small_vector.h

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/*
 * Copyright 2011-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * For high-level documentation and usage examples see
 * folly/docs/small_vector.md
 *
 * @author Jordan DeLong <delong.j@fb.com>
 */

#pragma once

#include <algorithm>
#include <cassert>
#include <cstdlib>
#include <cstring>
#include <iterator>
#include <stdexcept>
#include <type_traits>
#include <utility>

#include <boost/mpl/count.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/filter_view.hpp>
#include <boost/mpl/front.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/operators.hpp>

#include <folly/ConstexprMath.h>
#include <folly/FormatTraits.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Traits.h>
#include <folly/lang/Assume.h>
#include <folly/lang/Exception.h>
#include <folly/memory/Malloc.h>
#include <folly/portability/Malloc.h>

#if (FOLLY_X64 || FOLLY_PPC64)
#define FOLLY_SV_PACK_ATTR FOLLY_PACK_ATTR
#define FOLLY_SV_PACK_PUSH FOLLY_PACK_PUSH
#define FOLLY_SV_PACK_POP FOLLY_PACK_POP
#else
#define FOLLY_SV_PACK_ATTR
#define FOLLY_SV_PACK_PUSH
#define FOLLY_SV_PACK_POP
#endif

// Ignore shadowing warnings within this file, so includers can use -Wshadow.
FOLLY_PUSH_WARNING
FOLLY_GNU_DISABLE_WARNING("-Wshadow")

namespace folly {


namespace small_vector_policy {


/*
 * A flag which makes us refuse to use the heap at all.  If we
 * overflow the in situ capacity we throw an exception.
 */
struct NoHeap;


} // namespace small_vector_policy


template <class T, std::size_t M, class A, class B, class C>
class small_vector;


namespace detail {

/*
 * Move objects in memory to the right into some uninitialized
 * memory, where the region overlaps.  This doesn't just use
 * std::move_backward because move_backward only works if all the
 * memory is initialized to type T already.
 */
template <class T>
typename std::enable_if<
    std::is_default_constructible<T>::value &&
    !folly::is_trivially_copyable<T>::value>::type
moveObjectsRight(T* first, T* lastConstructed, T* realLast) {
  if (lastConstructed == realLast) {
    return;
  }

  T* end = first - 1; // Past the end going backwards.
  T* out = realLast - 1;
  T* in = lastConstructed - 1;
  try {
    for (; in != end && out >= lastConstructed; --in, --out) {
      new (out) T(std::move(*in));
    }
    for (; in != end; --in, --out) {
      *out = std::move(*in);
    }
    for (; out >= lastConstructed; --out) {
      new (out) T();
    }
  } catch (...) {
    // We want to make sure the same stuff is uninitialized memory
    // if we exit via an exception (this is to make sure we provide
    // the basic exception safety guarantee for insert functions).
    if (out < lastConstructed) {
      out = lastConstructed - 1;
    }
    for (auto it = out + 1; it != realLast; ++it) {
      it->~T();
    }
    throw;
  }
}

// Specialization for trivially copyable types.  The call to
// std::move_backward here will just turn into a memmove.  (TODO:
// change to std::is_trivially_copyable when that works.)
template <class T>
typename std::enable_if<
    !std::is_default_constructible<T>::value ||
    folly::is_trivially_copyable<T>::value>::type
moveObjectsRight(T* first, T* lastConstructed, T* realLast) {
  std::move_backward(first, lastConstructed, realLast);
}

/*
 * Populate a region of memory using `op' to construct elements.  If
 * anything throws, undo what we did.
 */
template <class T, class Function>
void populateMemForward(T* mem, std::size_t n, Function const& op) {
  std::size_t idx = 0;
  try {
    for (size_t i = 0; i < n; ++i) {
      op(&mem[idx]);
      ++idx;
    }
  } catch (...) {
    for (std::size_t i = 0; i < idx; ++i) {
      mem[i].~T();
    }
    throw;
  }
}

template <class SizeType, bool ShouldUseHeap>
struct IntegralSizePolicyBase {
  typedef SizeType InternalSizeType;

