inplace_vector

A dynamically-resizable vector with fixed capacity and embedded storage

Document number: P0843R7.
Date: 2023-06-16.
Authors: Gonzalo Brito Gadeschi, Timur Doumler, Nevin Liber, David Sankel <dsankel _at_ adobe.com>.
Reply to: Timur Doumler <papers _at_ timur.audio>.
Audience: LEWG.

Table of Contents

• inplace_vector
• Introduction
• Motivation and Scope
• Existing practice
• Design
  • Standalone or a special case another type?
  • Layout
  • Move semantics
  • constexpr support
  • Exception Safety
  • Fallible APIs
  • Fallible Unchecked APIs
  • Iterator invalidation
  • Freestanding
  • Return type of push_back
  • Summary of semantic differences with vector
• Technical specification
  • [library.requirements.organization.headers]
  • [library.requirements.organization.compliance]
  • [container.reqmts] General container requirements
  • [container.requirements.sequence.reqmts]
  • [containers.sequences.inplace_vector.syn] Header <inplace_vector> synopsis
  • [containers.sequences.inplace_vector] Class template inplace_vector
    • [containers.sequences.inplace_vector.overview] Overview`
    • [containers.sequences.inplace_vector.cons] Constructors, copy, and assignment
    • [containers.sequences.inplace_vector.destructor] Destruction
    • [containers.sequences.inplace_vector.capacity] Size and capacity
    • [containers.sequences.inplace_vector.data]
    • [containers.sequences.inplace_vector.modifiers]
    • [containers.sequences.inplace_vector.erasure] Erasure
    • [containers.sequences.inplace_vector.special] Specialized algorithms
• Acknowledgments

Changelog

• Revision 7 Varna 2023 draft
  • Rename static_vector to inplace_vector throughout.
  • Update try_push_back APIs to return T* with rationale.
  • Update push_back to throw std::bad_alloc with rationale .
  • Trivially-copyable if value_type is trivially-copyable.
  • Request LEWG poll regarding <vector> or <inplace_vector> header.
  • Make push_back return a reference
  • Update push_back_unchecked with benchmark data.
  • Add a specialization of hash like for std::vector.
  • Update operator!= to operator<=> using hidden friends for them.
  • Add deduction guides (there is only one, do we need more).
  • Update constexpr rationale to C++23.
• Revision 6: for Varna 2023 following Kona’s 2022 guidance
  • Updated push_back semantics to follow std::vector (note about exception to throw).
  • Added try_push_back returning an optional
  • Added push_back_unchecked: excedding capacity exhibits undefined behavior.
  • Added note about naming.
• Revision 5:
  • Update contact wording and contact data.
  • Removed naming discussion, since it was resolved (last available in P0843r4).
  • Removed future extensions discussion (last available in P0843r4).
  • Addressed LEWG feedback regarding move-semantics and exception-safety.
• Revision 4:
  • LEWG suggested that push_back should be UB when the capacity is exceeded
  • LEWG suggested that this should be a free-standing header
• Revision 3:
  • Include LWG design questions for LEWG.
  • Incorporates LWG feedback.
• Revision 2
  • Replace the placeholder name fixed_capacity_vector with static_vector
  • Remove at checked element access member function.
  • Add changelog section.
• Revision 1
  • Minor style changes and bugfixes.

Introduction

This paper proposes a modernized boost::static_vector<T, Capacity> [1]. That is, a dynamically-resizable vector with compile-time fixed capacity and contiguous embedded storage in which the elements are stored within the vector object itself.

Its API closely resembles that of std::vector<T, A>. It is a contiguous container with O(1) insertion and removal of elements at the end (non-amortized) and worst case O(size()) insertion and removal otherwise. Like std::vector, the elements are initialized on insertion and destroyed on removal. For trivial value_types, the vector is fully usable inside constexpr functions.

Motivation and Scope

The inplace_vector container is useful when:

• memory allocation is not possible, e.g., embedded environments without a free store, where only a stack and the static memory segment are available,
• memory allocation imposes an unacceptable performance penalty, e.g., with respect to latency,
• allocation of objects with complex lifetimes in the static-memory segment is required,
• std::array is not an option, e.g., if non-default constructible objects must be stored,
• a dynamically-resizable array is required within constexpr functions,
• the storage location of the inplace_vector elements is required to be within the inplace_vector object itself (e.g. to support memcpy for serialization purposes).

Existing practice

There are at least 3 widely used implementations of inplace_vector: Boost.Container [1], EASTL [2], and Folly [3]. Boost.Container implements inplace_vector as a standalone type with its own guarantees. EASTL and Folly implement it via an extra template parameter in their small_vector types.

Custom allocators like Howard Hinnant’s stack_alloc [4] emulate inplace_vector on top of std::vector, but as discussed in the next sections, this emulation is not great.

