C++ inheritance

C++ often uses inheritance. Sometimes a C++ API may also offer just a pure virtual class that the API user must implement to interface with the library. When faced with an API like this with FFI, the smart thing to do is to give up and do something else.

If you do not feel like being smart, I have good news for you: It is possible to do inheritance through an FFI interface layer, at the very least with GCC compiled C++. With other compilers your mileage may vary, though the basic idea is definitely the same.

C++ has two flavours of inheritance: Normal or concrete inheritance, and virtual inheritance. On top of this, inheritance can be done in the singular or plural. Here I'll be talking of normal inheritance in the singular. If you need to do multiple inheritance or virtual inheritance then you're on your own. Good luck.

Let's proceed with normal inheritance. At the core of C++ inheritance is the virtual table or vtable for short. A class that inherits from a class will have a vtable pointer at the beginning of its memory layout. A call to a virtual method will then indirectly find the instance's implementation of said method through the vtable pointer and call that instead of doing a direct call to a known function in the library.

Single inheritance

Before we being creating our own inherited C++ class in Deno, let's take a look at the memory layout of the vtable. This is the layout from a GCC compiled C++ library, so if you're using some other compiler then your vtable may look completely different.

We'll be using the inherited_class.cpp example as our base. Apologies for the TypeScript code being a mess.

Let's start with our base class (some parts omitted):

class PartiallyVirtualClass {
  PartiallyVirtualClass(int data);
  virtual ~PartiallyVirtualClass();

  virtual void doData(int data);
  virtual void useData(int data);
  virtual void maybeData() = 0;

We have here four virtual methods, one of which is pure virtual and one is the destructor. As we already learned in the C++ calling convention, C++ has three destructors of which 1 is often a duplicate. Thus, our actual number of virtual functions in this example is 5:

  • 2 for the destructors (deleting, and complete / base)
  • 1 for each of the virtual methods.

Additionally, our virtual table will need to hold a pointer to the object's type info, plus one extra pointer sized slot which holds the offset from this virtual table pointer (when in an instance) to the parent class. That information is relevant in either virtual or multiple inheritance but for our singular normal inheritance it is always 0.

The final vtable looks like this:

struct PartiallyVirtualClass_VTABLE {
    void* class_offset_, // This is 0 for our case
    void* type_info_,    // If library is built with `-fno-rtti` this is also 0
    void* destructor_D1, // complete / base object destructor: For base class this is often 0 as well
    void* destructor_D0, // deleting destructor: for base class this is often 0 as well
    void* doData_method,
    void* useData_method,
    void* maybeData_method,

If we read the values in our libinherited_class.so's vtable for the PartiallyVirtualClass then we find the following data:


The destructor pointers are zeroed out, presumably as unnecessary. The method calls that the class implements are then present in the vtable while finally the pure virtual function is replaced by a pointer to a C++ injected __cxa_pure_virtual function, which just prints out pure virtual method called and terminates the program.

The order of methods in the vtable depend on how they're declared in the class header, including the destructors'. If the destructors were moved to be the last item in the header, then they'd move to the bottom of the vtable though their internal order does not change (D1 first, D0 second).

Next let's take a look at a derived class:

class Derived : PartiallyVirtualClass {

  void doData(int data) override;
  void maybeData() override;

Our derived class overrides the destructor, doData() and maybeData() methods but leaves the useData() method untouched.

The size of the virtual table for this class is now the same size as the base class is, but with some data changes:


As we can see, the destructors are now found in the virtual table. Additionally, the maybeData() method is now there as well. Most interestingly, the useData() method is also found in the table but points to the base class method, since it was not overridden.

With this we now know how to implement our own derived class. First, we need to build ourselves a vtable of the same size as our base class:

const JS_DERIVED_VTABLE = new BigUint64Array(7);

The first item in the array we leave alone as 0. The second item is the type info pointer: If you're sure that the library doesn't use type info then you can safely leave it as 0. Otherwise, you need to implement a typeinfo struct as well, which includes at least one C string pointer and probably some other data. Good luck.

The third and fourth items are more interesting: We need to implement our own destructors. At their core, these should just call out to the base class' base object destructor (D2) and JavaScript's garbage collection will take care of the rest. But, if you've done other allocations using FFI in your "constructors" of this class instance, then these destructors are the place where you handle calling those destructors.

Note also that from Deno point of view, there is no difference between the deleting destructor (D0) and complete object destructor (D1): Deno should never manually deallocate memory as that is the garbage collection algorithm's job.

Then, the final three items are the methods. Here we can either inherit methods from the base class, or override them with our own functions. But how do we get the function pointers for inheriting base class methods? And how do we define our own functions?

Inheriting base class methods

Base class methods can be "inherited" by getting the pointer to a base class method and assigning said pointer to your vtable. You can get the pointer to a class method by declaring the method as a static in the Deno.dlopen call:

const lib = Deno.dlopen(
    "ptr__libexample__PartiallyVirtualClass__doData": {
      name: "_ZN11example_lib21PartiallyVirtualClass6doDataEi",
      type: "pointer",

Defining own class methods

Defining your own class methods is effectively the same act as defining FFI callbacks. As an example, we can implement our own doData() method like this:

const DO_DATA = new Deno.UnsafeCallback({
  parameters: ["buffer", "i32"],
  result: "void",
}, (pointer, data) => {
  console.log("doData:", pointer, data);

Since the C++ API for the doData() method takes one int, our callback needs to take two parameters: The first is the this argument, and the second is the int parameter passed in by the caller of the function.

Multiple inheritance

You're on your own. Good luck.