  IntegralSizePolicyBase() : size_(0) {}

 protected:
  static constexpr std::size_t policyMaxSize() {
    return SizeType(~kExternMask);
  }

  std::size_t doSize() const {
    return size_ & ~kExternMask;
  }

  std::size_t isExtern() const {
    return kExternMask & size_;
  }

  void setExtern(bool b) {
    if (b) {
      size_ |= kExternMask;
    } else {
      size_ &= ~kExternMask;
    }
  }

  void setSize(std::size_t sz) {
    assert(sz <= policyMaxSize());
    size_ = (kExternMask & size_) | SizeType(sz);
  }

  void swapSizePolicy(IntegralSizePolicyBase& o) {
    std::swap(size_, o.size_);
  }

 protected:
  static bool constexpr kShouldUseHeap = ShouldUseHeap;

 private:
  static SizeType constexpr kExternMask =
      kShouldUseHeap ? SizeType(1) << (sizeof(SizeType) * 8 - 1) : 0;

  SizeType size_;
};

template <class SizeType, bool ShouldUseHeap>
struct IntegralSizePolicy;

template <class SizeType>
struct IntegralSizePolicy<SizeType, true>
    : public IntegralSizePolicyBase<SizeType, true> {
 public:
  /*
   * Move a range to a range of uninitialized memory.  Assumes the
   * ranges don't overlap.
   */
  template <class T>
  typename std::enable_if<!folly::is_trivially_copyable<T>::value>::type
  moveToUninitialized(T* first, T* last, T* out) {
    std::size_t idx = 0;
    try {
      for (; first != last; ++first, ++idx) {
        new (&out[idx]) T(std::move(*first));
      }
    } catch (...) {
      // Even for callers trying to give the strong guarantee
      // (e.g. push_back) it's ok to assume here that we don't have to
      // move things back and that it was a copy constructor that
      // threw: if someone throws from a move constructor the effects
      // are unspecified.
      for (std::size_t i = 0; i < idx; ++i) {
        out[i].~T();
      }
      throw;
    }
  }

  // Specialization for trivially copyable types.
  template <class T>
  typename std::enable_if<folly::is_trivially_copyable<T>::value>::type
  moveToUninitialized(T* first, T* last, T* out) {
    std::memmove(out, first, (last - first) * sizeof *first);
  }

  /*
   * Move a range to a range of uninitialized memory. Assumes the
   * ranges don't overlap. Inserts an element at out + pos using
   * emplaceFunc(). out will contain (end - begin) + 1 elements on success and
   * none on failure. If emplaceFunc() throws [begin, end) is unmodified.
   */
  template <class T, class EmplaceFunc>
  void moveToUninitializedEmplace(
      T* begin,
      T* end,
      T* out,
      SizeType pos,
      EmplaceFunc&& emplaceFunc) {
    // Must be called first so that if it throws [begin, end) is unmodified.
    // We have to support the strong exception guarantee for emplace_back().
    emplaceFunc(out + pos);
    // move old elements to the left of the new one
    try {
      this->moveToUninitialized(begin, begin + pos, out);
    } catch (...) {
      out[pos].~T();
      throw;
    }
    // move old elements to the right of the new one
    try {
      if (begin + pos < end) {
        this->moveToUninitialized(begin + pos, end, out + pos + 1);
      }
    } catch (...) {
      for (SizeType i = 0; i <= pos; ++i) {
        out[i].~T();
      }
      throw;
    }
  }
};

template <class SizeType>
struct IntegralSizePolicy<SizeType, false>
    : public IntegralSizePolicyBase<SizeType, false> {
 public:
  template <class T>
  void moveToUninitialized(T* /*first*/, T* /*last*/, T* /*out*/) {
    assume_unreachable();
  }
  template <class T, class EmplaceFunc>
  void moveToUninitializedEmplace(
      T* /* begin */,
      T* /* end */,
      T* /* out */,
      SizeType /* pos */,
      EmplaceFunc&& /* emplaceFunc */) {
    assume_unreachable();
  }
};

/*
 * If you're just trying to use this class, ignore everything about
 * this next small_vector_base class thing.
 *
 * The purpose of this junk is to minimize sizeof(small_vector<>)
 * and allow specifying the template parameters in whatever order is
 * convenient for the user.  There's a few extra steps here to try
 * to keep the error messages at least semi-reasonable.
 *
 * Apologies for all the black magic.
 */
namespace mpl = boost::mpl;
template <
    class Value,
    std::size_t RequestedMaxInline,
    class InPolicyA,
    class InPolicyB,
    class InPolicyC>
struct small_vector_base {
  typedef mpl::vector<InPolicyA, InPolicyB, InPolicyC> PolicyList;