Other prior art includes:

• P0494R0: contiguous_container proposal [5]: proposes a Storage concept.
• P0597R0: std::constexpr_vector<T> [6]: proposes a vector that can only be used in constexpr contexts.

A reference implementation of this proposal is available here (godbolt).

Design

Standalone or a special case another type?

The EASTL [2] and Folly [3] special case small_vector, e.g., using a 4th template parameter, to make it become a inplace_vector. P0639R0: Changing attack vector of the constexpr_vector [7] proposes improving the Allocator concepts to allow implementing inplace_vector as a special case of vector with a custom allocator. Both approaches produce specializations of small_vector or vector whose methods subtly differ in terms of effects, exception-safety, iterator invalidation, and complexity guarantees.

This proposal closely follows boost::container::static_vector<T,Capacity> [1] and proposes inplace_vector as a standalone type.

Where possible, this proposal defines the semantics of inplace_vector to match vector. Providing the same programming model makes this type easier to teach, use, and makes it easy to “just change” one type on a program to, e.g., perform a performance experiment without accidentally introducing undefined behavior.

Layout

inplace_vector models ContiguousContainer. Its elements are stored and properly aligned within the inplace_vector object itself. If the Capacity is zero the container has zero size:

static_assert(is_empty_v<inplace_vector<T, 0>>); // for all T

The offset of the first element within inplace_vector is unspecified, and Ts are not allowed to overlap.

The layout differs from vector, since inplace_vector does not store the capacity field (it’s known from the template parameter).

If T is trivially-copyable or N == 0, then inplace_vector<T, N> is also trivially copyable to support HPC (such as copying one between a CPU and a GPU), serialization/deserialization, which is important for use cases like sending a vector via MPI_Send, etc.

// for all C:
static_assert(!is_trivially_copyable_v<T> || is_trivially_copyable_v<inplace_vector<T, C>> || N == 0);

Move semantics

A moved-from inplace_vector is left in a valid but unspecified state (option 3 below) unless T is trivially-copyable, in which case the size of the inplace_vector does not change (array semantics, option 2 below). That is:

inplace_vector a(10);
inplace_vector b(std::move(a));
assert(a.size() == 10); // MAY FAIL

moves a’s elements element-wise into b, and afterwards the size of the moved-from inplace_vector may have changed.

This prevents code from relying on the size staying the same (and therefore being incompatible with changing an inplace_vector type back to vector) without incuring the cost of having to clear the inplace_vector.

When T is trivially-copyable, array semantics are used to provide trivial move operations.

This is different from LEWG Kona '22 Polls (22 in person + 8 remote) and we’d like to poll on these semantics again:

• POLL: Moving a static_vector should empty it (vector semantics).

  Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
  9	10	4	2	2

• POLL: Moving a static_vector should leave it in a valid but unspecified state.

  Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
  6	9	1	5	6

Alternatives:

1. vector semantics: guarantees that inplace_vector is left empty (this happens with move assignment when using std::allocator<T> and always with move construction).
   • Pro: same programming model as vector.
   • Pro: increases safety by requiring users to re-initialize vector elements.
   • Con: clearing an inplace_vector is not free.
   • Con: inplace_vector<T, N> can no longer be made trivially copyable for a trivially copyable T, as the move operations can no longer be trivial.
2. array semantics: guarantees that size() of inplace_vector does not change, and that elements are left in their moved-from state.
   • Pro: no additional run-time cost incurred.
   • Con: different programming model than vector.
3. “valid but unspecified state”
   • Con: different programming model than vector and array, requires calling size()
   • Pro: code calling size() is correct for both vector and inplace_vector, enabling changing the type back and forth.

constexpr support

The API of inplace_vector<T, Capacity> can be used in constexpr-contexts if is_trivially_copyable_v<T>, is_default_constructible_v<T>, and is_trivially_destructible<T> are true.

The implementation cost of this is small. The prototye implementation specializes the storage to use a C array with value-initialized elements.

This negatively impacts the algorithmic complexity of inplace_vector constructors for these types from O(size) to O(capacity). When value-initialization takes place at run-time, this difference is significant.

Vectors with large capacity requirements are better served by vector instead.

Exception Safety

When using the inplace_vector APIs, the following types of failures are expected:

1. The value_type’s constructors/assignment/destructors/swap can potentially throw (depends on noexcept),
2. Mutating operations exceeding the capacity (push_back, insert, emplace, inplace_vector(value_type, size), inplace_vector(begin, end)…), and
3. Out-of-bounds unchecked access: front/back/pop_back when empty, operator[].

For inplace_vector, we choose the same semantics as for vector:

1. When value_type’s operations are invoked, inplace_vector exception-safety guarantees depend on whether these operations throw, which is detected with noexcept if the API has a narrow contract.
2. Mutating operations that exceed the capacity ask “allocator embedded within the inplace_vector”, which has run out of space, for more memory, and therefore throw std::bad_alloc like vector APIs do (e.g. a pmr “stack allocator” throws std::bad_alloc as well). This preserves the programming model from vector. std::bad_alloc also has the property that it never does an allocation itself.
3. Out-of-bounds unchecked access is a precondition violation.

