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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A template class for tagged unions. */
+
+#include <new>
+#include <stdint.h>
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Move.h"
+#include "mozilla/TypeTraits.h"
+
+#ifndef mozilla_Variant_h
+#define mozilla_Variant_h
+
+namespace mozilla {
+
+template<typename... Ts>
+class Variant;
+
+namespace detail {
+
+// MaxSizeOf computes the maximum sizeof(T) for each T in Ts.
+
+template<typename T, typename... Ts>
+struct MaxSizeOf
+{
+ static const size_t size = sizeof(T) > MaxSizeOf<Ts...>::size
+ ? sizeof(T)
+ : MaxSizeOf<Ts...>::size;
+};
+
+template<typename T>
+struct MaxSizeOf<T>
+{
+ static const size_t size = sizeof(T);
+};
+
+// The `IsVariant` helper is used in conjunction with static_assert and
+// `mozilla::EnableIf` to catch passing non-variant types to `Variant::is<T>()`
+// and friends at compile time, rather than at runtime. It ensures that the
+// given type `Needle` is one of the types in the set of types `Haystack`.
+
+template<typename Needle, typename... Haystack>
+struct IsVariant;
+
+template<typename Needle>
+struct IsVariant<Needle>
+{
+ static const bool value = false;
+};
+
+template<typename Needle, typename... Haystack>
+struct IsVariant<Needle, Needle, Haystack...>
+{
+ static const bool value = true;
+};
+
+template<typename Needle, typename T, typename... Haystack>
+struct IsVariant<Needle, T, Haystack...> : public IsVariant<Needle, Haystack...> { };
+
+/// SelectVariantTypeHelper is used in the implementation of SelectVariantType.
+template<typename T, typename... Variants>
+struct SelectVariantTypeHelper;
+
+template<typename T>
+struct SelectVariantTypeHelper<T>
+{ };
+
+template<typename T, typename... Variants>
+struct SelectVariantTypeHelper<T, T, Variants...>
+{
+ typedef T Type;
+};
+
+template<typename T, typename... Variants>
+struct SelectVariantTypeHelper<T, const T, Variants...>
+{
+ typedef const T Type;
+};
+
+template<typename T, typename... Variants>
+struct SelectVariantTypeHelper<T, const T&, Variants...>
+{
+ typedef const T& Type;
+};
+
+template<typename T, typename... Variants>
+struct SelectVariantTypeHelper<T, T&&, Variants...>
+{
+ typedef T&& Type;
+};
+
+template<typename T, typename Head, typename... Variants>
+struct SelectVariantTypeHelper<T, Head, Variants...>
+ : public SelectVariantTypeHelper<T, Variants...>
+{ };
+
+/**
+ * SelectVariantType takes a type T and a list of variant types Variants and
+ * yields a type Type, selected from Variants, that can store a value of type T
+ * or a reference to type T. If no such type was found, Type is not defined.
+ */
+template <typename T, typename... Variants>
+struct SelectVariantType
+ : public SelectVariantTypeHelper<typename RemoveConst<typename RemoveReference<T>::Type>::Type,
+ Variants...>
+{ };
+
+// Compute a fast, compact type that can be used to hold integral values that
+// distinctly map to every type in Ts.
+template<typename... Ts>
+struct VariantTag
+{
+private:
+ static const size_t TypeCount = sizeof...(Ts);
+
+public:
+ using Type =
+ typename Conditional<TypeCount < 3,
+ bool,
+ typename Conditional<TypeCount < (1 << 8),
+ uint_fast8_t,
+ size_t // stop caring past a certain point :-)
+ >::Type
+ >::Type;
+};
+
+// TagHelper gets the given sentinel tag value for the given type T. This has to
+// be split out from VariantImplementation because you can't nest a partial
+// template specialization within a template class.
+
+template<typename Tag, size_t N, typename T, typename U, typename Next, bool isMatch>
+struct TagHelper;
+
+// In the case where T != U, we continue recursion.
+template<typename Tag, size_t N, typename T, typename U, typename Next>
+struct TagHelper<Tag, N, T, U, Next, false>
+{
+ static Tag tag() { return Next::template tag<U>(); }
+};
+
+// In the case where T == U, return the tag number.
