static_vector

A dynamically-resizable vector with fixed capacity and embedded storage (revision 3)

Document number: P0843r3.

Date: 2019-01-20.

Project: Programming Language C++, Library Working Group.

Audience: LEWG.

Reply-to: Gonzalo Brito Gadeschi <gonzalo.gadeschi at rwth-aachen dot de>.

Table of contents

Changelog

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.

Design Questions from LWG to LEWG

LWG asks LEWG to re-consider the following two design decisions:

  • In this document, exceeding the capacity in methods like static_vector::push_back is a pre-condition violation, that is, if the capacity is exceeded, the behavior is undefined. LWG suggested that exceeding the capacity in these methods should std::abort instead. The trade-offs in this space are discussed in Section 4.4 Exception safety of this proposal.

  • In this document, <static_vector> is a free-standing header, this is now clarified in Section 5. Technical Specification. LWG suggests that static_vector should be included in <vector> instead.

1. Introduction

This paper proposes a modernized version of boost::container::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.

2. Motivation and Scope

The static_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 static_vector elements is required to be within the static_vector object itself (e.g. to support memcopy for serialization purposes).

3. Existing practice

There are at least 3 widely used implementations of static_vector: Boost.Container [1], EASTL [2], and Folly [3]. The main difference between these is that Boost.Container implements static_vector as a standalone type with its own guarantees, while both EASTL and Folly implement it by adding an extra template parameter to their small_vector types.

A static_vector can also be poorly emulated by using a custom allocator, like for example Howard Hinnant's stack_alloc [4], on top of std::vector.

This proposal shares a similar purpose with P0494R0 [5] and P0597R0: std::constexpr_vector<T> [6]. The main difference is that this proposal closely follows boost::container::static_vector [1] and proposes to standardize existing practice. A prototype implementation of this proposal for standardization purposes is provided here: http://github.com/gnzlbg/fixed_capacity_vector.

4. Design Decisions

The most fundamental question that must be answered is:

Should static_vector be a standalone type or a special case of some other type?

The EASTL [2] and Folly [3] special case small_vector, e.g., using a 4th template parameter, to make it become a static_vector. The paper P0639R0: Changing attack vector of the constexpr_vector [7] proposes improving the Allocator concepts to allow static_vector, among others, to be implemented as a special case of std::vector with a custom allocator.

Both approaches run into the same fundamental issue: static_vector methods are identically-named to those of std::vector yet they have subtly different effects, exception-safety, iterator invalidation, and complexity guarantees.

This proposal follows boost::container::static_vector<T,Capacity> [1] closely and specifies the semantics that static_vector ought to have as a standalone type. As a library component this delivers immediate value.

I hope that having the concise semantics of this type specified will also be helpful for those that want to generalize the Allocator interface to allow implementing static_vector as a std::vector with a custom allocator.

4.1 Storage/Memory Layout

The container models ContiguousContainer. The elements of the static_vector are contiguously stored and properly aligned within the static_vector object itself. The exact location of the contiguous elements within the static_vector is not specified. If the Capacity is zero the container has zero size:

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

This optimization is easily implementable, enables the EBO, and felt right.

4.2 Move semantics

The move semantics of static_vector<T, Capacity> are equal to those of std::array<T, Size>. That is, after

static_vector a(10);
static_vector b(std::move(a));

the elements of a have been moved element-wise into b, the elements of a are left in an initialized but unspecified state (have been moved from state), the size of a is not altered, and a.size() == b.size().

Note: this behavior differs from std::vector<T, Allocator>, in particular for the similar case in which std::allocator_traits<Allocator>::propagate_on_container_move_assignment is false. In this situation the state of std::vector is initialized but unspecified.

4.3 constexpr support

The API of static_vector<T, Capacity> is constexpr. If is_trivially_copyable_v<T> && is_default_constructible_v<T> is true, static_vectors can be seamlessly used from constexpr code. This allows using static_vector as a constexpr_vector to, e.g., implement other constexpr containers.

The implementation cost of this is small: the prototye implementation specializes the storage for trivial types to use a C array with value-initialized elements and a defaulted destructor.