Alternatives:

1. Throw std::bad_alloc when the inplace_vector is out-of-memory:
   • Pros: same programming model as vector.
2. Throw “some other exception” when the inplace_vector is out-of-memory:
   • Pros: to be determined.
   • Cons: different programming model as vector.
3. Abort the process
   • Pros: portability to embedded platforms without exception support
   • Cons: different programming model than vector
4. Precondition violation
   • Cons: different proramming model than vector, users responsible for checking before modifying vector size, etc.

Fallible APIs

We add the following new fallible APIs which, when the vector size equal its capacity, return nullptr (do not throw bad_alloc) without moving from the inputs, enabling them to be re-used:

constexpr T* inplace_vector<T, C>::try_push_back(const T& value);
constexpr T* inplace_vector<T, C>::try_push_back(T&& value);

template<class... Args>
constexpr T* try_emplace_back(Args&&... args);

They can be used as follows

T value = T();
if (!v.try_push_back(value)) {
    std::cerr << "Failed to insert " << value << std::endl; // value not moved-from
    std::terminate();
}

Fallible Unchecked APIs

We add the following new fallible unchecked APIs for which exceeding the capacity is a precondition violation:

constexpr T& inplace_vector<T, C>::push_back_unchecked(const T& value);
constexpr T& inplace_vector<T, C>::push_back_unchecked(T&& value);

template<class... Args>
constexpr T& emplace_back_unchecked(Args&&... args);

These APIs were requested in LEWG Kona '22 (22 in person + 8 remote):

• POLL: If static_vector has unchecked operations (e.g. push_back_unchecked), it is okay for checked operations (e.g. push_back) to throw when they run out of space.

  Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
  14	4	2	4	1

The potential impact of the three APIs on code generation is shown here, where the main difference between try_push_back and push_back_unchecked is the presenece of an extra branch in try_push_back.

The authors have made several attempts at micro-benchmarking the performance difference between push_back_unchecked and try_push_back, and the results will be presented to LEWG.

Iterator invalidation

inplace_vector iterator invalidation guarantees differ from std::vector:

• moving a inplace_vector invalidates all iterators, and
• swapping two inplace_vectors invalidates all iterators.

inplace_vector APIs that potentially invalidate iterators are: resize(n), resize(n, v), pop_back, erase, and swap.

Freestanding

The inplace_vector APIs are all freestanding.

LWG asked for inplace_vector to be part of the <vector> header.

Request LEWG to poll on that.

Return type of push_back

In C++20, both push_back and emplace_back were slated to return a reference (they used to both return void). Even with plenary approval, changing push_back turned out to be an ABI break that was backed out, leaving the situation where emplace_back returns a reference but push_back is still void. This ABI issue doesn’t apply to new types. Should push_back return a reference to be consistent with emplace_back, or should it be consistent with older containers?

Request LEWG to poll on that.

Summary of semantic differences with vector

Technical specification

Note to editor: This enhancement is a pure header-only addition to the C++ standard library as the freestanding <inplace_vector> header. It belongs in the “Sequence containers” (\ref{sequences}) part of the “Containers library” (\ref{containers}) as “Class template inplace_vector”.

• UNRESOLVED QUESTION: LWG recommends making this header part of <vector>.

Note: one of the primary use cases for this container is embedded/freestanding applications. Do we impact those by not putting it in a free-standing header and adding it to <vector> instead?

[library.requirements.organization.headers]

Add <inplace_vector> to [tab:headers.cpp].

[library.requirements.organization.compliance]

Add <inplace_vector> tp [tab:headers.cpp.fs]:

• [containers]
• Subclause: containers
• Headers: <inplace_vector>

[container.reqmts] General container requirements

http://eel.is/c++draft/container.reqmts

1. A type X meets the container requirements if the following types, statements, and expressions are well-formed and have the specified semantics.

typename X::value_type

• Result: T
• Preconditions: T is Cpp17Erasable from X (see [container.alloc.reqmts], below).

typename X::reference

• Result: T&

typename X::const_reference

• Result: const T&

typename X::iterator

• Result: A type that meets the forward iterator requirements ([forward.iterators]) with value type T. The type X::iterator is convertible to X::const_iterator.

typename X::const_iterator

• Result: A type that meets the requirements of a constant iterator and those of a forward iterator with value type T.

typename X::differcence_type

• Result: A signed integer type, identical to the difference type of X::iterator and X::const_iterator.

typename X::size_type

• Result: An unsigned integer type that can represent any non-negative value of X::difference_type.

X u;
X u = X();

• Postconditions: u.empty()
• Complexity: Constant.

