chromium/third_party/abseil-cpp/absl/types/variant.h

// Copyright 2018 The Abseil 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
//
//      https://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.
//
// -----------------------------------------------------------------------------
// variant.h
// -----------------------------------------------------------------------------
//
// This header file defines an `absl::variant` type for holding a type-safe
// value of some prescribed set of types (noted as alternative types), and
// associated functions for managing variants.
//
// The `absl::variant` type is a form of type-safe union. An `absl::variant`
// should always hold a value of one of its alternative types (except in the
// "valueless by exception state" -- see below). A default-constructed
// `absl::variant` will hold the value of its first alternative type, provided
// it is default-constructible.
//
// In exceptional cases due to error, an `absl::variant` can hold no
// value (known as a "valueless by exception" state), though this is not the
// norm.
//
// As with `absl::optional`, an `absl::variant` -- when it holds a value --
// allocates a value of that type directly within the `variant` itself; it
// cannot hold a reference, array, or the type `void`; it can, however, hold a
// pointer to externally managed memory.
//
// `absl::variant` is a C++11 compatible version of the C++17 `std::variant`
// abstraction and is designed to be a drop-in replacement for code compliant
// with C++17.

#ifndef ABSL_TYPES_VARIANT_H_
#define ABSL_TYPES_VARIANT_H_

#include "absl/base/config.h"
#include "absl/utility/utility.h"

#ifdef ABSL_USES_STD_VARIANT

#include <variant>  // IWYU pragma: export

namespace absl {
ABSL_NAMESPACE_BEGIN
using std::bad_variant_access;
using std::get;
using std::get_if;
using std::holds_alternative;
using std::monostate;
using std::variant;
using std::variant_alternative;
using std::variant_alternative_t;
using std::variant_npos;
using std::variant_size;
using std::variant_size_v;
using std::visit;
ABSL_NAMESPACE_END
}  // namespace absl

#else  // ABSL_USES_STD_VARIANT

#include <functional>
#include <new>
#include <type_traits>
#include <utility>

#include "absl/base/macros.h"
#include "absl/base/port.h"
#include "absl/meta/type_traits.h"
#include "absl/types/internal/variant.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

// -----------------------------------------------------------------------------
// absl::variant
// -----------------------------------------------------------------------------
//
// An `absl::variant` type is a form of type-safe union. An `absl::variant` --
// except in exceptional cases -- always holds a value of one of its alternative
// types.
//
// Example:
//
//   // Construct a variant that holds either an integer or a std::string and
//   // assign it to a std::string.
//   absl::variant<int, std::string> v = std::string("abc");
//
//   // A default-constructed variant will hold a value-initialized value of
//   // the first alternative type.
//   auto a = absl::variant<int, std::string>();   // Holds an int of value '0'.
//
//   // variants are assignable.
//
//   // copy assignment
//   auto v1 = absl::variant<int, std::string>("abc");
//   auto v2 = absl::variant<int, std::string>(10);
//   v2 = v1;  // copy assign
//
//   // move assignment
//   auto v1 = absl::variant<int, std::string>("abc");
//   v1 = absl::variant<int, std::string>(10);
//
//   // assignment through type conversion
//   a = 128;         // variant contains int
//   a = "128";       // variant contains std::string
//
// An `absl::variant` holding a value of one of its alternative types `T` holds
// an allocation of `T` directly within the variant itself. An `absl::variant`
// is not allowed to allocate additional storage, such as dynamic memory, to
// allocate the contained value. The contained value shall be allocated in a
// region of the variant storage suitably aligned for all alternative types.
template <typename... Ts>
class variant;

// swap()
//
// Swaps two `absl::variant` values. This function is equivalent to `v.swap(w)`
// where `v` and `w` are `absl::variant` types.
//
// Note that this function requires all alternative types to be both swappable
// and move-constructible, because any two variants may refer to either the same
// type (in which case, they will be swapped) or to two different types (in
// which case the values will need to be moved).
//
template <
    typename... Ts,
    absl::enable_if_t<
        absl::conjunction<std::is_move_constructible<Ts>...,
                          type_traits_internal::IsSwappable<Ts>...>::value,
        int> = 0>
void swap(variant<Ts...>& v, variant<Ts...>& w) noexcept(noexcept(v.swap(w))) { … }

// variant_size
//
// Returns the number of alternative types available for a given `absl::variant`
// type as a compile-time constant expression. As this is a class template, it
// is not generally useful for accessing the number of alternative types of
// any given `absl::variant` instance.
//
// Example:
//
//   auto a = absl::variant<int, std::string>;
//   constexpr int num_types =
//       absl::variant_size<absl::variant<int, std::string>>();
//
//   // You can also use the member constant `value`.
//   constexpr int num_types =
//       absl::variant_size<absl::variant<int, std::string>>::value;
//
//   // `absl::variant_size` is more valuable for use in generic code:
//   template <typename Variant>
//   constexpr bool IsVariantMultivalue() {
//       return absl::variant_size<Variant>() > 1;
//   }
//
// Note that the set of cv-qualified specializations of `variant_size` are
// provided to ensure that those specializations compile (especially when passed
// within template logic).
template <class T>
struct variant_size;

variant_size<variant<Ts...>>;

