const T ,
• [...]

So if your container has reference == int , then it does not meet the requirements of forward iterator. Which I suppose technically makes vector<bool>::iterator just an input iterator, even though it's tagged as a random access iterator.