X u(a);
X u = a;

• Preconditions: T is Cpp17CopyInsertable into X (see below).
• Postconditions: u == a
• Complexity: Linear.

X u(rv);
X u = rv;

• Postconditions: u is equal to the value that rv had before this construction.
• Complexity: Linear for array and inplace_vector and constant for all other standard containers.

a = rv

• Result: X&.
• Effects: All existing elements of a are either move assigned to or destroyed.
• Postconditions: If a and rv do not refer to the same object, a is equal to the value that rv had before this assignment.
• Complexity: Linear.

a.~X()

• Result: void
• Effects: Destroys every element of a; any memory obtained is deallocated.
• Complexity: Linear.

a.begin()

• Result: iterator; const_iterator for constant a.
• Returns: An iterator referring to the first element in the container.
• Complexity: Constant.

a.end()

• Result: iterator; const_iterator for constant a.
• Returns: An iterator which is the past-the-end value for the container.
• Complexity: Constant.

a.cbegin()

• Result: const_iterator.
• Returns: const_cast<X const&>(a).begin()
• Complexity: Constant.

a.cend()

• Result: const_iterator.
• Returns: const_cast<X const&>(a).end()
• Complexity: Constant.

i <=> j

• Result: strong_ordering.
• Constraints: X::iterator meets the random access iterator requirements.
• Complexity: Constant.

a == b

• Preconditions: T meets the Cpp17EqualityComparable requirements.
• Result: Convertible to bool.
• Returns: equal(a.begin(), a.end(), b.begin(), b.end()) [Note 1: The algorithm equal is defined in [alg.equal]. — end note]
• Complexity: Constant if a.size() != b.size(), linear otherwise.
• Remarks: == is an equivalence relation.

a != b

• Effects: Equivalent to !(a == b).

a.swap(b)

• Result: void
• Effects: Exchanges the contents of a and b.
• Complexity: Linear for array and inplace_vector, and constant for all other standard containers.

swap(a, b)

• Effects: Equivalent to a.swap(b).

r = a

• Result: X&.
• Postconditions: r == a.
• Complexity: Linear.

a.size()

• Result: size_type.
• Returns: distance(a.begin(), a.end()), i.e. the number of elements in the container.
• Complexity: Constant.
• Remarks: The number of elements is defined by the rules of constructors, inserts, and erases.

a.max_size()

• Result: size_type.
• Returns: distance(begin(), end()) for the largest possible container.
• Complexity: Constant.

a.empty()

• Result: Convertible to bool.
• Returns: a.begin() == a.end()
• Complexity: Constant.
• Remarks: If the container is empty, then a.empty() is true.

1. In the expressions

i == j
i != j
i < j
i <= j
i >= j
i > j
i <=> j
i - j

where i and j denote objects of a container’s iterator type, 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.

Unless otherwise specified, all containers defined in this Clause obtain memory using an allocator (see [allocator.requirements]).

[Note 2: In particular, containers and iterators do not store references to allocated elements other than through the allocator’s pointer type, i.e., as objects of type P or pointer_traits<P>::template rebind<unspecified>, where P is allocator_traits<allocator_type>::pointer. — end note]

Copy constructors for these container types obtain an allocator by calling allocator_traits<allocator_type>::select_on_container_copy_construction on the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. Such move construction of the allocator shall not exit via an exception. All other constructors for these container types take a const allocator_type& argument.

[Note 3: If an invocation of a constructor uses the default value of an optional allocator argument, then the allocator type must support value-initialization. — end note]

A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. The allocator may be replaced only via assignment or swap(). Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if

1. allocator_­traits<allocator_­type>​::​propagate_­on_­container_­copy_­assignment​::​value,
2. allocator_­traits<allocator_­type>​::​propagate_­on_­container_­move_­assignment​::​value, or
3. allocator_­traits<allocator_­type>​::​propagate_­on_­container_­swap​::​value
   is true within the implementation of the corresponding container operation. In all container types defined in this Clause, the member get_­allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.
4. The expression a.swap(b), for containers a and b of a standard container type other than array and inplace_vector, shall exchange the values of a and b without invoking any move, copy, or swap operations on the individual container elements. Lvalues of any Compare, Pred, or Hash types belonging to a and b shall be swappable and shall be exchanged by calling swap as described in [swappable.requirements]. If allocator_­traits<allocator_­type>​::​propagate_­on_­container_­swap​::​value is true, then lvalues of type allocator_­type shall be swappable and the allocators of a and b shall also be exchanged by calling swap as described in [swappable.requirements]. Otherwise, the allocators shall not be swapped, and the behavior is undefined unless a.get_­allocator() == b.get_­allocator(). Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value a.end() before the swap will have value b.end() after the swap.

