GCVariant.h

/* -*- 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