This changes the algorithmic complexity of static_vector constructors for trivial-types from "Linear in N" to "Constant in Capacity. When the value-initialization takes place at run-time, this difference in behavior might be signficiant: static_vector<non_trivial_type, 38721943228473>(4) will only initialize 4 elements but static_vector<trivial_type, 38721943228473>(4) must value-initialize the 38721943228473 - 4 excess elements to be a valid constexpr constructor.

Very large static_vector's are not the target use case of this container class and will have, in general, worse performance than, e.g., std::vector (e.g. due to moves being O(N)).

Future improvements to constexpr (e.g. being able to properly use std::aligned_storage in constexpr contexts) allow improving the performance of static_vector in a backwards compatible way.

4.4 Exception Safety

The only operations that can actually fail within static_vector<value_type, Capacity> are:

  1. value_type's constructors/assignment/destructors/swap can potentially throw,

  2. Mutating operations exceeding the capacity (push_back, insert, emplace, static_vector(value_type, size), static_vector(begin, end)...).

  3. Out-of-bounds unchecked access:

    • 3.1 front/back/pop_back when empty, operator[] (unchecked random-access).

When value_type's operations are invoked, the exception safety guarantees of static_vector depend on whether these operations can throw. This is detected with noexcept.

Since its Capacity is fixed at compile-time, static_vector never dynamically allocates memory, the answer to the following question determines the exception safety for all other operations:

What should static_vector do when its Capacity is exceeded?

Three main answers were explored in the prototype implementation:

  1. Throw an exception.
  2. Abort the process.
  3. Make this a precondition violation.

Throwing an exception is appealing because it makes the interface slightly more similar to that of std::vector. However, which exception should be thrown? It cannot be std::bad_alloc, because nothing is being allocated. It could throw either std::out_of_bounds or std::logic_error but in any case the interface does not end up being equal to that of std::vector.

Aborting the process avoids the perils of undefined behavior but comes at the cost of enforcing a particular "error handling" mechanism in the implementation, which would not allow extending it to use, e.g., Contracts, in the future.

The alternative is to make not exceeding the capacity a precondition on the static_vector's methods. This approach allows implementations to provide good run-time diagnostics if they so desire, e.g., on debug builds by means of an assertion, and makes implementation that avoid run-time checks conforming as well. Since the mutating methods have a precondition, they have narrow contracts, and are not conditionally noexcept. This provides implementations that desire throwing an exception the freedom to do so, and it also provides the standard the freedom to improve these APIs by using contracts in the future.

This proposal previously chooses this path and makes exceeding the static_vector's capacity a precondition violation that results in undefined behavior. Throwing checked_xxx methods can be provided in a backwards compatible way.

4.5 Iterator invalidation

The iterator invalidation rules are different than those for std::vector, since:

  • moving a static_vector invalidates all iterators,
  • swapping two static_vectors invalidates all iterators, and
  • inserting elements at the end of an static_vector never invalidates iterators.

The following functions can potentially invalidate the iterators of static_vectors: resize(n), resize(n, v), pop_back, erase, and swap.

4.6 Naming

The static_vector name was chosen after considering the following names via a poll in LEWG:

  • array_vector: a vector whose storage is backed up by a raw array.
  • bounded_vector: clearly indicates that the the size of the vector is bounded.
  • fixed_capacity_vector: clearly indicates that the capacity is fixed.
  • static_capacity_vector: clearly indicates that the capacity is fixed at compile time (static is overloaded).
  • static_vector (Boost.Container): due to "static" / compile-time allocation of the elements. The term static is, however, overloaded in C++ (e.g. static memory?).
  • embedded_vector<T, Capacity>: since the elements are "embedded" within the fixed_capacity_vector object itself. Sadly, the name embedded is overloaded, e.g., embedded systems.
  • inline_vector: the elements are stored "inline" within the fixed_capacity_vector object itself. The term inline is, however, already overloaded in C++ (e.g. inline functions => ODR, inlining, inline variables).
  • stack_vector: to denote that the elements can be stored on the stack. Is confusing since the elements can be on the stack, the heap, or the static memory segment. It also has a resemblance with std::stack.
  • limited_vector
  • vector_n

The names static_vector and vector_n tied in the number of votes. Many users are already familiar with the most widely implementation of this container (boost::container::static_vector), which gives static_vector an edge over a completely new name.