[container.requirements.sequence.reqmts]

http://eel.is/c++draft/sequence.reqmts

sequence.reqmts.1: The library provides fourthe following basic kinds of sequence containers: vector, inplace_vector, forward_list, list, and deque.

sequence.reqmts.2: […] vector is the type of sequence container that should be used by default. inplace_vector should be used when the container has a fixed capacity known during translation. array should be used when the container has a fixed size known during translation.

[…]

The following operations are provided for some types of sequence containers but not others. An implementation shall implement them so as to take amortized constant time.

a.front()

• Result: reference; const_reference for constant a.
• Returns: *a.begin()
• Remarks: Required for basic_string, array, deque, forward_list, list, inplace_vector, and vector.

a.back()

• Result: reference; const_reference for constant a.
• Effects: Equivalent to:

  auto tmp = a.end();
  --tmp;
  return *tmp;
  

• Remarks: Required for basic_string, array, deque, list, inplace_vector, and vector.

a.emplace_front(args)

• Result: reference
• Preconditions: T is Cpp17EmplaceConstructible into X from args.
• Effects: Prepends an object of type T constructed with `std::forward<Args>(args)…``.
• Returns: `a.front()``.
• Remarks: Required for deque, forward_list, and list.

a.emplace_back(args)

• Result: reference
• Preconditions: T is Cpp17EmplaceConstructible into X from args. For vector and inplace_vector, T is also Cpp17MoveInsertable into X.
• Effects: Appends an object of type T constructed with std::forward<Args>(args)....
• Returns: `a.back()``.
• Remarks: Required for deque, list, inplace_vector, and vector.

a.push_front(t)

• Result: void
• Preconditions: T is Cpp17CopyInsertable into X.
• Effects: Prepends a copy of t.
• Remarks: Required for deque, forward_list, and list.

a.push_front(rv)

• Result: void
• Preconditions: T is Cpp17MoveInsertable into X.
• Effects: Prepends a copy of rv.
• Remarks: Required for deque, forward_list, and list.

a.prepend_range(rg)

• Result: void
• Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).
• Effects: Inserts copies of elements in rg before begin(). Each iterator in the range rg is dereferenced exactly once. [Note 3: The order of elements in rg is not reversed. — end note]
• Remarks: Required for deque, forward_list, and list.

a.push_back(t)

• Result: Reference
• Returns: a.back().
• Preconditions: T is Cpp17CopyInsertable into X.
• Effects: Appends a copy of t.
• Remarks: Required for basic_string, deque, list, inplace_vector, and vector.

a.push_back(rv)

• Result: Reference
• Returns: a.back()
• Preconditions: T is Cpp17MoveInsertable into X.
• Effects: Appends a copy of rv.
• Remarks: Required for basic_string, deque, list, inplace_vector, and vector.

a.append_range(rg)

• Result: void
• Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg). For vector and inplace_vector, T is also Cpp17MoveInsertable into X.
• Effects: Inserts copies of elements in rg before end(). Each iterator in the range rg is dereferenced exactly once.
• Remarks: Required for deque, list,inplace_vector, and vector.

a.pop_front()

• Result: void
• Preconditions: a.empty() is false.
• Effects: Destroys the first element.
• Remarks: Required for deque, forward_list, and list.

a.pop_back()

• Result: void
• Preconditions: a.empty() is false.
• Effects: Destroys the last element.
• Remarks: Required for basic_string, deque, list,inplace_vector, and vector.

a[n]

• Result: reference; const_reference for constant a
• Returns: *(a.begin() + n)
• Remarks: Required for basic_string, array, deque, inplace_vector, and vector.

a.at(n)

• Result: reference; const_reference for constant a
• Returns: *(a.begin() + n)
• Throws: out_of_range if n >= a.size().
• Remarks: Required for basic_string, array, deque, inplace_vector, and vector.

[containers.sequences.inplace_vector.syn] Header <inplace_vector> synopsis

namespace std {

template <typename T, size_t N>
class inplace_vector {
public:
// types:
using value_type = T;
using pointer = T*;
using const_pointer = const T*;
using reference = value_type&;
using const_reference = const value_type&;
using size_type =  size_t;
using difference_type = ptrdiff_t;
using iterator = implementation-defined;  // see [container.requirements]
using const_iterator = implementation-defined; // see [container.requirements]
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;

// [containers.sequences.inplace_vector.cons]
constexpr inplace_vector() noexcept;
constexpr inplace_vector(const inplace_vector&);
constexpr inplace_vector(inplace_vector&&)
noexcept(!N || is_nothrow_move_constructible_v<value_type>);
constexpr explicit inplace_vector(size_type n);
constexpr inplace_vector(size_type n, const value_type& value);
template <class InputIterator>
constexpr inplace_vector(InputIterator first, InputIterator last);
constexpr inplace_vector(initializer_list<value_type> il);