// Specialization of `variant_size` for const qualified variants.
variant_size<const T>;

// Specialization of `variant_size` for volatile qualified variants.
variant_size<volatile T>;

// Specialization of `variant_size` for const volatile qualified variants.
variant_size<const volatile T>;

// variant_alternative
//
// Returns the alternative type for a given `absl::variant` at the passed
// index value as a compile-time constant expression. As this is a class
// template resulting in a type, it is not useful for access of the run-time
// value of any given `absl::variant` variable.
//
// Example:
//
//   // The type of the 0th alternative is "int".
//   using alternative_type_0
//     = absl::variant_alternative<0, absl::variant<int, std::string>>::type;
//
//   static_assert(std::is_same<alternative_type_0, int>::value, "");
//
//   // `absl::variant_alternative` is more valuable for use in generic code:
//   template <typename Variant>
//   constexpr bool IsFirstElementTrivial() {
//       return std::is_trivial_v<variant_alternative<0, Variant>::type>;
//   }
//
// Note that the set of cv-qualified specializations of `variant_alternative`
// are provided to ensure that those specializations compile (especially when
// passed within template logic).
template <std::size_t I, class T>
struct variant_alternative;

variant_alternative<I, variant<Types...>>;

// Specialization of `variant_alternative` for const qualified variants.
variant_alternative<I, const T>;

// Specialization of `variant_alternative` for volatile qualified variants.
variant_alternative<I, volatile T>;

// Specialization of `variant_alternative` for const volatile qualified
// variants.
variant_alternative<I, const volatile T>;

// Template type alias for variant_alternative<I, T>::type.
//
// Example:
//
//   using alternative_type_0
//     = absl::variant_alternative_t<0, absl::variant<int, std::string>>;
//   static_assert(std::is_same<alternative_type_0, int>::value, "");
variant_alternative_t;

// holds_alternative()
//
// Checks whether the given variant currently holds a given alternative type,
// returning `true` if so.
//
// Example:
//
//   absl::variant<int, std::string> foo = 42;
//   if (absl::holds_alternative<int>(foo)) {
//       std::cout << "The variant holds an integer";
//   }
template <class T, class... Types>
constexpr bool holds_alternative(const variant<Types...>& v) noexcept { … }

// get()
//
// Returns a reference to the value currently within a given variant, using
// either a unique alternative type amongst the variant's set of alternative
// types, or the variant's index value. Attempting to get a variant's value
// using a type that is not unique within the variant's set of alternative types
// is a compile-time error. If the index of the alternative being specified is
// different from the index of the alternative that is currently stored, throws
// `absl::bad_variant_access`.
//
// Example:
//
//   auto a = absl::variant<int, std::string>;
//
//   // Get the value by type (if unique).
//   int i = absl::get<int>(a);
//
//   auto b = absl::variant<int, int>;
//
//   // Getting the value by a type that is not unique is ill-formed.
//   int j = absl::get<int>(b);     // Compile Error!
//
//   // Getting value by index not ambiguous and allowed.
//   int k = absl::get<1>(b);

// Overload for getting a variant's lvalue by type.
template <class T, class... Types>
constexpr T& get(variant<Types...>& v) { … }

// Overload for getting a variant's rvalue by type.
template <class T, class... Types>
constexpr T&& get(variant<Types...>&& v) { … }

// Overload for getting a variant's const lvalue by type.
template <class T, class... Types>
constexpr const T& get(const variant<Types...>& v) { … }

// Overload for getting a variant's const rvalue by type.
template <class T, class... Types>
constexpr const T&& get(const variant<Types...>&& v) { … }

// Overload for getting a variant's lvalue by index.
template <std::size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>& get(
    variant<Types...>& v) { … }

// Overload for getting a variant's rvalue by index.
template <std::size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>&& get(
    variant<Types...>&& v) { … }

// Overload for getting a variant's const lvalue by index.
template <std::size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>& get(
    const variant<Types...>& v) { … }

// Overload for getting a variant's const rvalue by index.
template <std::size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>&& get(
    const variant<Types...>&& v) { … }

// get_if()
//
// Returns a pointer to the value currently stored within a given variant, if
// present, using either a unique alternative type amongst the variant's set of
// alternative types, or the variant's index value. If such a value does not
// exist, returns `nullptr`.
//
// As with `get`, attempting to get a variant's value using a type that is not
// unique within the variant's set of alternative types is a compile-time error.

