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checked_iterators.h
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checked_iterators.h
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// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_CONTAINERS_CHECKED_ITERATORS_H_
#define BASE_CONTAINERS_CHECKED_ITERATORS_H_
#include <iterator>
#include <memory>
#include <type_traits>
#include "base/check_op.h"
#include "base/containers/util.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "build/build_config.h"
namespace base {
template <typename T>
class CheckedContiguousIterator {
public:
using difference_type = std::ptrdiff_t;
using value_type = std::remove_cv_t<T>;
using pointer = T*;
using reference = T&;
using iterator_category = std::random_access_iterator_tag;
// Required for converting constructor below.
template <typename U>
friend class CheckedContiguousIterator;
// Required for certain libc++ algorithm optimizations that are not available
// for NaCl.
#if defined(_LIBCPP_VERSION) && !BUILDFLAG(IS_NACL)
template <typename Ptr>
friend struct std::pointer_traits;
#endif
constexpr CheckedContiguousIterator() = default;
constexpr CheckedContiguousIterator(T* start, const T* end)
: CheckedContiguousIterator(start, start, end) {}
constexpr CheckedContiguousIterator(const T* start, T* current, const T* end)
: start_(start), current_(current), end_(end) {
CHECK_LE(start, current);
CHECK_LE(current, end);
}
constexpr CheckedContiguousIterator(const CheckedContiguousIterator& other) =
default;
// Converting constructor allowing conversions like CCI<T> to CCI<const T>,
// but disallowing CCI<const T> to CCI<T> or CCI<Derived> to CCI<Base>, which
// are unsafe. Furthermore, this is the same condition as used by the
// converting constructors of std::span<T> and std::unique_ptr<T[]>.
// See https://wg21.link/n4042 for details.
template <
typename U,
std::enable_if_t<std::is_convertible<U (*)[], T (*)[]>::value>* = nullptr>
constexpr CheckedContiguousIterator(const CheckedContiguousIterator<U>& other)
: start_(other.start_), current_(other.current_), end_(other.end_) {
// We explicitly don't delegate to the 3-argument constructor here. Its
// CHECKs would be redundant, since we expect |other| to maintain its own
// invariant. However, DCHECKs never hurt anybody. Presumably.
DCHECK_LE(other.start_, other.current_);
DCHECK_LE(other.current_, other.end_);
}
~CheckedContiguousIterator() = default;
constexpr CheckedContiguousIterator& operator=(
const CheckedContiguousIterator& other) = default;
friend constexpr bool operator==(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ == rhs.current_;
}
friend constexpr bool operator!=(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ != rhs.current_;
}
friend constexpr bool operator<(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ < rhs.current_;
}
friend constexpr bool operator<=(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ <= rhs.current_;
}
friend constexpr bool operator>(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ > rhs.current_;
}
friend constexpr bool operator>=(const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ >= rhs.current_;
}
constexpr CheckedContiguousIterator& operator++() {
CHECK_NE(current_, end_);
++current_;
return *this;
}
constexpr CheckedContiguousIterator operator++(int) {
CheckedContiguousIterator old = *this;
++*this;
return old;
}
constexpr CheckedContiguousIterator& operator--() {
CHECK_NE(current_, start_);
--current_;
return *this;
}
constexpr CheckedContiguousIterator operator--(int) {
CheckedContiguousIterator old = *this;
--*this;
return old;
}
constexpr CheckedContiguousIterator& operator+=(difference_type rhs) {
if (rhs > 0) {
CHECK_LE(rhs, end_ - current_);
} else {
CHECK_LE(-rhs, current_ - start_);
}
current_ += rhs;
return *this;
}
constexpr CheckedContiguousIterator operator+(difference_type rhs) const {