  /*
   * Determine the size type
   */
  typedef typename mpl::filter_view<
      PolicyList,
      std::is_integral<mpl::placeholders::_1>>::type Integrals;
  typedef typename mpl::eval_if<
      mpl::empty<Integrals>,
      mpl::identity<std::size_t>,
      mpl::front<Integrals>>::type SizeType;

  static_assert(
      std::is_unsigned<SizeType>::value,
      "Size type should be an unsigned integral type");
  static_assert(
      mpl::size<Integrals>::value == 0 || mpl::size<Integrals>::value == 1,
      "Multiple size types specified in small_vector<>");

  /*
   * Determine whether we should allow spilling to the heap or not.
   */
  typedef typename mpl::count<PolicyList, small_vector_policy::NoHeap>::type
      HasNoHeap;

  static_assert(
      HasNoHeap::value == 0 || HasNoHeap::value == 1,
      "Multiple copies of small_vector_policy::NoHeap "
      "supplied; this is probably a mistake");

  /*
   * Make the real policy base classes.
   */
  typedef IntegralSizePolicy<SizeType, !HasNoHeap::value> ActualSizePolicy;

  /*
   * Now inherit from them all.  This is done in such a convoluted
   * way to make sure we get the empty base optimizaton on all these
   * types to keep sizeof(small_vector<>) minimal.
   */
  typedef boost::totally_ordered1<
      small_vector<Value, RequestedMaxInline, InPolicyA, InPolicyB, InPolicyC>,
      ActualSizePolicy>
      type;
};

template <class T>
T* pointerFlagSet(T* p) {
  return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(p) | 1);
}
template <class T>
bool pointerFlagGet(T* p) {
  return reinterpret_cast<uintptr_t>(p) & 1;
}
template <class T>
T* pointerFlagClear(T* p) {
  return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(p) & ~uintptr_t(1));
}
inline void* shiftPointer(void* p, size_t sizeBytes) {
  return static_cast<char*>(p) + sizeBytes;
}
} // namespace detail

FOLLY_SV_PACK_PUSH
template <
    class Value,
    std::size_t RequestedMaxInline = 1,
    class PolicyA = void,
    class PolicyB = void,
    class PolicyC = void>
class small_vector : public detail::small_vector_base<
                         Value,
                         RequestedMaxInline,
                         PolicyA,
                         PolicyB,
                         PolicyC>::type {
  typedef typename detail::
      small_vector_base<Value, RequestedMaxInline, PolicyA, PolicyB, PolicyC>::
          type BaseType;
  typedef typename BaseType::InternalSizeType InternalSizeType;

  /*
   * Figure out the max number of elements we should inline.  (If
   * the user asks for less inlined elements than we can fit unioned
   * into our value_type*, we will inline more than they asked.)
   */
  static constexpr std::size_t MaxInline{
      constexpr_max(sizeof(Value*) / sizeof(Value), RequestedMaxInline)};

 public:
  typedef std::size_t size_type;
  typedef Value value_type;
  typedef value_type& reference;
  typedef value_type const& const_reference;
  typedef value_type* iterator;
  typedef value_type* pointer;
  typedef value_type const* const_iterator;
  typedef value_type const* const_pointer;
  typedef std::ptrdiff_t difference_type;

  typedef std::reverse_iterator<iterator> reverse_iterator;
  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;

  small_vector() = default;
  // Allocator is unused here. It is taken in for compatibility with std::vector
  // interface, but it will be ignored.
  small_vector(const std::allocator<Value>&) {}

  small_vector(small_vector const& o) {
    auto n = o.size();
    makeSize(n);
    try {
      std::uninitialized_copy(o.begin(), o.end(), begin());
    } catch (...) {
      if (this->isExtern()) {
        u.freeHeap();
      }
      throw;
    }
    this->setSize(n);
  }

  small_vector(small_vector&& o) noexcept(
      std::is_nothrow_move_constructible<Value>::value) {
    if (o.isExtern()) {
      swap(o);
    } else {
      std::uninitialized_copy(
          std::make_move_iterator(o.begin()),
          std::make_move_iterator(o.end()),
          begin());
      this->setSize(o.size());
    }
  }

  small_vector(std::initializer_list<value_type> il) {
    constructImpl(il.begin(), il.end(), std::false_type());
  }

  explicit small_vector(size_type n) {
    doConstruct(n, [&](void* p) { new (p) value_type(); });
  }

  small_vector(size_type n, value_type const& t) {
    doConstruct(n, [&](void* p) { new (p) value_type(t); });
  }

  template <class Arg>
  explicit small_vector(Arg arg1, Arg arg2) {
    // Forward using std::is_arithmetic to get to the proper
    // implementation; this disambiguates between the iterators and
    // (size_t, value_type) meaning for this constructor.
    constructImpl(arg1, arg2, std::is_arithmetic<Arg>());
  }