// Overload for getting a pointer to the value stored in the given variant by
// index.
template <std::size_t I, class... Types>
constexpr absl::add_pointer_t<variant_alternative_t<I, variant<Types...>>>
get_if(variant<Types...>* v) noexcept { … }

// Overload for getting a pointer to the const value stored in the given
// variant by index.
template <std::size_t I, class... Types>
constexpr absl::add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
get_if(const variant<Types...>* v) noexcept { … }

// Overload for getting a pointer to the value stored in the given variant by
// type.
template <class T, class... Types>
constexpr absl::add_pointer_t<T> get_if(variant<Types...>* v) noexcept { … }

// Overload for getting a pointer to the const value stored in the given variant
// by type.
template <class T, class... Types>
constexpr absl::add_pointer_t<const T> get_if(
    const variant<Types...>* v) noexcept { … }

// visit()
//
// Calls a provided functor on a given set of variants. `absl::visit()` is
// commonly used to conditionally inspect the state of a given variant (or set
// of variants).
//
// The functor must return the same type when called with any of the variants'
// alternatives.
//
// Example:
//
//   // Define a visitor functor
//   struct GetVariant {
//       template<typename T>
//       void operator()(const T& i) const {
//         std::cout << "The variant's value is: " << i;
//       }
//   };
//
//   // Declare our variant, and call `absl::visit()` on it.
//   // Note that `GetVariant()` returns void in either case.
//   absl::variant<int, std::string> foo = std::string("foo");
//   GetVariant visitor;
//   absl::visit(visitor, foo);  // Prints `The variant's value is: foo'
template <typename Visitor, typename... Variants>
variant_internal::VisitResult<Visitor, Variants...> visit(Visitor&& vis,
                                                          Variants&&... vars) { … }

// monostate
//
// The monostate class serves as a first alternative type for a variant for
// which the first variant type is otherwise not default-constructible.
struct monostate { … };

// `absl::monostate` Relational Operators

constexpr bool operator<(monostate, monostate) noexcept { … }
constexpr bool operator>(monostate, monostate) noexcept { … }
constexpr bool operator<=(monostate, monostate) noexcept { … }
constexpr bool operator>=(monostate, monostate) noexcept { … }
constexpr bool operator==(monostate, monostate) noexcept { … }
constexpr bool operator!=(monostate, monostate) noexcept { … }


//------------------------------------------------------------------------------
// `absl::variant` Template Definition
//------------------------------------------------------------------------------
variant<T0, Tn...>;

// We need a valid declaration of variant<> for SFINAE and overload resolution
// to work properly above, but we don't need a full declaration since this type
// will never be constructed. This declaration, though incomplete, suffices.
template <>
class variant<>;

//------------------------------------------------------------------------------
// Relational Operators
//------------------------------------------------------------------------------
//
// If neither operand is in the `variant::valueless_by_exception` state:
//
//   * If the index of both variants is the same, the relational operator
//     returns the result of the corresponding relational operator for the
//     corresponding alternative type.
//   * If the index of both variants is not the same, the relational operator
//     returns the result of that operation applied to the value of the left
//     operand's index and the value of the right operand's index.
//   * If at least one operand is in the valueless_by_exception state:
//     - A variant in the valueless_by_exception state is only considered equal
//       to another variant in the valueless_by_exception state.
//     - If exactly one operand is in the valueless_by_exception state, the
//       variant in the valueless_by_exception state is less than the variant
//       that is not in the valueless_by_exception state.
//
// Note: The value 1 is added to each index in the relational comparisons such
// that the index corresponding to the valueless_by_exception state wraps around
// to 0 (the lowest value for the index type), and the remaining indices stay in
// the same relative order.

// Equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveEqualT<Types...> operator==(
    const variant<Types...>& a, const variant<Types...>& b) { … }

// Not equal operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveNotEqualT<Types...> operator!=(
    const variant<Types...>& a, const variant<Types...>& b) { … }

// Less-than operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveLessThanT<Types...> operator<(
    const variant<Types...>& a, const variant<Types...>& b) { … }

// Greater-than operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveGreaterThanT<Types...> operator>(
    const variant<Types...>& a, const variant<Types...>& b) { … }

// Less-than or equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveLessThanOrEqualT<Types...> operator<=(
    const variant<Types...>& a, const variant<Types...>& b) { … }

// Greater-than or equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveGreaterThanOrEqualT<Types...>
operator>=(const variant<Types...>& a, const variant<Types...>& b) { … }

ABSL_NAMESPACE_END
}  // namespace absl

namespace std {

// hash()
template <>  // NOLINT
struct hash<absl::monostate> { … };

hash<absl::variant<T...>>;

}  // namespace std

#endif  // ABSL_USES_STD_VARIANT

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace variant_internal {

// Helper visitor for converting a variant<Ts...>` into another type (mostly
// variant) that can be constructed from any type.
template <typename To>
struct ConversionVisitor { … };

}  // namespace variant_internal

// ConvertVariantTo()
//
// Helper functions to convert an `absl::variant` to a variant of another set of
// types, provided that the alternative type of the new variant type can be
// converted from any type in the source variant.
//
// Example:
//
//   absl::variant<name1, name2, float> InternalReq(const Req&);
//
//   // name1 and name2 are convertible to name
//   absl::variant<name, float> ExternalReq(const Req& req) {
//     return absl::ConvertVariantTo<absl::variant<name, float>>(
//              InternalReq(req));
//   }
template <typename To, typename Variant>
To ConvertVariantTo(Variant&& variant) { … }

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_TYPES_VARIANT_H_