C++ concepts: ForwardIterator
From cppreference.com
                    
                                        
                    
                    
                                                            
                    A ForwardIterator is an Iterator that can read data from the pointed-to element.
Unlike InputIterator and OutputIterator, it can be used in multipass algorithms.
| Contents | 
[edit] Requirements
The type It satisfies ForwardIterator if
-  The type ItsatisfiesInputIterator
-  The type ItsatisfiesDefaultConstructible
-  Objects of the type Itprovide multipass guarantee described below
-  The type std::iterator_traits<It>::referencemust be exactly
-  T&ifItsatisfiesOutputIterator(Itis mutable)
-  const T&otherwise (Itis constant),
 
-  
- (where Tis the type denoted by std::iterator_traits<It>::value_type)
- Equality and inequality comparison is defined over all iterators for the same underlying sequence and the value initialized-iterators (since C++14).
And, given
-  i, dereferenceable iterator of typeIt
-  reference,the type denoted bystd::iterator_traits<It>::reference
The following expressions must be valid and have their specified effects
| Expression | Return type | Equivalent expression | Notes | 
|---|---|---|---|
| i++ | It | It ip=i; ++i; return ip; | |
| *i++ | reference | 
A mutable ForwardIterator is a ForwardIterator that additionally satisfies the OutputIterator requirements.
[edit] Multipass guarantee
Given a and b, dereferenceable iterators of type It
-  If aandbcompare equal (a == bis contextually convertible totrue) then either they are both non-dereferenceable or*aand*bare references bound to the same object
-  Assignment through a mutable ForwardIteratoriterator cannot invalidate the iterator (implicit due toreferencedefined as a true reference)
-  incrementing a copy of adoes not change the value read froma(formally, eitherItis a raw pointer type or the expression (void)++It(a), *a is equivalent to the expression *a)
-  a == bimplies++a == ++b
| Singular iteratorsA value-initializedForwardIteratorbehaves like the past-the-end iterator of some unspecified empty container: it compares equal to all value-initializedForwardIterators of the same type. | (since C++14) | 


