C++ named requirements: SequenceContainer

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C++
C++ named requirements

A SequenceContainer is a Container that stores objects of the same type in a linear arrangement.

Requirements

Legend
X A sequence container class
T The element type of X
a A value of type X
u The name of a declared variable
A The allocator type of X :
i , j LegacyInputIterator s such that [ i , j ) is a valid range and that the iterators refer to elements implicitly convertible to value_type
rg (since C++23) A value of a type R that models container-compatible-range <T>
il (since C++11) An object of type std:: initializer_list < value_type >
n A value of type X::size_type
p A valid const iterator into a
q A valid dereferenceable const iterator into a
q1 , q2 Two const iterators into a such that [ q1 , q2 ) is a valid range
t An lvalue or const rvalue (since C++11) of type X::value_type
rv (since C++11) A non-const rvalue of type X::value_type
Args (since C++11) A template parameter pack
args (since C++11) A function parameter pack with the pattern Arg&&

The type X satisfies SequenceContainer if

  • The type X satisfies Container , and
  • The following statements and expressions must be valid and have their specified effects for all sequence containers except std::array (see notes ) (since C++11) :
Statement Effects Conditions [1]
X u ( n, t ) Constructs the sequence container holding n copies of t Pre T is CopyInsertable into X
Post std:: distance ( u. begin ( ) , u. end ( ) )
== n
X u ( i, j ) Constructs the sequence container equal, element-wise, to the range [ i , j ) Pre T is EmplaceConstructible from * i into X
Post std:: distance ( u. begin ( ) , u. end ( ) )
== std:: distance ( i, j )
Expression Type Effects Conditions
X ( std:: from_range , rg )
(since C++23) 		X Constructs the sequence container equal, element-wise, to the range rg Pre T is EmplaceConstructible into X from * ranges:: begin ( rg )
Post
X ( il )
(since C++11) 		X Equivalent to X ( il. begin ( ) ,
il. end ( ) ) 		No explicit requirement
a = il
(since C++11) 		X& Assigns the range represented by il into a . [2] Pre T is CopyInsertable and CopyAssignable
Post Existing elements of a are destroyed or assigned to
a. emplace ( p, args )
(since C++11) 		iterator Insert an object of type T , constructed with std:: forward < Args > ( args ) before p Pre T is EmplaceConstructible
Post The returned iterator points at the element constructed from args into a
a. insert ( p, t ) iterator Inserts a copy of t before p Pre T is CopyInsertable
Post The returned iterator points at the copy of t inserted into a
a. insert ( p, rv )
(since C++11) 		iterator Inserts a copy of rv before p , possibly using move semantics Pre T is MoveInsertable
Post The returned iterator points at the copy of rv inserted into a
a. insert ( p, n, t ) iterator Inserts n copies of t before p Pre T is CopyInsertable and CopyAssignable
Post The returned iterator points at the copy of the first element inserted into a or is p for n == 0
a. insert ( p, i, j ) iterator Inserts copies of elements in
[ i , j ) before p 		Pre T is EmplaceConstructible and i and j are not in a
Post
  • Each iterator in [ i , j ) is dereferenced once.
  • The returned iterator points at the copy of the first element inserted into a or is p for i == j
a. insert_range ( p, rg )
(since C++23) 		iterator Inserts copies of elements in rg before p Pre
Post
  • Each iterator in the range rg is dereferenced once
  • The returned iterator points at the copy of the first element inserted into a or at p if rg is empty
a. insert ( p, il )
(since C++11) 		iterator Equivalent to a. insert ( p,
il. begin ( ) ,
il. end ( ) ) 		Pre No explicit requirement
Post The returned iterator points at the copy of the first element inserted into a or is p if il is empty
a. erase ( q ) iterator Erases the element pointed to by q Pre No explicit requirement
Post The returned iterator points at the element that was immediately following q prior to erasure, or a. end ( ) if no such element exists
a. erase ( q1, q2 ) iterator Erases elements in [ q1 , q2 ) Pre No explicit requirement
Post The returned iterator points at the element that was pointed by q2 prior to any erasure, or a. end ( ) if no such element exists
a. clear ( ) void Destroys all elements in a Pre No explicit requirement
Post
  • All references, pointers, and iterators are invalidated, including the end iterator
  • a. empty ( ) is true
a. assign ( i, j ) void Replaces elements in a with a copy of [ i , j ) Pre
Post Each iterator in [ i , j ) is dereferenced once.
a. assign_range ( rg )
(since C++23) 		void Replaces elements in a with a copy of each element in rg Pre
Post
  • Each iterator in the range rg is dereferenced once.
  • All references, pointers, and iterators are invalidated.
a. assign ( il )
(since C++11) 		void Equivalent to

a. assign ( il. begin ( ) ,
il. end ( ) )

No explicit requirement
a. assign ( n, t ) void Replaces elements in a with n copies of t Pre T is CopyInsertable and CopyAssignable
Post No explicit requirement
Notes
  1. ↑ For an expression whose effect is equivalent to some other operations, the conditions of the expressions inside those operations are inherited on top of the conditions listed in the table.
  2. ↑ std::array supports assignment from a braced-init-list, but not from an std::initializer_list .
  3. ↑ T and R are types such that std:: assignable_from < T & , ranges:: range_reference_t < R >> is modeled.