// [containers.sequences.inplace_vector.cons]
constexpr inplace_vector& operator=(const inplace_vector& other);
constexpr inplace_vector& operator=(inplace_vector&& other)
  noexcept(!N || is_nothrow_move_assignable_v<value_type>);
template <class InputIterator>
constexpr void assign(InputIterator first, InputIterator last);
constexpr void assign(size_type n, const value_type& u);
constexpr void assign(initializer_list<value_type> il);

// [containers.sequences.inplace_vector.destructor]
constexpr ~inplace_vector();

// iterators
constexpr iterator               begin()         noexcept;
constexpr const_iterator         begin()   const noexcept;
constexpr iterator               end()           noexcept;
constexpr const_iterator         end()     const noexcept;
constexpr reverse_iterator       rbegin()        noexcept;
constexpr const_reverse_iterator rbegin()  const noexcept;
constexpr reverse_iterator       rend()          noexcept;
constexpr const_reverse_iterator rend()    const noexcept;
constexpr const_iterator         cbegin()  const noexcept;
constexpr const_iterator         cend()    const noexcept;
constexpr const_reverse_iterator crbegin() const noexcept;
constexpr const_reverse_iterator crend()   const noexcept;

// [containers.sequences.inplace_vector.members] size/capacity
[[nodiscard]] constexpr bool empty() const noexcept;
constexpr size_type size() const noexcept;
static constexpr size_type max_size() noexcept;
static constexpr size_type capacity() noexcept;
constexpr void resize(size_type sz);
constexpr void resize(size_type sz, const value_type& c);

// [containers.sequences.inplace_vector.members] element and data access:
constexpr reference       operator[](size_type n);
constexpr const_reference operator[](size_type n) const;
constexpr reference       front();
constexpr const_reference front() const;
constexpr reference       back();
constexpr const_reference back() const;
constexpr       T* data()       noexcept;
constexpr const T* data() const noexcept;

// 5.7, modifiers:
constexpr iterator insert(const_iterator position, const value_type& x);
constexpr iterator insert(const_iterator position, value_type&& x);
constexpr iterator insert(const_iterator position, size_type n, const value_type& x);
template <class InputIterator>
  constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last);
constexpr iterator insert(const_iterator position, initializer_list<value_type> il);

template <class... Args>
  constexpr iterator emplace(const_iterator position, Args&&... args);
template <class... Args>
  constexpr reference emplace_back(Args&&... args);
constexpr T& push_back(const value_type& x);
constexpr T& push_back(value_type&& x);

constexpr void pop_back();
constexpr iterator erase(const_iterator position);
constexpr iterator erase(const_iterator first, const_iterator last);

constexpr void clear() noexcept;

constexpr void swap(inplace_vector& x)
  noexcept(is_nothrow_swappable_v<value_type> &&
           is_nothrow_move_constructible_v<value_type>);
};

template<class InputIterator>
inplace_vector(InputIterator, InputIterator)
-> inplace_vector<iter-value-type<InputIterator>>;

template <typename T, size_t N>
constexpr bool operator==(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);
template <typename T, size_t N>
constexpr bool operator!=(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);
template <typename T, size_t N>
constexpr bool operator<(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);
template <typename T, size_t N>
constexpr bool operator<=(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);
template <typename T, size_t N>
constexpr bool operator>(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);
template <typename T, size_t N>
constexpr bool operator>=(const inplace_vector<T, N>& a, const inplace_vector<T, N>& b);

// 5.8, specialized algorithms:
template <typename T, size_t N>
constexpr void swap(inplace_vector<T, N>& x, inplace_vector<T, N>& y)
  noexcept(noexcept(x.swap(y)));

}  // namespace std

[containers.sequences.inplace_vector] Class template inplace_vector

[containers.sequences.inplace_vector.overview] Overview`

1. A inplace_vector is a contiguous container that supports constant time insert and erase operations at the end; insert and erase in the middle take linear time. Its capacity is part of its type and its elements are stored within the inplace_vector object itself, meaning that that if v is a inplace_vector<T, N> then it obeys the identity &v[n] == &v[0] + n for all 0 <= n <= v.size().
2. A inplace_vector satisfies the container requirements
   (\ref{container.requirements}) with the exception of the swap member function, whose complexity is linear instead of constant. It satisfies the sequence container requirements, including the optional sequence container requirements (\ref{sequence.reqmts}), with the exception of the push_front, pop_front, and emplace_front member functions, which are not provided. It satisfies the reversible container ([container.requirements]) and contiguous container ([container.requirements.general]) requirements. Descriptions are provided here only for operations on inplace_vector that are not described in one of these tables or for operations where there is additional semantic information.
3. Class inplace_vector relies on the implicitly-declared special member functions [class.default.ctor], [class.dtor], and [class.copy.ctor] to conform to the container requirements table in [container.requirements]. In addition to the requirements specified in the container requirements table, the move constructor and move assignment operator for array require that T be Cpp17MoveConstructible or Cpp17MoveAssignable, respectively.
4. The types iterator and const_iterator meet the constexpr iterator
   requirements ([iterator.requirements.general])."

