C++ named requirements: LegacyInputIterator

A LegacyInputIterator is a LegacyIterator that can read from the pointed-to element. LegacyInputIterators only guarantee validity for single pass algorithms: once a LegacyInputIterator i

Requirements

Type	Definition
`X`	An input iterator type
`T`	The value type of `X` (i.e. std::iterator_traits <X> :: value_type
`R`	std::iterator_traits <X> :: reference
Value	Definition
i, j	Values of type `X` or const X
r	A value of type `X&`
Other	Definition
`m`	An identifier which possibly denotes a data member or member function

X satisfies LegacyInputIterator if all following conditions are satisfied:

X satisfies LegacyIterator.
X satisfies EqualityComparable.
The following expressions are well-formed and have the specified semantics:

Expression Type Semantics

i != j

a type satisfying BooleanTestable	(until C++20)
a type modeling `boolean-testable`	(since C++20)

Precondition

i and j are in the domain of ==

Effect Equivalent to !(i == j).

*i R, convertible to T Precondition i is dereferenceable.

Effect

The expression (void)*i, *i is equivalent to *i
If i and j are in the domain of ==, and i == j, then *i is equivalent to *j

i->m Precondition i is dereferenceable.

Effect Equivalent to (*i).m.

++r X& Precondition r is dereferenceable.

Postcondition

r is dereferenceable or r is past-the-end.
Any copies of the previous value of r are no longer required to be either dereferenceable or to be in the domain of ==

(void)r++ Effect Equivalent to (void)++r.

*r++ convertible to T Effect Equivalent to T x = *r; ++r; return x;

Equality domain

The term the domain of == is used in the ordinary mathematical sense to denote the set of values which can be compared using ==

Each algorithm places additional requirements on the equality domain for the iterator values it uses. These requirements can be inferred from the uses that algorithm makes of == and !=

Notes

For an input iterator X that is not a LegacyForwardIterator, std::iterator_traits <X> :: reference does not have to be a reference type: dereferencing an input iterator may return a proxy object or std::iterator_traits <X> :: value_type itself by value (as in the case of std::istreambuf_iterator

Concept

For the definition of std::iterator_traits, the following exposition-only concept is defined.

template < class I> concept __LegacyInputIterator = __LegacyIterator<I> && std::equality_comparable <I> && requires(I i) { typename std::incrementable_traits <I> :: difference_type ; typename std::indirectly_readable_traits <I> :: value_type ; typename std::common_reference_t < std::iter_reference_t <I> &&, typename std::indirectly_readable_traits <I> :: value_type & > ; i++ ; typename std::common_reference_t <decltype( i++ ) &&, typename std::indirectly_readable_traits <I> :: value_type & > ; requires std::signed_integral < typename std::incrementable_traits <I> :: difference_type > ; } ;

where the exposition-only concept __LegacyIterator is described in LegacyIterator.

(since C++20)

Defect reports

The following behavior-changing 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 required to be `T`	it can be any type convertible to `T`
LWG 2114 (P2167R3)	C++98	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

input_iterator (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 post-incremented (concept)
Iterator library	provides definitions for iterators, iterator traits, adaptors, and utility functions