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C++ named requirements: SequenceContainer

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`: `X::allocator_type` if it exists, otherwise std::allocator<T>
`i`, `j`	LegacyInputIterators 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`.
`X u(n, t)`			Post	`std::distance(u.begin(), u.end()) == n` is `true`.
`X u(i, j)`		Constructs the sequence container equal, element-wise, to the range `[i,` `j)`.	Pre	`T` is EmplaceConstructible from `*i` into `X`.
`X u(i, j)`			Post	`std::distance(u.begin(), u.end()) == std::distance(i, j)` is `true`.
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)`.
`X(std::from_range, rg)` (since C++23)	`X`		Post	Each iterator in the range `rg` is dereferenced once. `std::distance(begin(), end()) == ranges::distance(rg)` is `true`.
`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.
`a = il` (since C++11)	`X&`	Assigns the range represented by `il` into `a`.^[2]	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.
`a.emplace(p, args)` (since C++11)	`iterator`		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.
`a.insert(p, t)`	`iterator`	Inserts a copy of `t` before `p`.	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.
`a.insert(p, rv)` (since C++11)	`iterator`		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.
`a.insert(p, n, t)`	`iterator`	Inserts `n` copies of `t` before `p`.	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`.
`a.insert(p, i, j)`	`iterator`	Inserts copies of elements in `[i,` `j)` before `p`.	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	`T` is EmplaceConstructible into `X` from `*ranges::begin(rg)`. `rg` and `a` do not overlap.
`a.insert_range(p, rg)` (since C++23)	`iterator`	Inserts copies of elements in `rg` before `p`.	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
`a.insert(p, il)` (since C++11)	`iterator`	Equivalent to `a.insert(p, il.begin(), il.end())`.	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
`a.erase(q)`	`iterator`	Erases the element pointed to by `q`.	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
`a.erase(q1, q2)`	`iterator`	Erases elements in `[q1,` `q2)`.	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
`a.clear()`	void	Destroys all elements in `a`.	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	`T` is EmplaceConstructible. `i` and `j` are not in `a`.
`a.assign(i, j)`	void	Replaces elements in `a` with a copy of `[i,` `j)`.	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	`T`^[3] is EmplaceConstructible into `X` from `*ranges::begin(rg)`. `rg` and `a` do not overlap.
`a.assign_range(rg)` (since C++23)	void		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.
`a.assign(n, t)`	void	Replaces elements in `a` with `n` copies of `t`.	Post	No explicit requirement
Notes
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. `std::array` supports assignment from a braced-init-list, but not from an `std::initializer_list`. `T` and `R` are types such that `ranges::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::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::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::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`.	`std::deque` `std::forward_list` `std::list`
`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::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`.	`std::basic_string` `std::deque` `std::list` `std::vector`
`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::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::list` `std::vector`
`a[n]`	`reference`, or `const_reference` for const `a`	Equivalent to `return *(a.begin() + n);`.	No explicit requirement	`std::basic_string` `std::array` `std::deque` `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	`std::basic_string` `std::array` `std::deque` `std::vector`
Notes
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. 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)

Sequence containers in the standard library

basic_string	stores and manipulates sequences of characters (class template)
array (C++11)	static contiguous array (class template)
vector	dynamic contiguous array (class template)
deque	double-ended queue (class template)
forward_list (C++11)	singly-linked list (class template)
list	doubly-linked list (class template)

Trade-offs / usage notes

`std::vector`	Fast access but mostly inefficient insertions/deletions
`std::array`	Fast access but fixed number of elements
`std::list` `std::forward_list`	Efficient insertion/deletion in the middle of the sequence
`std::deque`	Efficient insertion/deletion at the beginning and at the end of the sequence

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 3927	C++98	operator[] had no implicit requirement	added the implicit requirement

It is a defect because it makes the behavior of a.erase(a.begin(), a.end()) undefined is a is an empty container.
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.