  ~small_vector() {
    for (auto& t : *this) {
      (&t)->~value_type();
    }
    if (this->isExtern()) {
      u.freeHeap();
    }
  }

  small_vector& operator=(small_vector const& o) {
    if (FOLLY_LIKELY(this != &o)) {
      assign(o.begin(), o.end());
    }
    return *this;
  }

  small_vector& operator=(small_vector&& o) {
    // TODO: optimization:
    // if both are internal, use move assignment where possible
    if (FOLLY_LIKELY(this != &o)) {
      clear();
      swap(o);
    }
    return *this;
  }

  bool operator==(small_vector const& o) const {
    return size() == o.size() && std::equal(begin(), end(), o.begin());
  }

  bool operator<(small_vector const& o) const {
    return std::lexicographical_compare(begin(), end(), o.begin(), o.end());
  }

  static constexpr size_type max_size() {
    return !BaseType::kShouldUseHeap ? static_cast<size_type>(MaxInline)
                                     : BaseType::policyMaxSize();
  }

  size_type size() const {
    return this->doSize();
  }
  bool empty() const {
    return !size();
  }

  iterator begin() {
    return data();
  }
  iterator end() {
    return data() + size();
  }
  const_iterator begin() const {
    return data();
  }
  const_iterator end() const {
    return data() + size();
  }
  const_iterator cbegin() const {
    return begin();
  }
  const_iterator cend() const {
    return end();
  }

  reverse_iterator rbegin() {
    return reverse_iterator(end());
  }
  reverse_iterator rend() {
    return reverse_iterator(begin());
  }

  const_reverse_iterator rbegin() const {
    return const_reverse_iterator(end());
  }

  const_reverse_iterator rend() const {
    return const_reverse_iterator(begin());
  }

  const_reverse_iterator crbegin() const {
    return rbegin();
  }
  const_reverse_iterator crend() const {
    return rend();
  }

  /*
   * Usually one of the simplest functions in a Container-like class
   * but a bit more complex here.  We have to handle all combinations
   * of in-place vs. heap between this and o.
   *
   * Basic guarantee only.  Provides the nothrow guarantee iff our
   * value_type has a nothrow move or copy constructor.
   */
  void swap(small_vector& o) {
    using std::swap; // Allow ADL on swap for our value_type.

    if (this->isExtern() && o.isExtern()) {
      this->swapSizePolicy(o);

      auto thisCapacity = this->capacity();
      auto oCapacity = o.capacity();

      auto* tmp = u.pdata_.heap_;
      u.pdata_.heap_ = o.u.pdata_.heap_;
      o.u.pdata_.heap_ = tmp;

      this->setCapacity(oCapacity);
      o.setCapacity(thisCapacity);

      return;
    }

    if (!this->isExtern() && !o.isExtern()) {
      auto& oldSmall = size() < o.size() ? *this : o;
      auto& oldLarge = size() < o.size() ? o : *this;

      for (size_type i = 0; i < oldSmall.size(); ++i) {
        swap(oldSmall[i], oldLarge[i]);
      }

      size_type i = oldSmall.size();
      const size_type ci = i;
      try {
        for (; i < oldLarge.size(); ++i) {
          auto addr = oldSmall.begin() + i;
          new (addr) value_type(std::move(oldLarge[i]));
          oldLarge[i].~value_type();
        }
      } catch (...) {
        oldSmall.setSize(i);
        for (; i < oldLarge.size(); ++i) {
          oldLarge[i].~value_type();
        }
        oldLarge.setSize(ci);
        throw;
      }
      oldSmall.setSize(i);
      oldLarge.setSize(ci);
      return;
    }