CheckedContiguousIterator it = *this;
it += rhs;
return it;
}
constexpr CheckedContiguousIterator& operator-=(difference_type rhs) {
if (rhs < 0) {
CHECK_LE(-rhs, end_ - current_);
} else {
CHECK_LE(rhs, current_ - start_);
}
current_ -= rhs;
return *this;
}
constexpr CheckedContiguousIterator operator-(difference_type rhs) const {
CheckedContiguousIterator it = *this;
it -= rhs;
return it;
}
constexpr friend difference_type operator-(
const CheckedContiguousIterator& lhs,
const CheckedContiguousIterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.current_ - rhs.current_;
}
constexpr reference operator*() const {
CHECK_NE(current_, end_);
return *current_;
}
constexpr pointer operator->() const {
CHECK_NE(current_, end_);
return current_;
}
constexpr reference operator[](difference_type rhs) const {
CHECK_GE(rhs, 0);
CHECK_LT(rhs, end_ - current_);
return current_[rhs];
}
[[nodiscard]] static bool IsRangeMoveSafe(
const CheckedContiguousIterator& from_begin,
const CheckedContiguousIterator& from_end,
const CheckedContiguousIterator& to) {
if (from_end < from_begin)
return false;
const auto from_begin_uintptr = get_uintptr(from_begin.current_);
const auto from_end_uintptr = get_uintptr(from_end.current_);
const auto to_begin_uintptr = get_uintptr(to.current_);
const auto to_end_uintptr =
get_uintptr((to + std::distance(from_begin, from_end)).current_);
return to_begin_uintptr >= from_end_uintptr ||
to_end_uintptr <= from_begin_uintptr;
}
private:
constexpr void CheckComparable(const CheckedContiguousIterator& other) const {
CHECK_EQ(start_, other.start_);
CHECK_EQ(end_, other.end_);
}
// This field is not a raw_ptr<> because it was filtered by the rewriter for:
// #union, #constexpr-ctor-field-initializer
RAW_PTR_EXCLUSION const T* start_ = nullptr;
// This field is not a raw_ptr<> because it was filtered by the rewriter for:
// #union, #constexpr-ctor-field-initializer
RAW_PTR_EXCLUSION T* current_ = nullptr;
// This field is not a raw_ptr<> because it was filtered by the rewriter for:
// #union, #constexpr-ctor-field-initializer
RAW_PTR_EXCLUSION const T* end_ = nullptr;
};
template <typename T>
using CheckedContiguousConstIterator = CheckedContiguousIterator<const T>;
} // namespace base
#if defined(_LIBCPP_VERSION) && !BUILDFLAG(IS_NACL)
// Specialize both std::__is_cpp17_contiguous_iterator and std::pointer_traits
// for CCI in case we compile with libc++ outside of NaCl. The former is
// required to enable certain algorithm optimizations (e.g. std::copy can be a
// simple std::memmove under certain circumstances), and is a precursor to
// C++20's std::contiguous_iterator concept [1]. Once we actually use C++20 it
// will be enough to add `using iterator_concept = std::contiguous_iterator_tag`
// to the iterator class [2], and we can get rid of this non-standard
// specialization.
//
// The latter is required to obtain the underlying raw pointer without resulting
// in CHECK failures. The important bit is the `to_address(pointer)` overload,
// which is the standard blessed way to customize `std::to_address(pointer)` in
// C++20 [3].
//
// [1] https://wg21.link/iterator.concept.contiguous
// [2] https://wg21.link/std.iterator.tags
// [3] https://wg21.link/pointer.traits.optmem
namespace std {
template <typename T>
struct __is_cpp17_contiguous_iterator<::base::CheckedContiguousIterator<T>>
: true_type {};
template <typename T>
struct pointer_traits<::base::CheckedContiguousIterator<T>> {
using pointer = ::base::CheckedContiguousIterator<T>;
using element_type = T;
using difference_type = ptrdiff_t;
template <typename U>
using rebind = ::base::CheckedContiguousIterator<U>;
static constexpr pointer pointer_to(element_type& r) noexcept {
return pointer(&r, &r);
}
static constexpr element_type* to_address(pointer p) noexcept {
return p.current_;
}
};
} // namespace std
#endif
#endif // BASE_CONTAINERS_CHECKED_ITERATORS_H_