implementing-jsc-classes-cpp

Implementing JavaScript Classes in C++

Class Structure

For publicly accessible Constructor and Prototype, create 3 classes:

1. class Foo : public JSC::DestructibleObject - if C++ fields exist; otherwise use JSC::constructEmptyObject with putDirectOffset
2. class FooPrototype : public JSC::JSNonFinalObject
3. class FooConstructor : public JSC::InternalFunction

No public constructor? Only Prototype and class needed.

Iso Subspaces

Classes with C++ fields need subspaces in:

• src/jsc/bindings/webcore/DOMClientIsoSubspaces.h
• src/jsc/bindings/webcore/DOMIsoSubspaces.h

template<typename MyClassT, JSC::SubspaceAccess mode>
static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) {
    if constexpr (mode == JSC::SubspaceAccess::Concurrently)
        return nullptr;
    return WebCore::subspaceForImpl<MyClassT, WebCore::UseCustomHeapCellType::No>(
        vm,
        [](auto& spaces) { return spaces.m_clientSubspaceForMyClassT.get(); },
        [](auto& spaces, auto&& space) { spaces.m_clientSubspaceForMyClassT = std::forward<decltype(space)>(space); },
        [](auto& spaces) { return spaces.m_subspaceForMyClassT.get(); },
        [](auto& spaces, auto&& space) { spaces.m_subspaceForMyClassT = std::forward<decltype(space)>(space); });
}

Property Definitions

static JSC_DECLARE_HOST_FUNCTION(jsFooProtoFuncMethod);
static JSC_DECLARE_CUSTOM_GETTER(jsFooGetter_property);

static const HashTableValue JSFooPrototypeTableValues[] = {
    { "property"_s, static_cast<unsigned>(PropertyAttribute::ReadOnly | PropertyAttribute::CustomAccessor), NoIntrinsic, { HashTableValue::GetterSetterType, jsFooGetter_property, 0 } },
    { "method"_s, static_cast<unsigned>(PropertyAttribute::Function), NoIntrinsic, { HashTableValue::NativeFunctionType, jsFooProtoFuncMethod, 1 } },
};

Prototype Class

class JSFooPrototype final : public JSC::JSNonFinalObject {
public:
    using Base = JSC::JSNonFinalObject;
    static constexpr unsigned StructureFlags = Base::StructureFlags;

    static JSFooPrototype* create(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::Structure* structure) {
        JSFooPrototype* prototype = new (NotNull, allocateCell<JSFooPrototype>(vm)) JSFooPrototype(vm, structure);
        prototype->finishCreation(vm);
        return prototype;
    }

    template<typename, JSC::SubspaceAccess>
    static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) { return &vm.plainObjectSpace(); }

    DECLARE_INFO;

    static JSC::Structure* createStructure(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::JSValue prototype) {
        auto* structure = JSC::Structure::create(vm, globalObject, prototype, JSC::TypeInfo(JSC::ObjectType, StructureFlags), info());
        structure->setMayBePrototype(true);
        return structure;
    }

private:
    JSFooPrototype(JSC::VM& vm, JSC::Structure* structure) : Base(vm, structure) {}
    void finishCreation(JSC::VM& vm);
};

void JSFooPrototype::finishCreation(VM& vm) {
    Base::finishCreation(vm);
    reifyStaticProperties(vm, JSFoo::info(), JSFooPrototypeTableValues, *this);
    JSC_TO_STRING_TAG_WITHOUT_TRANSITION();
}