[containers.sequences.inplace_vector.cons] Constructors, copy, and assignment

constexpr inplace_vector() noexcept;

• Effects: Constructs an empty inplace_vector.
• Postconditions: empty() is true.
• Complexity: Constant.

constexpr inplace_vector(inplace_vector&& rv);

• Effects: Constructs a inplace_vector by move-inserting the elements of rv.
• Mandates: is_move_constructible_v<value_type> is true.
• Postconditions: The inplace_vector is equal to the value that rv had before this construction.
• Complexity: Linear in rv.size().

constexpr explicit inplace_vector(size_type n);

• Effects: Constructs a inplace_vector with n default-inserted elements.
• Mandates: is_default_constructible_v<value_type> is true.
• Preconditions: n <= capacity() is true.
• Complexity: Linear in n.

constexpr inplace_vector(size_type n, const value_type& value);

• Effects: Constructs a inplace_vector with n copies of value.
• Mandates: is_copy_constructible_v<value_type> is true.
• Preconditions: n <= capacity() is true.
• Complexity: Linear in n.

template <class InputIterator>
constexpr inplace_vector(InputIterator first, InputIterator last);

• Effects: Constructs a inplace_vector equal to the range [first, last).
• Mandates: is_constructible_v<value_type, decltype(*first)> is true.
• Preconditions: distance(first, last) <= capacity() is true.
• Complexity: Linear in distance(first, last).

[containers.sequences.inplace_vector.destructor] Destruction

constexpr ~inplace_vector();

• Effects: Destroys the inplace_vector and its elements.
• Remarks: This destructor is trivial if the destructor of value_type is trivial.

[containers.sequences.inplace_vector.capacity] Size and capacity

static constexpr size_type capacity() noexcept
static constexpr size_type max_size() noexcept

• Returns: N.

constexpr void resize(size_type sz);

• Effects: If sz < size(), erases the last size() - sz elements from the sequence. Otherwise, appends sz - size() default-constructed elements.
• Mandates: is_default_constructible_v<value_type> is true.
• Preconditions: sz <= capacity() is true.
• Complexity: Linear in sz.

constexpr void resize(size_type sz, const value_type& c);

• Effects: If sz < size(), erases the last size() - sz elements from the sequence. Otherwise, appends sz - size() copies of c.
• Mandates: is_copy_constructible_v<value_type> is true.
• Preconditions: sz <= capacity() is true.
• Complexity: Linear in sz.

[containers.sequences.inplace_vector.data]

Element and data access

constexpr       T* data()       noexcept;
constexpr const T* data() const noexcept;

• Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty inplace_vector, data() == addressof(front()).
• Complexity: Constant time.

[containers.sequences.inplace_vector.modifiers]

Modifiers

Note to LWG: All modifiers have a pre-condition on not exceeding the
capacity() when inserting elements. That is, exceeding the capacity() of the vector is undefined behavior. This supports some of the major use cases of this container (embedded, freestanding, etc.) and was required by the stakeholders during LEWG review. Currently, this provides maximum freedom to the implementation to choose an appropriate behavior: abort, assert, throw an exception (which exception? bad_alloc? std::length_error? logic_error? out_of_bounds? etc.). In the future, this freedom allows us to specify these pre-conditions using contracts.

Note to LWG: Because all modifiers have preconditions, they all have narrow contracts and are not unconditionally noexcept.

constexpr iterator insert(const_iterator position, const value_type& x);

• Effects: Inserts x at position and invalidates all references to elements after position.
• Preconditions: size() < capacity() is true.
• Mandates: is_copy_constructible_v<value_type> is true.
• Complexity: Linear in size().
• Remarks: If an exception is thrown by value_type’s copy constructor and is_nothrow_move_constructible_v<value_type> is true there are no effects. Otherwise, if an exception is thrown by value_type’s copy constructor the effects are unspecified.

constexpr iterator insert(const_iterator position, size_type n, const value_type& x);

• Effects: Inserts n copies of x at position and invalidates all references to elements after position.
• Preconditions: n <= capacity() - size() is true.
• Mandates: is_copy_constructible_v<value_type> is true.
• Complexity: Linear in size() and n.
• Remarks: If an exception is thrown by value_type’s copy constructor and is_nothrow_move_constructible_v<value_type> is true there are no effects. Otherwise, if an exception is thrown by value_type’s copy constructor the effects are unspecified.

constexpr iterator insert(const_iterator position, value_type&& x);

• Effects: Inserts x at position and invalidates all references to elements after position.
• Preconditions: size() < capacity() is true.
• Mandates: is_move_constructible_v<value_type> is true.
• Complexity: Linear in size().
• Remarks: If an exception is thrown by value_type’s move constructor the effects are unspecified.

