C++ named requirements: LegacyRandomAccessIterator

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C++ named requirements

A LegacyRandomAccessIterator is a LegacyBidirectionalIterator that can be moved to point to any element in constant time.

If a LegacyRandomAccessIterator it originates from a Container , then it 's value_type is the same as the container's, so dereferencing ( * it ) obtains the container's value_type .

A pointer to an element of an array satisfies all requirements of LegacyRandomAccessIterator .

Requirements

The type It satisfies LegacyRandomAccessIterator if

And, given

The following expressions must be valid and have their specified effects:

Expression Return type Operational semantics Notes
r + = n It& difference_type m = n ;

if ( m >= 0 ) while ( m -- ) ++ r ;
else while ( m ++ ) -- r ;
return r ;

  • n can be both positive or negative
  • The complexity is constant (that is, the implementation cannot actually execute the while loop shown in operational semantics)
a + n

n + a

It It temp = a ;

return temp + = n ;

  • n can be both positive or negative
  • a + n == n + a
r - = n It& return r + = - n ; The absolute value of n must be within the range of representable values of difference_type .
i - n It It temp = i ;
return temp - = n ;
b - a difference_type return n ;
(see the precondition)

Precondition:

  • there exists a value n of type difference_type such that a + n == b

Postcondition:

  • b == a + ( b - a ) .
i [ n ] convertible to reference * ( i + n )
a < b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
Equivalent to return b - a > 0 ; Precondition:
  • same as of b - a

Strict total ordering relation:

  • ! ( a < a )
  • if a < b then ! ( b < a )
  • if a < b and b < c then a < c
  • a < b or b < a or a == b
    (exactly one of the expressions is true)
a > b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
b < a Total ordering relation opposite to a < b
a >= b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
! ( a < b )
a <= b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
! ( a > b )

The above rules imply that LegacyRandomAccessIterator also implements LessThanComparable .

A mutable LegacyRandomAccessIterator is a LegacyRandomAccessIterator that additionally satisfies the LegacyOutputIterator requirements.

Concept

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

template < class I >

concept __LegacyRandomAccessIterator =
__LegacyBidirectionalIterator < I > && std:: totally_ordered < I > &&
requires ( I i, typename std:: incrementable_traits < I > :: difference_type n )
{
{ i + = n } - > std:: same_as < I & > ;
{ i - = n } - > std:: same_as < I & > ;
{ i + n } - > std:: same_as < I > ;
{ n + i } - > std:: same_as < I > ;
{ i - n } - > std:: same_as < I > ;
{ i - i } - > std:: same_as < decltype ( n ) > ;
{ i [ n ] } - > std:: convertible_to < std:: iter_reference_t < I >> ;

} ;

where the exposition-only concept __LegacyBidirectionalIterator is described in LegacyBidirectionalIterator .

(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 299
( N3066 ) 		C++98 the return type of a [ n ] was required
to be convertible to const value_type & 		the return type is required to
be convertible to reference
LWG 448 C++98 the return type of a [ n ] was required
to be convertible to value_type 		the return type is required to be
convertible to const value_type & [1]
LWG 1079 C++98 b - a was defined using a < b ,
resulted in circular definition 		removed a < b from the definition
LWG 2114
( P2167R3 ) 		C++98 convertibility to bool was too weak to reflect the expectation of implementations requirements strengthened
  1. ↑ LWG issue 299 was reopened after this resolution.

See also

(C++20)
specifies that a bidirectional_iterator is a random-access iterator, supporting advancement in constant time and subscripting
(concept)
Iterator library provides definitions for iterators, iterator traits, adaptors, and utility functions
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