25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.5 C++17 iterator requirements [iterator.cpp17] Input iterators [input.iterators]

A class or pointer type X meets the requirements of an input iterator for the value type T if X meets the Cpp17Iterator ([iterator.iterators]) and Cpp17EqualityComparable (Table 28) requirements and the expressions in Table 90 are valid and have the indicated semantics.
In Table 90, the term the domain of == is used in the ordinary mathematical sense to denote the set of values over which == is (required to be) defined.
This set can change over time.
Each algorithm places additional requirements on the domain of == for the iterator values it uses.
These requirements can be inferred from the uses that algorithm makes of == and !=.
[Example 1: 
The call find(a,b,x) is defined only if the value of a has the property p defined as follows: b has property p and a value i has property p if (*i==x) or if (*i!=x and ++i has property p).
— end example]
Table 90: Cpp17InputIterator requirements (in addition to Cpp17Iterator) [tab:inputiterator]
Return type
a != b
decltype(a != b) models boolean-testable
!(a == b)
Preconditions: (a, b) is in the domain of ==.
reference, convertible to T
Preconditions: a is dereferenceable.

The expression
(void)*a, *a is equivalent to *a.

If a == b and (a, b) is in the domain of == then *a is equivalent to *b.
Preconditions: a is dereferenceable.
Preconditions: r is dereferenceable.

Postconditions: r is dereferenceable or r is past-the-end;
any copies of the previous value of r are no longer required to be dereferenceable nor to be in the domain of ==.
equivalent to (void)++r
convertible to T
{ T tmp = *r;
return tmp; }
Recommended practice: The implementation of an algorithm on input iterators should never attempt to pass through the same iterator twice; such an algorithm should be a single pass algorithm.
[Note 1: 
For input iterators, a == b does not imply ++a == ++b.
(Equality does not guarantee the substitution property or referential transparency.)
Value type T is not required to be a Cpp17CopyAssignable type (Table 34).
Such an algorithm can be used with istreams as the source of the input data through the istream_iterator class template.
— end note]