build command.txt

clang++ -I/Users/xaxxon/v8  -std=c++14 -Wall -Werror bar.cpp -o bar -L/Users/xaxxon/v8/out/x64.debug/  libv8toolkit.a -lv8_base -lv8_libbase -lv8_base -lv8_libplatform -lv8_nosnapshot -licudata -licuuc -licui18n

output.txt

In some_function
Allocating int at 0x7f9c12700520
uncasted internal field: 0x7f9c12700520
In some_function
Allocating int at 0x7f9c127004b0
uncasted internal field: 0x0

repro.cpp

#include "include/libplatform/libplatform.h"
#include "include/v8.h"

class ArrayBufferAllocator : public v8::ArrayBuffer::Allocator {
 public:
  inline virtual void* Allocate(size_t length) override {
    void* data = AllocateUninitialized(length);
    return data == NULL ? data : memset(data, 0, length);
  }
  inline virtual void* AllocateUninitialized(size_t length) override { return malloc(length); }
  inline virtual void Free(void* data, size_t) override { free(data); }
};



void MyClass(const v8::FunctionCallbackInfo<v8::Value> & info) {
  printf("In some_function\n");
  auto js_object = info.This();
  int * i = new int();
  printf("Allocating int at %p\n", i);


  js_object->SetInternalField(0, v8::External::New(info.GetIsolate(),i));

}

void do_something(const v8::FunctionCallbackInfo<v8::Value> & info) {
  auto self = info.Holder();
  auto wrap = v8::Local<v8::External>::Cast(self->GetInternalField(0));
  printf("uncasted internal field: %p\n", wrap->Value());
  

}

int main(int argc, char ** argv)
{
      v8::V8::InitializeICU();
      auto platform = v8::platform::CreateDefaultPlatform();
      v8::V8::InitializePlatform(platform);
      v8::V8::Initialize();

      v8::Isolate::CreateParams create_params;
      create_params.array_buffer_allocator = (v8::ArrayBuffer::Allocator *) new ArrayBufferAllocator();

      auto i = v8::Isolate::New(create_params);

      {
	v8::Isolate::Scope is(i);
	v8::HandleScope hs(i);
	auto got = v8::ObjectTemplate::New(i);
	
	auto ft = v8::FunctionTemplate::New(i, MyClass);
	ft->InstanceTemplate()->SetInternalFieldCount(1);
	got->Set(i, "MyClass", ft);
	
	auto ft2 = v8::FunctionTemplate::New(i, do_something);
	ft->InstanceTemplate()->Set(i, "do_something", ft2);
	
	
	auto c = v8::Context::New(i, nullptr, got);

	v8::Context::Scope cs(c);


	auto s = v8::Script::Compile(c, v8::String::NewFromUtf8(i,"new MyClass().do_something(); Object.create(new MyClass()).do_something();")).ToLocalChecked();

	(void)s->Run(c);
      }

}