+template<typename Tag, size_t N, typename T, typename U, typename Next>
+struct TagHelper<Tag, N, T, U, Next, true>
+{
+ static Tag tag() { return Tag(N); }
+};
+
+// The VariantImplementation template provides the guts of mozilla::Variant. We
+// create a VariantImplementation for each T in Ts... which handles
+// construction, destruction, etc for when the Variant's type is T. If the
+// Variant's type isn't T, it punts the request on to the next
+// VariantImplementation.
+
+template<typename Tag, size_t N, typename... Ts>
+struct VariantImplementation;
+
+// The singly typed Variant / recursion base case.
+template<typename Tag, size_t N, typename T>
+struct VariantImplementation<Tag, N, T>
+{
+ template<typename U>
+ static Tag tag() {
+ static_assert(mozilla::IsSame<T, U>::value,
+ "mozilla::Variant: tag: bad type!");
+ return Tag(N);
+ }
+
+ template<typename Variant>
+ static void copyConstruct(void* aLhs, const Variant& aRhs) {
+ new (aLhs) T(aRhs.template as<T>());
+ }
+
+ template<typename Variant>
+ static void moveConstruct(void* aLhs, Variant&& aRhs) {
+ new (aLhs) T(aRhs.template extract<T>());
+ }
+
+ template<typename Variant>
+ static void destroy(Variant& aV) {
+ aV.template as<T>().~T();
+ }
+
+ template<typename Variant>
+ static bool
+ equal(const Variant& aLhs, const Variant& aRhs) {
+ return aLhs.template as<T>() == aRhs.template as<T>();
+ }
+
+ template<typename Matcher, typename ConcreteVariant>
+ static auto
+ match(Matcher&& aMatcher, ConcreteVariant& aV)
+ -> decltype(aMatcher.match(aV.template as<T>()))
+ {
+ return aMatcher.match(aV.template as<T>());
+ }
+};
+
+// VariantImplementation for some variant type T.
+template<typename Tag, size_t N, typename T, typename... Ts>
+struct VariantImplementation<Tag, N, T, Ts...>
+{
+ // The next recursive VariantImplementation.
+ using Next = VariantImplementation<Tag, N + 1, Ts...>;
+
+ template<typename U>
+ static Tag tag() {
+ return TagHelper<Tag, N, T, U, Next, IsSame<T, U>::value>::tag();
+ }
+
+ template<typename Variant>
+ static void copyConstruct(void* aLhs, const Variant& aRhs) {
+ if (aRhs.template is<T>()) {
+ new (aLhs) T(aRhs.template as<T>());
+ } else {
+ Next::copyConstruct(aLhs, aRhs);
+ }
+ }
+
+ template<typename Variant>
+ static void moveConstruct(void* aLhs, Variant&& aRhs) {
+ if (aRhs.template is<T>()) {
+ new (aLhs) T(aRhs.template extract<T>());
+ } else {
+ Next::moveConstruct(aLhs, aRhs);
+ }
+ }
+
+ template<typename Variant>
+ static void destroy(Variant& aV) {
+ if (aV.template is<T>()) {
+ aV.template as<T>().~T();
+ } else {
+ Next::destroy(aV);
+ }
+ }
+
+ template<typename Variant>
+ static bool equal(const Variant& aLhs, const Variant& aRhs) {
+ if (aLhs.template is<T>()) {
+ MOZ_ASSERT(aRhs.template is<T>());
+ return aLhs.template as<T>() == aRhs.template as<T>();
+ } else {
+ return Next::equal(aLhs, aRhs);
+ }
+ }
+
+ template<typename Matcher, typename ConcreteVariant>
+ static auto
+ match(Matcher&& aMatcher, ConcreteVariant& aV)
+ -> decltype(aMatcher.match(aV.template as<T>()))
+ {
+ if (aV.template is<T>()) {
+ return aMatcher.match(aV.template as<T>());
+ } else {
+ // If you're seeing compilation errors here like "no matching
+ // function for call to 'match'" then that means that the
+ // Matcher doesn't exhaust all variant types. There must exist a
+ // Matcher::match(T&) for every variant type T.
