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Update the jsc class skill for Bun::allocatePlainObjectCell #47840

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Update the jsc class skill for Bun::allocatePlainObjectCell #47840

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a668e42
Update the jsc class skill for Bun::allocatePlainObjectCell
robobun Nov 3, 2026
333628d
jsc class skill: call Bun::createClassStructure in the constructor sn…
robobun Nov 3, 2026
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18 changes: 10 additions & 8 deletions .claude/skills/implementing-jsc-classes-cpp/SKILL.md
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Original file line number Diff line number Diff line change
Expand Up @@ -55,18 +55,15 @@ public:
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);
JSFooPrototype* prototype = new (NotNull, Bun::allocatePlainObjectCell(vm, sizeof(JSFooPrototype))) 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());
auto* structure = Bun::createClassStructure(vm, globalObject, prototype, JSC::TypeInfo(JSC::ObjectType, StructureFlags), info());
structure->setMayBePrototype(true);
return structure;
}
Expand All @@ -75,14 +72,19 @@ private:
JSFooPrototype(JSC::VM& vm, JSC::Structure* structure) : Base(vm, structure) {}
void finishCreation(JSC::VM& vm);
};
STATIC_ASSERT_ISO_SUBSPACE_SHARABLE(JSFooPrototype, JSFooPrototype::Base);

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

A prototype class adds no C++ field and no destructor to `JSC::JSNonFinalObject`, so its object fits in the shared `vm.plainObjectSpace()`. `Bun::allocatePlainObjectCell` allocates there and never asks the class for a subspace, so the class has no `subspaceFor` member. `STATIC_ASSERT_ISO_SUBSPACE_SHARABLE` fails the build when the class gains a field or its own `destroy`. Keep the assert at file scope, after the class: the compiler does not evaluate it inside a member template that nothing instantiates.

`Bun::allocatePlainObjectCell`, `Bun::createClassStructure`, `Bun::reifyStaticPropertyTable` and `Bun::putToStringTagWithoutTransition` are declared in `src/jsc/bindings/BunClientData.h`. They are the out-of-line forms of `JSC::allocateCell`, `JSC::Structure::create`, `JSC::reifyStaticProperties` and `JSC_TO_STRING_TAG_WITHOUT_TRANSITION()`. Use them so that a new class does not inline its own copy of that once-per-class setup.

## Getter/Setter/Function Definitions

```cpp
Expand Down Expand Up @@ -131,7 +133,7 @@ public:
static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) { return &vm.internalFunctionSpace(); }

static JSC::Structure* createStructure(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::JSValue prototype) {
return JSC::Structure::create(vm, globalObject, prototype, JSC::TypeInfo(JSC::InternalFunctionType, StructureFlags), info());
return Bun::createClassStructure(vm, globalObject, prototype, JSC::TypeInfo(JSC::InternalFunctionType, StructureFlags), info());
}

private:
Expand Down
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