template <typename InputIterator>
constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last);

• Effects: Inserts elements in range [first, last) at position and invalidates all references to elements after position.
• Preconditions: distance(first, last) <= capacity() - size() is true.
• Mandates: is_constructible_v<value_type, decltype(*first)> is true.
• Complexity: Linear in size() and distance(first, last).
• Remarks: If an exception is thrown by value_type constructor from decltype(*first) and is_nothrow_move_constructible_v<value_type> is true there are no effects. Otherwise, if an exception is thrown by value_type’s constructor from decltype(*first) the effects are unspecified.

constexpr iterator insert(const_iterator position, initializer_list<value_type> il);

• Effects: Inserts elements of il at position and invalidates all references to elements after position.
• Preconditions: il.size() <= capacity() - size() is true.
• Mandates: is_copy_constructible_v<value_type> is true.
• Complexity: Linear in size() and il.size().
• Remarks: If an exception is thrown by value_type’s copy constructor and is_nothrow_move_constructible_v<value_type> is true there are no effects. Otherwise, if an exception is thrown by value_type’s copy constructor the effects are unspecified.

template <class... Args>
constexpr iterator emplace(const_iterator position, Args&&... args);

• Effects: Inserts an element constructed from args... at position and invalidates all references to elements after position.
• Preconditions: size() < capacity() is true.
• Mandates: is_constructible_v<value_type, Args...> is true.
• Complexity: Linear in size().
• Remarks: If an exception is thrown by value_type’s constructor from args... and is_nothrow_move_constructible_v<value_type> is true there are no effects. Otherwise, if an exception is thrown by value_type’s constructor from args... the effects are unspecified.

template <class... Args>
constexpr reference emplace_back(Args&&... args);

• Effects: Inserts an element constructed from args... at the end.
• Preconditions: size() < capacity() is true.
• Mandates: is_constructible_v<value_type, Args...> is true.
• Complexity: Constant.
• Remarks: If an exception is thrown by value_type’s constructor from args... there are no effects.

constexpr reference push_back(const value_type& x);

• Effects: Inserts a copy of x at the end.
• Preconditions: size() < capacity() is true.
• Mandates: is_copy_constructible_v<value_type> is true.
• Complexity: Constant.
• Remarks: If an exception is thrown by value_type’s copy constructor there are no effects.

constexpr reference push_back(value_type&& x);

• Effects: Moves x to the end.
• Preconditions: size() < capacity() is true.
• Mandates: is_move_constructible_v<value_type> is true.
• Complexity: Constant.
• Remarks: If an exception is thrown by value_type’s move constructor there are no effects.

constexpr void pop_back();

• Effects: Removes the last element of the container and destroys it.
• Preconditions: !empty() is true.
• Complexity: Constant.

[containers.sequences.inplace_vector.erasure] Erasure

constexpr iterator erase(const_iterator position);

• Effects: Removes the element at position, destroys it, and invalidates references to elements after position.
• Preconditions: position in range [begin(), end()) is true.
• Complexity: Linear in size().
• Remarks: If an exception is thrown by value_type’s move constructor the effects are unspecified.

constexpr iterator erase(const_iterator first, const_iterator last);

• Effects: Removes the elements in range [first, last), destroying them, and invalidating references to elements after last.
• Preconditions: [first, last) in range [begin(), end()) is true.
• Complexity: Linear in size() and distance(first, last).
• Remarks: If an exception is thrown by value_type’s move constructor the effects are unspecified.

constexpr void swap(inplace_vector& x)
  noexcept(is_nothrow_swappable_v<value_type> &&
           is_nothrow_move_constructible_v<value_type>);

• Effects: Exchanges the contents of *this with x. All references to the elements of *this and x are invalidated.
• Complexity: Linear in size() and x.size().
• Remarks: Shall not participate in overload resolution unless is_move_constructible_v<value_type> is true and is_swappable_v<value_type> is true.

[containers.sequences.inplace_vector.special] Specialized algorithms

template <typename T, size_t N>
constexpr void swap(inplace_vector<T, N>& x,
                    inplace_vector<T, N>& y)
  noexcept(noexcept(x.swap(y)));

• Constraints: This function shall not participate in overload resolution unless is_swappable_v<T> is true.
• Effects: As if by x.swap(y).
• Complexity: Linear in size() and x.size().

Acknowledgments

This proposal is based on Boost.Container’s boost::container::static_vector, mainly authored by Adam Wulkiewicz, Andrew Hundt, and Ion Gaztanaga. The reference implementation is based on Howard Hinnant std::vector implementation in libc++ and its test-suite. The following people provided valuable feedback that influenced some aspects of this proposal: Walter Brown, Zach Laine, Rein Halbersma, Andrzej Krzemieński, Casey Carter and many others.