+ //
+ // If you're seeing compilation errors here like "cannot
+ // initialize return object of type <...> with an rvalue of type
+ // <...>" then that means that the Matcher::match(T&) overloads
+ // are returning different types. They must all return the same
+ // Matcher::ReturnType type.
+ return Next::match(aMatcher, aV);
+ }
+ }
+};
+
+/**
+ * AsVariantTemporary stores a value of type T to allow construction of a
+ * Variant value via type inference. Because T is copied and there's no
+ * guarantee that the copy can be elided, AsVariantTemporary is best used with
+ * primitive or very small types.
+ */
+template <typename T>
+struct AsVariantTemporary
+{
+ explicit AsVariantTemporary(const T& aValue)
+ : mValue(aValue)
+ {}
+
+ template<typename U>
+ explicit AsVariantTemporary(U&& aValue)
+ : mValue(Forward<U>(aValue))
+ {}
+
+ AsVariantTemporary(const AsVariantTemporary& aOther)
+ : mValue(aOther.mValue)
+ {}
+
+ AsVariantTemporary(AsVariantTemporary&& aOther)
+ : mValue(Move(aOther.mValue))
+ {}
+
+ AsVariantTemporary() = delete;
+ void operator=(const AsVariantTemporary&) = delete;
+ void operator=(AsVariantTemporary&&) = delete;
+
+ typename RemoveConst<typename RemoveReference<T>::Type>::Type mValue;
+};
+
+} // namespace detail
+
+/**
+ * # mozilla::Variant
+ *
+ * A variant / tagged union / heterogenous disjoint union / sum-type template
+ * class. Similar in concept to (but not derived from) `boost::variant`.
+ *
+ * Sometimes, you may wish to use a C union with non-POD types. However, this is
+ * forbidden in C++ because it is not clear which type in the union should have
+ * its constructor and destructor run on creation and deletion
+ * respectively. This is the problem that `mozilla::Variant` solves.
+ *
+ * ## Usage
+ *
+ * A `mozilla::Variant` instance is constructed (via move or copy) from one of
+ * its variant types (ignoring const and references). It does *not* support
+ * construction from subclasses of variant types or types that coerce to one of
+ * the variant types.
+ *
+ * Variant<char, uint32_t> v1('a');
+ * Variant<UniquePtr<A>, B, C> v2(MakeUnique<A>());
+ *
+ * Because specifying the full type of a Variant value is often verbose,
+ * AsVariant() can be used to construct a Variant value using type inference in
+ * contexts such as expressions or when returning values from functions. Because
+ * AsVariant() must copy or move the value into a temporary and this cannot
+ * necessarily be elided by the compiler, it's mostly appropriate only for use
+ * with primitive or very small types.
+ *
+ *
+ * Variant<char, uint32_t> Foo() { return AsVariant('x'); }
+ * // ...
+ * Variant<char, uint32_t> v1 = Foo(); // v1 holds char('x').
+ *
+ * All access to the contained value goes through type-safe accessors.
+ *
+ * void
+ * Foo(Variant<A, B, C> v)
+ * {
+ * if (v.is<A>()) {
+ * A& ref = v.as<A>();
+ * ...
+ * } else {
+ * ...
+ * }
+ * }
+ *
+ * Attempting to use the contained value as type `T1` when the `Variant`
+ * instance contains a value of type `T2` causes an assertion failure.
+ *
+ * A a;
+ * Variant<A, B, C> v(a);
+ * v.as<B>(); // <--- Assertion failure!
+ *
+ * Trying to use a `Variant<Ts...>` instance as some type `U` that is not a
+ * member of the set of `Ts...` is a compiler error.
+ *
+ * A a;
+ * Variant<A, B, C> v(a);
+ * v.as<SomeRandomType>(); // <--- Compiler error!
