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