tor-browser

GCVariant.h (5576B)

---

/* -*- 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/. */

#ifndef js_GCVariant_h
#define js_GCVariant_h

#include "mozilla/Variant.h"

#include <type_traits>

#include "js/GCPolicyAPI.h"
#include "js/RootingAPI.h"
#include "js/TracingAPI.h"

namespace JS {

// These template specializations allow Variant to be used inside GC wrappers.
//
// When matching on GC wrappers around Variants, matching should be done on
// the wrapper itself. The matcher class's methods should take Handles or
// MutableHandles. For example,
//
//   struct MyMatcher
//   {
//        using ReturnType = const char*;
//        ReturnType match(HandleObject o) { return "object"; }
//        ReturnType match(HandleScript s) { return "script"; }
//   };
//
//   Rooted<Variant<JSObject*, JSScript*>> v(cx, someScript);
//   MyMatcher mm;
//   v.match(mm);
//
// If you get compile errors about inability to upcast subclasses (e.g., from
// NativeObject* to JSObject*) and are inside js/src, be sure to also include
// "gc/Policy.h".

namespace detail {

template <typename... Ts>
struct GCVariantImplementation;

// The base case.
template <typename T>
struct GCVariantImplementation<T> {
 template <typename ConcreteVariant>
 static void trace(JSTracer* trc, ConcreteVariant* v, const char* name) {
   T& thing = v->template as<T>();
   GCPolicy<T>::trace(trc, &thing, name);
 }

 template <typename Matcher, typename ConcreteVariant>
 static typename Matcher::ReturnType match(Matcher& matcher,
                                           Handle<ConcreteVariant> v) {
   const T& thing = v.get().template as<T>();
   return matcher.match(Handle<T>::fromMarkedLocation(&thing));
 }

 template <typename Matcher, typename ConcreteVariant>
 static typename Matcher::ReturnType match(Matcher& matcher,
                                           MutableHandle<ConcreteVariant> v) {
   T& thing = v.get().template as<T>();
   return matcher.match(MutableHandle<T>::fromMarkedLocation(&thing));
 }
};

// The inductive case.
template <typename T, typename... Ts>
struct GCVariantImplementation<T, Ts...> {
 using Next = GCVariantImplementation<Ts...>;

 template <typename ConcreteVariant>
 static void trace(JSTracer* trc, ConcreteVariant* v, const char* name) {
   if (v->template is<T>()) {
     T& thing = v->template as<T>();
     GCPolicy<T>::trace(trc, &thing, name);
   } else {
     Next::trace(trc, v, name);
   }
 }

 template <typename Matcher, typename ConcreteVariant>
 static typename Matcher::ReturnType match(Matcher& matcher,
                                           Handle<ConcreteVariant> v) {
   if (v.get().template is<T>()) {
     const T& thing = v.get().template as<T>();
     return matcher.match(Handle<T>::fromMarkedLocation(&thing));
   }
   return Next::match(matcher, v);
 }

 template <typename Matcher, typename ConcreteVariant>
 static typename Matcher::ReturnType match(Matcher& matcher,
                                           MutableHandle<ConcreteVariant> v) {
   if (v.get().template is<T>()) {
     T& thing = v.get().template as<T>();
     return matcher.match(MutableHandle<T>::fromMarkedLocation(&thing));
   }
   return Next::match(matcher, v);
 }
};

}  // namespace detail

template <typename... Ts>
struct GCPolicy<mozilla::Variant<Ts...>> {
 using Impl = detail::GCVariantImplementation<Ts...>;

 static void trace(JSTracer* trc, mozilla::Variant<Ts...>* v,
                   const char* name) {
   Impl::trace(trc, v, name);
 }

 static bool isValid(const mozilla::Variant<Ts...>& v) {
   return v.match([](auto& v) {
     return GCPolicy<std::remove_reference_t<decltype(v)>>::isValid(v);
   });
 }
};

}  // namespace JS

namespace js {

template <typename Wrapper, typename... Ts>
class WrappedPtrOperations<mozilla::Variant<Ts...>, Wrapper> {
 using Impl = JS::detail::GCVariantImplementation<Ts...>;
 using Variant = mozilla::Variant<Ts...>;

 const Variant& variant() const {
   return static_cast<const Wrapper*>(this)->get();
 }

public:
 template <typename T>
 bool is() const {
   return variant().template is<T>();
 }

 template <typename T>
 JS::Handle<T> as() const {
   return JS::Handle<T>::fromMarkedLocation(&variant().template as<T>());
 }

 template <typename Matcher>
 typename Matcher::ReturnType match(Matcher& matcher) const {
   return Impl::match(matcher,
                      JS::Handle<Variant>::fromMarkedLocation(&variant()));
 }
};

template <typename Wrapper, typename... Ts>
class MutableWrappedPtrOperations<mozilla::Variant<Ts...>, Wrapper>
   : public WrappedPtrOperations<mozilla::Variant<Ts...>, Wrapper> {
 using Impl = JS::detail::GCVariantImplementation<Ts...>;
 using Variant = mozilla::Variant<Ts...>;

 const Variant& variant() const {
   return static_cast<const Wrapper*>(this)->get();
 }
 Variant& variant() { return static_cast<Wrapper*>(this)->get(); }

public:
 template <typename T>
 JS::MutableHandle<T> as() {
   return JS::MutableHandle<T>::fromMarkedLocation(
       &variant().template as<T>());
 }

 template <typename Matcher>
 typename Matcher::ReturnType match(Matcher& matcher) {
   return Impl::match(
       matcher, JS::MutableHandle<Variant>::fromMarkedLocation(&variant()));
 }
};

}  // namespace js

#endif  // js_GCVariant_h