+ *
+ * Additionally, you can turn a `Variant` that `is<T>` into a `T` by moving it
+ * out of the containing `Variant` instance with the `extract<T>` method:
+ *
+ * Variant<UniquePtr<A>, B, C> v(MakeUnique<A>());
+ * auto ptr = v.extract<UniquePtr<A>>();
+ *
+ * Finally, you can exhaustively match on the contained variant and branch into
+ * different code paths depending which type is contained. This is preferred to
+ * manually checking every variant type T with is<T>() because it provides
+ * compile-time checking that you handled every type, rather than runtime
+ * assertion failures.
+ *
+ * // Bad!
+ * char* foo(Variant<A, B, C, D>& v) {
+ * if (v.is<A>()) {
+ * return ...;
+ * } else if (v.is<B>()) {
+ * return ...;
+ * } else {
+ * return doSomething(v.as<C>()); // Forgot about case D!
+ * }
+ * }
+ *
+ * // Good!
+ * struct FooMatcher
+ * {
+ * // The return type of all matchers must be identical.
+ * char* match(A& a) { ... }
+ * char* match(B& b) { ... }
+ * char* match(C& c) { ... }
+ * char* match(D& d) { ... } // Compile-time error to forget D!
+ * }
+ * char* foo(Variant<A, B, C, D>& v) {
+ * return v.match(FooMatcher());
+ * }
+ *
+ * ## Examples
+ *
+ * A tree is either an empty leaf, or a node with a value and two children:
+ *
+ * struct Leaf { };
+ *
+ * template<typename T>
+ * struct Node
+ * {
+ * T value;
+ * Tree<T>* left;
+ * Tree<T>* right;
+ * };
+ *
+ * template<typename T>
+ * using Tree = Variant<Leaf, Node<T>>;
+ *
+ * A copy-on-write string is either a non-owning reference to some existing
+ * string, or an owning reference to our copy:
+ *
+ * class CopyOnWriteString
+ * {
+ * Variant<const char*, UniquePtr<char[]>> string;
+ *
+ * ...
+ * };
+ */
+template<typename... Ts>
+class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Variant
+{
+ using Tag = typename detail::VariantTag<Ts...>::Type;
+ using Impl = detail::VariantImplementation<Tag, 0, Ts...>;
+ using RawData = AlignedStorage<detail::MaxSizeOf<Ts...>::size>;
+
+ // Raw storage for the contained variant value.
+ RawData raw;
+
+ // Each type is given a unique tag value that lets us keep track of the
+ // contained variant value's type.
+ Tag tag;
+
+ void* ptr() {
+ return reinterpret_cast<void*>(&raw);
+ }
+
+public:
+ /** Perfect forwarding construction for some variant type T. */
+ template<typename RefT,
+ // RefT captures both const& as well as && (as intended, to support
+ // perfect forwarding), so we have to remove those qualifiers here
+ // when ensuring that T is a variant of this type, and getting T's
+ // tag, etc.
+ typename T = typename detail::SelectVariantType<RefT, Ts...>::Type>
+ explicit Variant(RefT&& aT)
+ : tag(Impl::template tag<T>())
+ {
+ new (ptr()) T(Forward<RefT>(aT));
+ }
+
+ /**
+ * Constructs this Variant from an AsVariantTemporary<T> such that T can be
+ * stored in one of the types allowable in this Variant. This is used in the
+ * implementation of AsVariant().
