A LegacyInputIterator is a LegacyIterator that can read from the pointedto element. LegacyInputIterators only guarantee validity for single pass algorithms: once a LegacyInputIterator i
has been incremented, all copies of its previous value may be invalidated.
The type It
satisfies LegacyInputIterator if.
It
satisfies LegacyIterator It
satisfies EqualityComparable And, given.
i
and j
, values of type It
or const It
reference
, the type denoted by std::iterator_traits<It>::reference
value_type
, the type denoted by std::iterator_traits<It>::value_type
The following expressions must be valid and have their specified effects.
Expression  Return type  Equivalent expression  Notes 

i != j  contextually convertible to bool
 !(i == j) 
Precondition: (i, j) is in the domain of == . 
*i 
reference , convertible to value_type
 If i == j and (i, j) is in the domain of == then this is equivalent to *j .  Precondition: The expression 
i>m  (*i).m 
Precondition: i is dereferenceable. 

++i  It&  Precondition: Postcondition: Postcondition: Any copies of the previous value of 

(void)i++  (void)++i  
*i++  convertible to value_type 
value_type x = *i; ++i; return x; 
"in the domain of ==
" means equality comparison is defined between the two iterator values. For input iterators, equality comparison does not need to be defined for all values, and the set of the values in the domain of ==
may change over time.
The reference
type for an input iterator that is not also a LegacyForwardIterator does not have to be a reference type: dereferencing an input iterator may return a proxy object or value_type
itself by value (as in the case of std::istreambuf_iterator
).
ConceptFor the definition of
where the expositiononly concept  (since C++20) 
The following behaviorchanging defect reports were applied retroactively to previously published C++ standards.
DR  Applied to  Behavior as published  Correct behavior 

LWG 98  C++98  the return type of *i++ was value_type  it can be any type convertible to value_type 
(C++20)  specifies that a type is an input iterator, that is, its referenced values can be read and it can be both pre and postincremented (concept) 
Iterator library  provides definitions for iterators, iterator traits, adaptors, and utility functions 
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