Optional operations

The following expressions must be valid and have their specified effects for the sequence containers named, all operations except prepend_range and append_range (since C++23) take amortized constant time:

Expression Type Effects Preconditions [1] Containers
a. front ( ) reference , or

const_reference for const a

Returns * a. begin ( ) No explicit requirement std::basic_string , std::array , std::deque , std::forward_list , std::inplace_vector , std::list , std::vector
a. back ( ) reference , or

const_reference for const a

Equivalent to auto tmp = a. end ( ) ;
-- tmp ;
return * tmp ; 		No explicit requirement std::basic_string , std::array , std::deque , std::inplace_vector , std::list , std::vector
a. emplace_front ( args )
(since C++11) 		void Prepends a T constructed with std:: forward < Args >
( args ) ... 		T is EmplaceConstructible into X from args std::deque , std::forward_list , std::list
a. emplace_back ( args )
(since C++11) 		void Appends a T constructed with std:: forward < Args >
( args ) ... 		T is EmplaceConstructible into X from args std::deque , std::inplace_vector , std::list , std::vector
a. push_front ( t ) void Prepends a copy of t T is CopyInsertable into X std::deque , std::forward_list , std::list
a. push_front ( rv )
(since C++11) 		void Prepends a copy of rv , possibly using move semantics T is MoveInsertable into X
a. prepend_range ( rg )
(since C++23) 		void Inserts [2] copies of elements in rg before begin ( ) , each iterator in rg is dereferenced once T is EmplaceConstructible into X from * ranges:: begin ( rg ) std::deque , std::forward_list , std::list
a. push_back ( t ) void Appends a copy of t T is CopyInsertable into X std::basic_string , std::deque , std::inplace_vector , std::list , std::vector
a. push_back ( rv )
(since C++11) 		void Appends a copy of rv , possibly using move semantics T is MoveInsertable into X
a. append_range ( rg )
(since C++23) 		void Inserts [2] copies of elements in rg before end ( ) dereferencing each iterator in rg once T is EmplaceConstructible into X from * ranges:: begin ( rg ) std::deque , std::inplace_vector , std::list , std::vector
a. pop_front ( ) void Destroys the first element a. empty ( ) is false std::deque , std::forward_list , std::list
a. pop_back ( ) void Destroys the last element a. empty ( ) is false std::basic_string , std::deque , std::inplace_vector , std::list , std::vector
a [ n ] reference , or

const_reference for const a

Returns * ( a. begin ( ) + n ) No explicit requirement std::basic_string , std::array , std::deque , std::inplace_vector , std::vector
a. at ( n ) reference , or

const_reference for const a

Returns * ( a. begin ( ) + n ) , throws std::out_of_range if n >= size ( ) No explicit requirement
Notes
  1. ↑ For an expression whose effect is equivalent to some other operations, the preconditions of the expressions inside those operations are inherited on top of the preconditions listed in the table.
  2. ↑ 2.0 2.1 Insertion order, relative to order of elements in rg , is non-reversing.

Additionally, for every sequence container:

  • A constructor template that takes two input iterators and the member function template overloads of insert , append , assign , replace that take two input iterators do not participate in overload resolution if the corresponding template argument does not satisfy LegacyInputIterator .
  • A deduction guide that has either a LegacyInputIterator or an Allocator template parameter does not participate in overload resolution if the type that does not qualify as an input iterator or an allocator respectively is deduced for that parameter.
(since C++17)

Standard library

The following standard library string types and containers satisfy the SequenceContainer requirements:

stores and manipulates sequences of characters
(class template)
(C++11)
fixed-sized inplace contiguous array
(class template)
dynamic contiguous array
(class template)
(C++26)
dynamically-resizable, fixed capacity, inplace contiguous array
(class template)
double-ended queue
(class template)
(C++11)
singly-linked list
(class template)
doubly-linked list
(class template)

Usage notes

Container Pros Cons
std::vector Fast access Mostly inefficient insertions/deletions
std:: inplace_vector Fast access, inplace contiguous storage Fixed capacity and mostly inefficient insertions/deletions
std::array Fast access, inplace contiguous storage Fixed number of elements and no insertion/deletion
std::deque Fast access, efficient insertion/deletion at the beginning/end Inefficient insertion/deletion in the middle of the sequence
std::list
std::forward_list 		Efficient insertion/deletion in the middle of the sequence Access is mostly linear-time

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 139 C++98 the optional operations were not required to
be implemented for the designated containers 		required with
amortized time
LWG 149 C++98 a. insert ( p, t ) returned iterator while
a. insert ( p, n, t ) and a. insert ( p, n, t ) returned void 		they all return
iterator
LWG 151 C++98 q1 was required to be dereferenceable [1] it can be non-dereferenceable
LWG 355 C++98 calling a. back ( ) or a. pop_back ( ) would
execute -- a. end ( ) , which is dangerous [2] 		decrements a copy
of a. end ( ) instead
LWG 589 C++98 the elements that i and j refer to
might not be convertible to value_type 		they are implicitly
convertible to value_type
LWG 2194 C++11 std::queue , std::priority_queue and
std::stack were also SequenceContainer s [3] 		they are not SequenceContainer s
LWG 3927 C++98 operator [ ] had no implicit requirement added the implicit requirement
  1. ↑ It is a defect because it makes the behavior of a. erase ( a. begin ( ) , a. end ( ) ) undefined is a is an empty container.
  2. ↑ If the type of a. end ( ) is a fundamental type, -- a. end ( ) is ill-formed. It is dangerous when the type of a is templated, in this case this bug can be difficult to be found.
  3. ↑ They were not documented as SequenceContainer s in C++98.
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