4.7 Future extensions

The following extensions could be added in a backwards compatible way:

  • utilities for hiding the concrete type of vector-like containers (e.g. any_vector_ref<T>/any_vector<T>).

  • default-initialization of the vector elements (as opposed to value-initialization): e.g. by using a tagged constructor with a default_initialized_t tag.

  • tagged-constructor of the form static_vector(with_size_t, std::size_t N, T const& t = T()) to avoid the complexity introduced by initializer lists and braced initialization:

using vec_t = static_vector<std::size_t, Capacity>;
vec_t v0(2);  // two-elements: 0, 0
vec_t v1{2};  // one-element: 2
vec_t v2(2, 1);  // two-elements: 1, 1
vec_t v3{2, 1};  // two-elements: 2, 1

All these extensions are generally useful and not part of this proposal.

5. Technical specification

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

Note to LWG: one of the primary use cases for this container is embedded/freestanding. An alternative to adding a new <static_vector> header would be to add static_vector to any of the freestanding headers. None of the current freestanding headers is a good semantic fit.

5. Class template static_vector

Changes to library.requirements.organization.headers table "C++ library headers": add <static_vector>.

Changes to library.requirements.organization.compliance table "C++ headers for freestanding implementations": add row:

[static_vector] Static vector <static_vector>

Changes to container.requirements.general.

The note of Table "Container Requirements" should be changed to contain static_vector as well:

Those entries marked “(Note A)” or “(Note B)” have linear complexity for array and static_vector and have constant complexity for all other standard containers. [ Note: The algorithm equal() is defined in [algorithms]. — end note ]

5.1 Class template static_vector overview

    1. A static_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 static_vector object itself, meaning that that if v is a static_vector<T, N> then it obeys the identity &v[n] == &v[0] + n for all 0 <= n <= v.size().
    1. A static_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 (\ref{container.requirements}) and contiguous container (\ref{container.requirements.general}) requirements. Descriptions are provided here only for operations on static_vector that are not described in one of these tables or for operations where there is additional semantic information.
    1. Class static_vector relies on the implicitly-declared special member functions (\ref{class.default.ctor}, \ref{class.dtor}, and \ref{class.copy.ctor}) to conform to the container requirements table in \ref{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.
namespace std {

template <typename T, size_t N>
class static_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>;

// 5.2, copy/move construction:
constexpr static_vector() noexcept;
constexpr static_vector(const static_vector&);
constexpr static_vector(static_vector&&);
constexpr explicit static_vector(size_type n);
constexpr static_vector(size_type n, const value_type& value);
template <class InputIterator>
constexpr static_vector(InputIterator first, InputIterator last);
constexpr static_vector(const static_vector& other);
constexpr static_vector(static_vector&& other);
constexpr static_vector(initializer_list<value_type> il);

// 5.3, copy/move assignment:
constexpr static_vector& operator=(const static_vector& other)
  noexcept(is_nothrow_copy_assignable_v<value_type>);
constexpr static_vector& operator=(static_vector&& other);
  noexcept(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);

// 5.4, destruction
~static_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()        noexcept;
constexpr const_iterator         cend()    const noexcept;
constexpr const_reverse_iterator crbegin()       noexcept;
constexpr const_reverse_iterator crend()   const noexcept;

// 5.5, size/capacity:
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);

// 5.6, 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 void push_back(const value_type& x);
constexpr void 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(static_vector& x)
  noexcept(is_nothrow_swappable_v<value_type> &&
           is_nothrow_move_constructible_v<value_type>);
};

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

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

5.2 static_vector constructors

constexpr static_vector() noexcept;

  • Effects: Constructs an empty static_vector.

  • Ensures: empty().

  • Complexity: Constant.

constexpr static_vector(static_vector&& rv);

  • Effects: Constructs a static_vector by move-inserting the elements of rv.

