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// Copyright 2020 The Pigweed Authors
// 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
// 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.
#pragma once
#include <iterator>
#include <type_traits>
namespace pw::kvs {
namespace internal {
// This borrows the `make_span` function from Chromium and uses to see if a type
// can be represented as a span. See:
// Simplified implementation of C++20's std::iter_reference_t.
// As opposed to std::iter_reference_t, this implementation does not restrict
// the type of `Iter`.
// Reference:
template <typename Iter>
using iter_reference_t = decltype(*std::declval<Iter&>());
template <typename T>
struct ExtentImpl : std::integral_constant<size_t, std::dynamic_extent> {};
template <typename T, size_t N>
struct ExtentImpl<T[N]> : std::integral_constant<size_t, N> {};
template <typename T, size_t N>
struct ExtentImpl<std::array<T, N>> : std::integral_constant<size_t, N> {};
template <typename T, size_t N>
struct ExtentImpl<std::span<T, N>> : std::integral_constant<size_t, N> {};
template <typename T>
using Extent = ExtentImpl<std::remove_cv_t<std::remove_reference_t<T>>>;
// Type-deducing helpers for constructing a span.
template <int&... ExplicitArgumentBarrier, typename It, typename EndOrSize>
constexpr auto make_span(It it, EndOrSize end_or_size) noexcept {
using T = std::remove_reference_t<iter_reference_t<It>>;
return std::span<T>(it, end_or_size);
// make_span utility function that deduces both the span's value_type and extent
// from the passed in argument.
// Usage: auto span = base::make_span(...);
template <int&... ExplicitArgumentBarrier,
typename Container,
typename T = std::remove_pointer_t<
constexpr auto make_span(Container&& container) noexcept {
return std::span<T, Extent<Container>::value>(
// The make_span functions above don't seem to work correctly with arrays of
// non-const values, so add const to the type. That is fine for KVS's Put
// method, since the values can be accepted as const.
template <typename T,
typename = decltype(make_span(std::declval<std::add_const_t<T>>()))>
constexpr bool ConvertsToSpan(int) {
return true;
// If the expression std::span(T) fails, then the type can't be converted to a
// std::span.
template <typename T>
constexpr bool ConvertsToSpan(...) {
return false;
} // namespace internal
// Traits class to detect if the type converts to a std::span.
template <typename T>
struct ConvertsToSpan
: public std::bool_constant<
internal::ConvertsToSpan<std::remove_reference_t<T>>(0)> {};
} // namespace pw::kvs