    // isExtern != o.isExtern()
    auto& oldExtern = o.isExtern() ? o : *this;
    auto& oldIntern = o.isExtern() ? *this : o;

    auto oldExternCapacity = oldExtern.capacity();
    auto oldExternHeap = oldExtern.u.pdata_.heap_;

    auto buff = oldExtern.u.buffer();
    size_type i = 0;
    try {
      for (; i < oldIntern.size(); ++i) {
        new (&buff[i]) value_type(std::move(oldIntern[i]));
        oldIntern[i].~value_type();
      }
    } catch (...) {
      for (size_type kill = 0; kill < i; ++kill) {
        buff[kill].~value_type();
      }
      for (; i < oldIntern.size(); ++i) {
        oldIntern[i].~value_type();
      }
      oldIntern.setSize(0);
      oldExtern.u.pdata_.heap_ = oldExternHeap;
      oldExtern.setCapacity(oldExternCapacity);
      throw;
    }
    oldIntern.u.pdata_.heap_ = oldExternHeap;
    this->swapSizePolicy(o);
    oldIntern.setCapacity(oldExternCapacity);
  }

  void resize(size_type sz) {
    if (sz < size()) {
      erase(begin() + sz, end());
      return;
    }
    makeSize(sz);
    detail::populateMemForward(
        begin() + size(), sz - size(), [&](void* p) { new (p) value_type(); });
    this->setSize(sz);
  }

  void resize(size_type sz, value_type const& v) {
    if (sz < size()) {
      erase(begin() + sz, end());
      return;
    }
    makeSize(sz);
    detail::populateMemForward(
        begin() + size(), sz - size(), [&](void* p) { new (p) value_type(v); });
    this->setSize(sz);
  }

  value_type* data() noexcept {
    return this->isExtern() ? u.heap() : u.buffer();
  }

  value_type const* data() const noexcept {
    return this->isExtern() ? u.heap() : u.buffer();
  }

  template <class... Args>
  iterator emplace(const_iterator p, Args&&... args) {
    if (p == cend()) {
      emplace_back(std::forward<Args>(args)...);
      return end() - 1;
    }

    /*
     * We implement emplace at places other than at the back with a
     * temporary for exception safety reasons.  It is possible to
     * avoid having to do this, but it becomes hard to maintain the
     * basic exception safety guarantee (unless you respond to a copy
     * constructor throwing by clearing the whole vector).
     *
     * The reason for this is that otherwise you have to destruct an
     * element before constructing this one in its place---if the
     * constructor throws, you either need a nothrow default
     * constructor or a nothrow copy/move to get something back in the
     * "gap", and the vector requirements don't guarantee we have any
     * of these.  Clearing the whole vector is a legal response in
     * this situation, but it seems like this implementation is easy
     * enough and probably better.
     */
    return insert(p, value_type(std::forward<Args>(args)...));
  }

  void reserve(size_type sz) {
    makeSize(sz);
  }

  size_type capacity() const {
    if (this->isExtern()) {
      if (u.hasCapacity()) {
        return u.getCapacity();
      }
      return malloc_usable_size(u.pdata_.heap_) / sizeof(value_type);
    }
    return MaxInline;
  }

  void shrink_to_fit() {
    if (!this->isExtern()) {
      return;
    }

    small_vector tmp(begin(), end());
    tmp.swap(*this);
  }

  template <class... Args>
  void emplace_back(Args&&... args) {
    if (capacity() == size()) {
      // Any of args may be references into the vector.
      // When we are reallocating, we have to be careful to construct the new
      // element before modifying the data in the old buffer.
      makeSize(
          size() + 1,
          [&](void* p) { new (p) value_type(std::forward<Args>(args)...); },
          size());
    } else {
      new (end()) value_type(std::forward<Args>(args)...);
    }
    this->setSize(size() + 1);
  }

  void push_back(value_type&& t) {
    return emplace_back(std::move(t));
  }

  void push_back(value_type const& t) {
    emplace_back(t);
  }

  void pop_back() {
    erase(end() - 1);
  }

  iterator insert(const_iterator constp, value_type&& t) {
    iterator p = unconst(constp);

    if (p == end()) {
      push_back(std::move(t));
      return end() - 1;
    }

    auto offset = p - begin();

    if (capacity() == size()) {
      makeSize(
          size() + 1,
          [&t](void* ptr) { new (ptr) value_type(std::move(t)); },
          offset);
      this->setSize(this->size() + 1);
    } else {
      detail::moveObjectsRight(
          data() + offset, data() + size(), data() + size() + 1);
      this->setSize(size() + 1);
      data()[offset] = std::move(t);
    }
    return begin() + offset;
  }