+ */
+ template<typename RefT,
+ typename T = typename detail::SelectVariantType<RefT, Ts...>::Type>
+ MOZ_IMPLICIT Variant(detail::AsVariantTemporary<RefT>&& aValue)
+ : tag(Impl::template tag<T>())
+ {
+ new (ptr()) T(Move(aValue.mValue));
+ }
+
+ /** Copy construction. */
+ Variant(const Variant& aRhs)
+ : tag(aRhs.tag)
+ {
+ Impl::copyConstruct(ptr(), aRhs);
+ }
+
+ /** Move construction. */
+ Variant(Variant&& aRhs)
+ : tag(aRhs.tag)
+ {
+ Impl::moveConstruct(ptr(), Move(aRhs));
+ }
+
+ /** Copy assignment. */
+ Variant& operator=(const Variant& aRhs) {
+ MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
+ this->~Variant();
+ new (this) Variant(aRhs);
+ return *this;
+ }
+
+ /** Move assignment. */
+ Variant& operator=(Variant&& aRhs) {
+ MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
+ this->~Variant();
+ new (this) Variant(Move(aRhs));
+ return *this;
+ }
+
+ /** Move assignment from AsVariant(). */
+ template <typename T>
+ Variant& operator=(detail::AsVariantTemporary<T>&& aValue)
+ {
+ this->~Variant();
+ new (this) Variant(Move(aValue));
+ return *this;
+ }
+
+ ~Variant()
+ {
+ Impl::destroy(*this);
+ }
+
+ /** Check which variant type is currently contained. */
+ template<typename T>
+ bool is() const {
+ static_assert(detail::IsVariant<T, Ts...>::value,
+ "provided a type not found in this Variant's type list");
+ return Impl::template tag<T>() == tag;
+ }
+
+ /**
+ * Operator == overload that defers to the variant type's operator==
+ * implementation if the rhs is tagged as the same type as this one.
+ */
+ bool operator==(const Variant& aRhs) const {
+ return tag == aRhs.tag && Impl::equal(*this, aRhs);
+ }
+
+ /**
+ * Operator != overload that defers to the negation of the variant type's
+ * operator== implementation if the rhs is tagged as the same type as this
+ * one.
+ */
+ bool operator!=(const Variant& aRhs) const {
+ return !(*this == aRhs);
+ }
+
+ // Accessors for working with the contained variant value.
+
+ /** Mutable reference. */
+ template<typename T>
+ T& as() {
+ static_assert(detail::IsVariant<T, Ts...>::value,
+ "provided a type not found in this Variant's type list");
+ MOZ_ASSERT(is<T>());
+ return *reinterpret_cast<T*>(&raw);
+ }
+
+ /** Immutable const reference. */
+ template<typename T>
+ const T& as() const {
+ static_assert(detail::IsVariant<T, Ts...>::value,
+ "provided a type not found in this Variant's type list");
+ MOZ_ASSERT(is<T>());
+ return *reinterpret_cast<const T*>(&raw);
+ }
+
+ /**
+ * Extract the contained variant value from this container into a temporary
+ * value. On completion, the value in the variant will be in a
+ * safely-destructible state, as determined by the behavior of T's move
+ * constructor when provided the variant's internal value.
+ */
+ template<typename T>
+ T extract() {
+ static_assert(detail::IsVariant<T, Ts...>::value,
+ "provided a type not found in this Variant's type list");
+ MOZ_ASSERT(is<T>());
+ return T(Move(as<T>()));
+ }
+
+ // Exhaustive matching of all variant types on the contained value.
+
+ /** Match on an immutable const reference. */
+ template<typename Matcher>
+ auto
+ match(Matcher&& aMatcher) const
+ -> decltype(Impl::match(aMatcher, *this))
+ {
+ return Impl::match(aMatcher, *this);
+ }
+
+ /** Match on a mutable non-const reference. */
+ template<typename Matcher>
+ auto
+ match(Matcher&& aMatcher)
+ -> decltype(Impl::match(aMatcher, *this))
+ {
+ return Impl::match(aMatcher, *this);
+ }
+};
+
+/*
+ * AsVariant() is used to construct a Variant<T,...> value containing the
+ * provided T value using type inference. It can be used to construct Variant
+ * values in expressions or return them from functions without specifying the
+ * entire Variant type.
+ *
+ * Because AsVariant() must copy or move the value into a temporary and this
+ * cannot necessarily be elided by the compiler, it's mostly appropriate only
+ * for use with primitive or very small types.
+ *
+ * AsVariant() returns a AsVariantTemporary value which is implicitly
+ * convertible to any Variant that can hold a value of type T.
+ */
+template<typename T>
+detail::AsVariantTemporary<T>
+AsVariant(T&& aValue)
+{
+ return detail::AsVariantTemporary<T>(Forward<T>(aValue));
+}
+
+} // namespace mozilla
+
+#endif /* mozilla_Variant_h */
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