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Difference between revisions of "cpp/named req/RandomAccessIterator"

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< cpp‎ | named req
(Undo revision 143724 by 2600:1700:4900:1340:A9E3:6B24:FE2B:7D1C (talk))
m (unifying fmt to, hopefully, reduce a confusion)
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A {{named req|RandomAccessIterator}} is a {{named req|BidirectionalIterator}} that can be moved to point to any element in constant time.
 
A {{named req|RandomAccessIterator}} is a {{named req|BidirectionalIterator}} that can be moved to point to any element in constant time.
  
−
If a {{named req|RandomAccessIterator}} {{tt|it}} originates from a {{named req|Container}}, then {{tt|it}}'s {{c|value_type}} is the same as the container's, so dereferencing ({{c|*it}}) obtains the container's {{c|value_type}}.
+
If a {{named req|RandomAccessIterator}} {{c|it}} originates from a {{named req|Container}}, then {{c|it}}'s {{tt|value_type}} is the same as the container's, so dereferencing ({{c|*it}}) obtains the container's {{tt|value_type}}.
  
 
A pointer to an element of an array satisfies all requirements of {{named req/core|RandomAccessIterator}}
 
A pointer to an element of an array satisfies all requirements of {{named req/core|RandomAccessIterator}}
  
 
===Requirements===
 
===Requirements===
−
The type {{tt|It}} satisfies {{named req/core|RandomAccessIterator}} if
+
The type {{c|It}} satisfies {{named req/core|RandomAccessIterator}} if
−
 
+
* The type {{c|It}} satisfies {{named req|BidirectionalIterator}}
−
* The type {{tt|It}} satisfies {{named req|BidirectionalIterator}}
+
  
 
And, given
 
And, given
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* {{tt|difference_type}}, the type denoted by {{c|std::iterator_traits<It>::difference_type}}
 
* {{tt|difference_type}}, the type denoted by {{c|std::iterator_traits<It>::difference_type}}
 
* {{tt|reference}}, the type denoted by {{c|std::iterator_traits<It>::reference}}
 
* {{tt|reference}}, the type denoted by {{c|std::iterator_traits<It>::reference}}
−
* {{tt|i}}, {{tt|a}}, {{tt|b}}, objects of type {{tt|It}} or {{tt|const It}}
+
* {{c|i}}, {{c|a}}, {{c|b}}, objects of type {{c|It}} or {{c|const It}}
−
* {{tt|r}}, a value of type {{tt|It&}}
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* {{c|r}}, a value of type {{c|It&}}
−
* {{tt|n}}, an integer of type {{tt|difference_type}}
+
* {{c|n}}, an integer of type {{tt|difference_type}}
  
 
The following expressions must be valid and have their specified effects  
 
The following expressions must be valid and have their specified effects  
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*{{c|1=a + n == n + a}}
 
*{{c|1=a + n == n + a}}
 
|-
 
|-
−
|{{c|r -{{=}} n}}||{{c|It&}}||{{c|return r +{{=}} -n;}}
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|{{c|1=r -= n}}||{{c|It&}}||{{c|1=return r += -n;}}
−
|The absolute value of {{tt|n}} must be within the range of representable values of {{tt|difference_type}}. {{mark unreviewed dr|LWG|2519}}
+
|The absolute value of {{c|n}} must be within the range of representable values of {{tt|difference_type}}. {{mark unreviewed dr|LWG|2519}}
 
|-
 
|-
 
|{{c|i - n}}||{{c|It}}||{{c|1=It temp = i;
 
|{{c|i - n}}||{{c|It}}||{{c|1=It temp = i;
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|{{c|b - a}}||{{tt|difference_type}}||{{c|return n;}}||
 
|{{c|b - a}}||{{tt|difference_type}}||{{c|return n;}}||
 
Precondition:
 
Precondition:
−
* there exists a value {{tt|n}} of type {{tt|difference_type}} such that {{c|1=a+n==b}}
+
* there exists a value {{c|n}} of type {{tt|difference_type}} such that {{c|1=a+n==b}}
 
Postcondition:
 
Postcondition:
 
* {{c|1=b == a + (b - a)}}.
 
* {{c|1=b == a + (b - a)}}.
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* same as of {{c|b - a}}<!-- LWG3236 -->
 
* same as of {{c|b - a}}<!-- LWG3236 -->
 
Strict total ordering relation:
 
Strict total ordering relation:
−
* {{ttb|!(a < a)}}
+
* {{c|!(a < a)}}
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* if {{ttb|a < b}} then {{ttb|!(b < a)}}
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* if {{c|a < b}} then {{c|!(b < a)}}
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* if {{ttb|a < b}} and {{ttb|b < c}} then {{ttb|a < c}}
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* if {{c|a < b}} and {{c|b < c}} then {{c|a < c}}
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* {{ttb|a < b}} or {{ttb|b < a}} or {{ttb|a {{==}} b}} <br> (exactly one of the expressions is true)
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* {{c|a < b}} or {{c|b < a}} or {{c|1=a == b}}<br> (exactly one of the expressions is true)
 
|-
 
|-
 
|{{c|a > b}}||contextually convertible to {{c|bool}}||{{c|b < a}}||Total ordering relation opposite to {{c|a < b}}
 
|{{c|a > b}}||contextually convertible to {{c|bool}}||{{c|b < a}}||Total ordering relation opposite to {{c|a < b}}
 
|-
 
|-
−
|{{c|a >{{=}} b}}||contextually convertible to {{c|bool}}||{{c|!(a < b)}}||
+
|{{c|1=a >= b}}||contextually convertible to {{c|bool}}||{{c|!(a < b)}}||
 
|-
 
|-
−
|{{c|a <{{=}} b}}||contextually convertible to {{c|bool}}||{{c|!(a > b)}}||
+
|{{c|1=a <= b}}||contextually convertible to {{c|bool}}||{{c|!(a > b)}}||
 
|}
 
|}
  

Revision as of 10:27, 25 September 2022

 
C++
 
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

  • value_type, the type denoted by std::iterator_traits<It>::value_type
  • difference_type, the type denoted by std::iterator_traits<It>::difference_type
  • reference, the type denoted by std::iterator_traits<It>::reference
  • i, a, b, objects of type It or const It
  • r, a value of type It&
  • n, an integer of type difference_type

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;

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 contextually convertible to bool 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 contextually convertible to bool b < a Total ordering relation opposite to a < b
a >= b contextually convertible to bool !(a < b)
a <= b contextually convertible to bool !(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#Concept.

(since C++20)

See also

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