  iterator insert(const_iterator p, value_type const& t) {
    // Make a copy and forward to the rvalue value_type&& overload
    // above.
    return insert(p, value_type(t));
  }

  iterator insert(const_iterator pos, size_type n, value_type const& val) {
    auto offset = pos - begin();
    makeSize(size() + n);
    detail::moveObjectsRight(
        data() + offset, data() + size(), data() + size() + n);
    this->setSize(size() + n);
    std::generate_n(begin() + offset, n, [&] { return val; });
    return begin() + offset;
  }

  template <class Arg>
  iterator insert(const_iterator p, Arg arg1, Arg arg2) {
    // Forward using std::is_arithmetic to get to the proper
    // implementation; this disambiguates between the iterators and
    // (size_t, value_type) meaning for this function.
    return insertImpl(unconst(p), arg1, arg2, std::is_arithmetic<Arg>());
  }

  iterator insert(const_iterator p, std::initializer_list<value_type> il) {
    return insert(p, il.begin(), il.end());
  }

  iterator erase(const_iterator q) {
    std::move(unconst(q) + 1, end(), unconst(q));
    (data() + size() - 1)->~value_type();
    this->setSize(size() - 1);
    return unconst(q);
  }

  iterator erase(const_iterator q1, const_iterator q2) {
    if (q1 == q2) {
      return unconst(q1);
    }
    std::move(unconst(q2), end(), unconst(q1));
    for (auto it = (end() - std::distance(q1, q2)); it != end(); ++it) {
      it->~value_type();
    }
    this->setSize(size() - (q2 - q1));
    return unconst(q1);
  }

  void clear() {
    erase(begin(), end());
  }

  template <class Arg>
  void assign(Arg first, Arg last) {
    clear();
    insert(end(), first, last);
  }

  void assign(std::initializer_list<value_type> il) {
    assign(il.begin(), il.end());
  }

  void assign(size_type n, const value_type& t) {
    clear();
    insert(end(), n, t);
  }

  reference front() {
    assert(!empty());
    return *begin();
  }
  reference back() {
    assert(!empty());
    return *(end() - 1);
  }
  const_reference front() const {
    assert(!empty());
    return *begin();
  }
  const_reference back() const {
    assert(!empty());
    return *(end() - 1);
  }

  reference operator[](size_type i) {
    assert(i < size());
    return *(begin() + i);
  }

  const_reference operator[](size_type i) const {
    assert(i < size());
    return *(begin() + i);
  }

  reference at(size_type i) {
    if (i >= size()) {
      throw_exception<std::out_of_range>("index out of range");
    }
    return (*this)[i];
  }

  const_reference at(size_type i) const {
    if (i >= size()) {
      throw_exception<std::out_of_range>("index out of range");
    }
    return (*this)[i];
  }

 private:
  static iterator unconst(const_iterator it) {
    return const_cast<iterator>(it);
  }

  // The std::false_type argument is part of disambiguating the
  // iterator insert functions from integral types (see insert().)
  template <class It>
  iterator insertImpl(iterator pos, It first, It last, std::false_type) {
    typedef typename std::iterator_traits<It>::iterator_category categ;
    if (std::is_same<categ, std::input_iterator_tag>::value) {
      auto offset = pos - begin();
      while (first != last) {
        pos = insert(pos, *first++);
        ++pos;
      }
      return begin() + offset;
    }

    auto distance = std::distance(first, last);
    auto offset = pos - begin();
    makeSize(size() + distance);
    detail::moveObjectsRight(
        data() + offset, data() + size(), data() + size() + distance);
    this->setSize(size() + distance);
    std::copy_n(first, distance, begin() + offset);
    return begin() + offset;
  }

  iterator
  insertImpl(iterator pos, size_type n, const value_type& val, std::true_type) {
    // The true_type means this should call the size_t,value_type
    // overload.  (See insert().)
    return insert(pos, n, val);
  }

