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

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< cpp‎ | named req
m (Added `i <=> j` per [eel.is/c++draft/container.requirements.general#container.reqmts-itemdecl:18]. +#Optional container requirements from [eel.is/c++draft/container.opt.reqmts].)
m (→‎Optional container requirements: fmt)
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in the expressions {{c|1=i == j}}, {{c|1=i != j}}, {{c|i < j}}, {{c|1=i <= j}}, {{c|1=i >= j}}, {{c|i > j}}, {{c|i - j}}, either or both may be replaced by an object of the container's {{tt|const_iterator}} type referring to the same element with no change in semantics.
 
in the expressions {{c|1=i == j}}, {{c|1=i != j}}, {{c|i < j}}, {{c|1=i <= j}}, {{c|1=i >= j}}, {{c|i > j}}, {{c|i - j}}, either or both may be replaced by an object of the container's {{tt|const_iterator}} type referring to the same element with no change in semantics.
  
−
===Optional container requirements===
+
{{anchor|Optional container requirements}}
 +
===Optional container requirements {{mark since c++20}}===
 
The following operations are provided only for some types of containers.
 
The following operations are provided only for some types of containers.
  
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|-
 
|-
 
!Expression
 
!Expression
−
|{{c|1=a <=> b}}<br>{{mark since c++20}}
+
|{{c|1=a <=> b}}
 
|-
 
|-
 
!Returns
 
!Returns
 
|{{c multi|
 
|{{c multi|
−
std::lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(),|
+
std::lexicographical_compare_three_way(a.begin(), a.end(),|
 +
                                      b.begin(), b.end(),|
 
                                       /*synth-three-way*/)
 
                                       /*synth-three-way*/)
 
}}
 
}}

Revision as of 04:41, 24 May 2024

 
C++
 
C++ named requirements
 

A Container is an object used to store other objects and taking care of the management of the memory used by the objects it contains.

Contents

Requirements

  • T, an element type;
  • C, a Container type containing elements of type T;
  • a and b, objects of type C;
  • i and j, values of (possibly const) type C::iterator;
  • rv, a prvalue expression of type C.

Types

Name Type Requirements
value_type T CopyConstructible(until C++11)Erasable(since C++11)
reference T&
const_reference const T&
iterator Iterator whose value type is T LegacyForwardIterator
convertible to const_iterator
const_iterator Constant iterator whose value type is T LegacyForwardIterator
difference_type Signed integer Must be the same as iterator_traits::difference_type for iterator and const_iterator
size_type Unsigned integer Large enough to represent all positive values of difference_type

Member functions and operators

Expression Return type Semantics Conditions Complexity
C() C Creates an empty container Post: C().empty()
== true		 Constant
C(a) C Creates a copy of a Pre: T must be CopyInsertable
Post: a == C(a)		 Linear
C(rv)
(since C++11)		 C Moves rv Post: equal to the value rv had before this construction Constant[1]
a = b C& Destroys or copy-assigns all elements of a from elements of b Post: a == b Linear
a = rv
(since C++11)		 C& Destroys or move-assigns all elements of a from elements of rv Post: if a and rv do not refer the same object, a is equal to the value rv had before this assignment Linear
a.~C() void Destroys all elements of a and frees all memory Linear
a.begin() (const_)iterator Iterator to the first element of a Constant
a.end() (const_)iterator Iterator to one past the last element of a Constant
a.cbegin()
(since C++11)		 const_iterator const_cast<const C&>(a).begin() Constant
a.cend()
(since C++11)		 const_iterator const_cast<const C&>(a).end() Constant
i <=> j
(since C++20)		 strong_ordering Three-way comparison of container iterators C::​iterator meets the random access iterator requirements Constant
a == b Convertible to bool
a.size() == b.size() &&

    std::equal(a.begin(),

        a.end(), b.begin())		 (until C++14)
std::equal(a.begin(), a.end(),
    b.begin(), b.end())		 (since C++14)
 Pre: T must be EqualityComparable Constant[2] if a.size() !=
b.size(), linear otherwise
a != b convertible to bool !(a == b) Linear
a.swap(b) void Exchanges the values of a and b Constant[1][3]
swap(a, b) void a.swap(b) Constant[1]
a.size() size_type std::distance(a.begin(), a.end()) Constant[3]
a.max_size() size_type b.size() where b is the largest possible container Constant[3]
a.empty() Convertible to bool a.begin() == a.end() Constant
Notes
  1. ↑ 1.0 1.1 1.2 (since C++11) Linear for std::array
  2. ↑ Always linear for std::forward_list
  3. ↑ 3.0 3.1 3.2 (until C++11) Not strictly constant

Given

  • i and j, objects of a container's iterator type,

in the expressions i == j, i != j, i < j, i <= j, i >= j, i > j, i - j, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

Optional container requirements (since C++20)

The following operations are provided only for some types of containers.

If the iterators passed to std::lexicographical_compare_three_way meet the ConstexprIterator then the operations described below are implemented by constexpr functions.

Category Description
Expression a <=> b
Returns std::lexicographical_compare_three_way(a.begin(), a.end(),
                                       b.begin(), b.end(),
                                       /*synth-three-way*/)
Result /*synth-three-way-result*/<C::value_type>
Preconditions Either T models three_way_comparable, or < is defined for values of (possibly const) type T and < is a total ordering relationship.
Complexity Linear

Container data races

see container thread safety

Other requirements

C (Container)

T (Type)

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 179 C++98 iterator and const_iterator types might be incomparable required to be comparable
LWG 276 C++98 T was required to be CopyAssignable T is required to be
CopyConstructible
LWG 322 C++98 the value types of iterator and const_iterator were not specified specified as T
LWG 774 C++98 there was no requirement on swap(a, b) added
LWG 883 C++98 a.swap(b) was defined as swap(a, b),
resulted in circular definition 		defined as exchanging
the values of a and b
LWG 1319 C++98 iterator and const_iterator
might not have multipass guarantee 		they are required to satisfy
the requirements of
LegacyForwardIterator
LWG 2263 C++11 the resolution of LWG issue 179 was accidentally dropped in C++11 restored
LWG 2839 C++11 self move assignment of standard containers was not allowed allowed but the
result is unspecified

See also

C++ documentation for Containers library
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