  • Requires: std::is_move_constructivle<value_type>.

  • Ensures: The static_vector is equal to the value that rv had before this construction.

  • Complexity: Linear in rv.size().

constexpr explicit static_vector(size_type n);

  • Effects: Constructs a static_vector with n default-inserted elements.

  • Requires: std::is_default_constructible<value_type>.

  • Expects: n <= capacity().

  • Complexity: Linear in n.

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

  • Effects: Constructs a static_vector with n copies of value.

  • Requires: std::is_copy_constructible<value_type>

  • Expects: n <= capacity().

  • Complexity: Linear in n.

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

  • Effects: Constructs a static_vector equal to the range [first, last)

  • Requires: std::is_constructible<value_type, decltype(*first)>

  • Expects: distance(first, last) <= capacity()

  • Complexity: Linear in distance(first, last).

5.3 Destruction

~static_vector();

Effects: Destroys the static_vector and its elements.

Requires: This destructor is trivial if the destructor of value_type is trivial.

5.4 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.

  • Requires: std::is_default_constructible<value_type>.

  • Expects: sz <= capacity().

  • 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.

  • Requires: std::is_copy_constructible<value_type>.

  • Expects: sz <= capacity().

  • Complexity: Linear in sz.

5.5 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 static_vector, data() == addressof(front()).

  • Complexity: Constant time.

5.6 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? 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.

  • Expects: size() < capacity().

  • Requires: std::is_copy_constructible<value_type>.

  • 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.

  • Expects: n <= capacity() - size().

  • Requires: std::is_copy_constructible<value_type>.

  • 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.

  • Expects: size() < capacity().

  • Requires: std::is_move_constructible<value_type>.

  • 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.

  • Expects: distance(first, last) <= capacity() - size().

  • Requires: std::is_constructible<value_type, decltype(*first)>.

  • 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.

  • Expects: il.size() <= capacity() - size().

  • Requires: std::is_copy_constructible<value_type>.

  • 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.

  • Expects: size() < capacity().

  • Requires: std::is_constructible<value_type, Args...>.

  • 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.

  • Expects: size() < capacity().

  • Requires: std::is_constructible<value_type, Args...>.

  • Complexity: Constant.

  • Remarks: If an exception is thrown by value_type's constructor from args... there are no effects.

constexpr void push_back(const value_type& x);

  • Effects: Inserts a copy of x at the end.

  • Expects: size() < capacity().

  • Requires: std::is_copy_constructible<value_type>.

  • Complexity: Constant.

  • Remarks: If an exception is thrown by value_type's copy constructor there are no effects.

constexpr void push_back(value_type&& x);

  • Effects: Moves x to the end.

  • Expects: size() < capacity().

  • Requires: std::is_move_constructible<value_type>.

  • 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.

  • Expects: !empty().

  • Complexity: Constant.

constexpr iterator erase(const_iterator position);

  • Effects: Removes the element at position, destroys it, and invalidates references to elements after position.

  • Expects: position in range [begin(), end()].

  • 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.

  • Expects: [first, last] in range [begin(), end()].

  • 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(static_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().

5.7 static_vector specialized algorithms

template <typename T, size_t N>
constexpr void swap(static_vector<T, N>& x, 
                    static_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().

6. Acknowledgments

The following people have significantly contributed to the development of this proposal. This proposal is based on Boost.Container's boost::container::static_vector and my extensive usage of this class over the years. As a consequence the authors of Boost.Container (Adam Wulkiewicz, Andrew Hundt, and Ion Gaztanaga) have had a very significant indirect impact on this proposal. The implementation of libc++ std::vector and the libc++ test-suite have been used extensively while prototyping this proposal, such that its author, Howard Hinnant, has had a significant indirect impact on the result of this proposal as well. The following people provided valuable feedback that influenced some aspects of this proposal: Walter Brown, Zach Laine, Rein Halbersma, and Andrzej Krzemieński. But I want to wholeheartedly acknowledge Casey Carter for taking the time to do a very detailed analysis of the whole proposal, which was invaluable and reshaped it in fundamental ways.

7. References