  // The std::false_type argument came from std::is_arithmetic as part
  // of disambiguating an overload (see the comment in the
  // constructor).
  template <class It>
  void constructImpl(It first, It last, std::false_type) {
    typedef typename std::iterator_traits<It>::iterator_category categ;
    if (std::is_same<categ, std::input_iterator_tag>::value) {
      // With iterators that only allow a single pass, we can't really
      // do anything sane here.
      while (first != last) {
        emplace_back(*first++);
      }
      return;
    }

    auto distance = std::distance(first, last);
    makeSize(distance);
    this->setSize(distance);
    try {
      detail::populateMemForward(
          data(), distance, [&](void* p) { new (p) value_type(*first++); });
    } catch (...) {
      if (this->isExtern()) {
        u.freeHeap();
      }
      throw;
    }
  }

  template <typename InitFunc>
  void doConstruct(size_type n, InitFunc&& func) {
    makeSize(n);
    this->setSize(n);
    try {
      detail::populateMemForward(data(), n, std::forward<InitFunc>(func));
    } catch (...) {
      if (this->isExtern()) {
        u.freeHeap();
      }
      throw;
    }
  }

  // The true_type means we should forward to the size_t,value_type
  // overload.
  void constructImpl(size_type n, value_type const& val, std::true_type) {
    doConstruct(n, [&](void* p) { new (p) value_type(val); });
  }

  /*
   * Compute the size after growth.
   */
  size_type computeNewSize() const {
    return std::min((3 * capacity()) / 2 + 1, max_size());
  }

  void makeSize(size_type newSize) {
    makeSizeInternal(newSize, false, [](void*) { assume_unreachable(); }, 0);
  }

  template <typename EmplaceFunc>
  void makeSize(size_type newSize, EmplaceFunc&& emplaceFunc, size_type pos) {
    assert(size() == capacity());
    makeSizeInternal(
        newSize, true, std::forward<EmplaceFunc>(emplaceFunc), pos);
  }

  /*
   * Ensure we have a large enough memory region to be size `newSize'.
   * Will move/copy elements if we are spilling to heap_ or needed to
   * allocate a new region, but if resized in place doesn't initialize
   * anything in the new region.  In any case doesn't change size().
   * Supports insertion of new element during reallocation by given
   * pointer to new element and position of new element.
   * NOTE: If reallocation is not needed, insert must be false,
   * because we only know how to emplace elements into new memory.
   */
  template <typename EmplaceFunc>
  void makeSizeInternal(
      size_type newSize,
      bool insert,
      EmplaceFunc&& emplaceFunc,
      size_type pos) {
    if (newSize > max_size()) {
      throw std::length_error("max_size exceeded in small_vector");
    }
    if (newSize <= capacity()) {
      assert(!insert);
      return;
    }

    assert(this->kShouldUseHeap);
    // This branch isn't needed for correctness, but allows the optimizer to
    // skip generating code for the rest of this function in NoHeap
    // small_vectors.
    if (!this->kShouldUseHeap) {
      return;
    }

    newSize = std::max(newSize, computeNewSize());

    auto needBytes = newSize * sizeof(value_type);
    // If the capacity isn't explicitly stored inline, but the heap
    // allocation is grown to over some threshold, we should store
    // a capacity at the front of the heap allocation.
    bool heapifyCapacity =
        !kHasInlineCapacity && needBytes > kHeapifyCapacityThreshold;
    if (heapifyCapacity) {
      needBytes += kHeapifyCapacitySize;
    }
    auto const sizeBytes = goodMallocSize(needBytes);
    void* newh = checkedMalloc(sizeBytes);
    // We expect newh to be at least 2-aligned, because we want to
    // use its least significant bit as a flag.
    assert(!detail::pointerFlagGet(newh));

    value_type* newp = static_cast<value_type*>(
        heapifyCapacity ? detail::shiftPointer(newh, kHeapifyCapacitySize)
                        : newh);

    try {
      if (insert) {
        // move and insert the new element
        this->moveToUninitializedEmplace(
            begin(), end(), newp, pos, std::forward<EmplaceFunc>(emplaceFunc));
      } else {
        // move without inserting new element
        this->moveToUninitialized(begin(), end(), newp);
      }
    } catch (...) {
      free(newh);
      throw;
    }
    for (auto& val : *this) {
      val.~value_type();
    }

    if (this->isExtern()) {
      u.freeHeap();
    }
    auto availableSizeBytes = sizeBytes;
    if (heapifyCapacity) {
      u.pdata_.heap_ = detail::pointerFlagSet(newh);
      availableSizeBytes -= kHeapifyCapacitySize;
    } else {
      u.pdata_.heap_ = newh;
    }
    this->setExtern(true);
    this->setCapacity(availableSizeBytes / sizeof(value_type));
  }

