folly/folly/portability/Constexpr.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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.
 */

#pragma once

#include <folly/CPortability.h>
#include <folly/Portability.h>

#include <cstdint>
#include <cstring>
#include <type_traits>

namespace folly {

namespace detail {

#if FOLLY_HAS_FEATURE(cxx_constexpr_string_builtins) || \
    FOLLY_HAS_BUILTIN(__builtin_strlen) || defined(_MSC_VER)
#define FOLLY_DETAIL_STRLEN …
#else
#define FOLLY_DETAIL_STRLEN …
#endif

#if FOLLY_HAS_FEATURE(cxx_constexpr_string_builtins) || \
    FOLLY_HAS_BUILTIN(__builtin_strcmp)
#define FOLLY_DETAIL_STRCMP …
#else
#define FOLLY_DETAIL_STRCMP …
#endif

// This overload is preferred if Char is char and if FOLLY_DETAIL_STRLEN
// yields a compile-time constant.
template <
    typename Char,
    std::size_t = FOLLY_DETAIL_STRLEN(static_cast<const Char*>(""))>
constexpr std::size_t constexpr_strlen_internal(const Char* s, int) noexcept { … }
template <typename Char>
constexpr std::size_t constexpr_strlen_internal(
    const Char* s, unsigned) noexcept { … }

template <typename Char>
constexpr std::size_t constexpr_strlen_fallback(const Char* s) noexcept { … }

static_assert …;

// This overload is preferred if Char is char and if FOLLY_DETAIL_STRCMP
// yields a compile-time constant.
template <
    typename Char,
    int = FOLLY_DETAIL_STRCMP(static_cast<const Char*>(""), "")>
constexpr int constexpr_strcmp_internal(
    const Char* s1, const Char* s2, int) noexcept { … }
template <typename Char>
constexpr int constexpr_strcmp_internal(
    const Char* s1, const Char* s2, unsigned) noexcept { … }

template <typename Char>
constexpr int constexpr_strcmp_fallback(
    const Char* s1, const Char* s2) noexcept { … }

#undef FOLLY_DETAIL_STRCMP
#undef FOLLY_DETAIL_STRLEN

} // namespace detail

template <typename Char>
constexpr std::size_t constexpr_strlen(const Char* s) noexcept { … }

template <typename Char>
constexpr int constexpr_strcmp(const Char* s1, const Char* s2) noexcept { … }

namespace detail {

template <typename V>
struct is_constant_evaluated_or_constinit_ { … };

} // namespace detail

//  is_constant_evaluated_or
//
//  Similar in spirit to std::is_constant_evaluated (c++20), with differences:
//  * Takes an argument to be used as the default return value, if the code is
//    unable to tell whether it is in a constant context.
constexpr bool is_constant_evaluated_or(
    detail::is_constant_evaluated_or_constinit_<bool> const def) noexcept { … }

} // namespace folly