  /*
   * This will set the capacity field, stored inline in the storage_ field
   * if there is sufficient room to store it.
   */
  void setCapacity(size_type newCapacity) {
    assert(this->isExtern());
    if (u.hasCapacity()) {
      assert(newCapacity < std::numeric_limits<InternalSizeType>::max());
      u.setCapacity(newCapacity);
    }
  }

 private:
  struct HeapPtrWithCapacity {
    void* heap_;
    InternalSizeType capacity_;

    InternalSizeType getCapacity() const {
      return capacity_;
    }
    void setCapacity(InternalSizeType c) {
      capacity_ = c;
    }
  } FOLLY_SV_PACK_ATTR;

  struct HeapPtr {
    // Lower order bit of heap_ is used as flag to indicate whether capacity is
    // stored at the front of the heap allocation.
    void* heap_;

    InternalSizeType getCapacity() const {
      assert(detail::pointerFlagGet(heap_));
      return *static_cast<InternalSizeType*>(detail::pointerFlagClear(heap_));
    }
    void setCapacity(InternalSizeType c) {
      *static_cast<InternalSizeType*>(detail::pointerFlagClear(heap_)) = c;
    }
  } FOLLY_SV_PACK_ATTR;

  typedef typename std::aligned_storage<
      sizeof(value_type) * MaxInline,
      alignof(value_type)>::type InlineStorageDataType;

  typedef typename std::conditional<
      sizeof(value_type) * MaxInline != 0,
      InlineStorageDataType,
      void*>::type InlineStorageType;

  static bool constexpr kHasInlineCapacity =
      sizeof(HeapPtrWithCapacity) < sizeof(InlineStorageType);

  // This value should we multiple of word size.
  static size_t constexpr kHeapifyCapacitySize = sizeof(
      typename std::
          aligned_storage<sizeof(InternalSizeType), alignof(value_type)>::type);

  // Threshold to control capacity heapifying.
  static size_t constexpr kHeapifyCapacityThreshold =
      100 * kHeapifyCapacitySize;

  typedef typename std::
      conditional<kHasInlineCapacity, HeapPtrWithCapacity, HeapPtr>::type
          PointerType;

  union Data {
    explicit Data() {
      pdata_.heap_ = nullptr;
    }

    PointerType pdata_;
    InlineStorageType storage_;

    value_type* buffer() noexcept {
      void* vp = &storage_;
      return static_cast<value_type*>(vp);
    }
    value_type const* buffer() const noexcept {
      return const_cast<Data*>(this)->buffer();
    }
    value_type* heap() noexcept {
      if (kHasInlineCapacity || !detail::pointerFlagGet(pdata_.heap_)) {
        return static_cast<value_type*>(pdata_.heap_);
      } else {
        return static_cast<value_type*>(detail::shiftPointer(
            detail::pointerFlagClear(pdata_.heap_), kHeapifyCapacitySize));
      }
    }
    value_type const* heap() const noexcept {
      return const_cast<Data*>(this)->heap();
    }

    bool hasCapacity() const {
      return kHasInlineCapacity || detail::pointerFlagGet(pdata_.heap_);
    }
    InternalSizeType getCapacity() const {
      return pdata_.getCapacity();
    }
    void setCapacity(InternalSizeType c) {
      pdata_.setCapacity(c);
    }

    void freeHeap() {
      auto vp = detail::pointerFlagClear(pdata_.heap_);
      free(vp);
    }
  } u;
};
FOLLY_SV_PACK_POP


// Basic guarantee only, or provides the nothrow guarantee iff T has a
// nothrow move or copy constructor.
template <class T, std::size_t MaxInline, class A, class B, class C>
void swap(
    small_vector<T, MaxInline, A, B, C>& a,
    small_vector<T, MaxInline, A, B, C>& b) {
  a.swap(b);
}


namespace detail {

// Format support.
template <class T, size_t M, class A, class B, class C>
struct IndexableTraits<small_vector<T, M, A, B, C>>
    : public IndexableTraitsSeq<small_vector<T, M, A, B, C>> {};

} // namespace detail

} // namespace folly

FOLLY_POP_WARNING

#undef FOLLY_SV_PACK_ATTR
#undef FOLLY_SV_PACK_PUSH
#undef FOLLY_SV_PACK_POP