C++ Tutorial

A follow-along series of coding experiments that allowed me to explore and understand concepts like the vptr, vtable, and how the compiler organizes base and derived members in memory by seeing them in action through code.

I’ve struggled to understand the exact memory layout of C++ objects. Each time I thought I grasped it, I later came back to realize I didn’t. So, I decided to make a series of experiments to make sure that this time I really got it. Hopefully this post will be useful to you if you’re like me, someone who loves lots of diagrams and experiments to visualize concepts.

You can find all the code and experiments on GitHub.

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Series Parts

1. Part 1 — Single Inheritance (current one)
2. Part 2 — Multiple Non-Virtual Inheritance (Diamond Without Virtual) (Coming soon)
3. Part 3 — Virtual Inheritance (Diamond With Virtual Base) (Coming soon)

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Before going further, it’s worth mentioning one important assumption: all the experiments in this series use the Itanium C++ ABI, which is what GCC and Clang follow on most platforms.

The ABI (Application Binary Interface) defines things the C++ standard doesn’t, like how vtables are laid out, where the vptr lives, how virtual calls are dispatched, and how RTTI (runtime type information) works.

Different compilers and platforms can follow different ABIs. For example, MSVC on Windows uses its own. So, the exact memory layouts we’ll see here apply specifically to the Itanium C++ ABI.

Simple Single Inheritance

We’ll start with single inheritance, so we can then build on top of this intuition.

struct Base {
Base(int b = 0) : b_data(b) {}

virtual void f() {};
int b_data;
};

struct Derived : Base {
Derived(int b = 0, int d = 0)
: Base(b), d_data(d) {}

int d_data;
};

Let’s create a couple of objects from the classes we just defined:

Base base {1};
Derived derived {2, 3};

We initialize these data members so that we can later see them in memory.

Memory Layout of Derived

Because Base has a virtual function, each Base or Derived object includes a hidden pointer to the vtable (vptr), typically the first word in the object layout.

Every polymorphic object (a class with at least one virtual function) contains a hidden pointer called the vptr. This pointer points to a vtable, a table of function pointers corresponding to the virtual functions of that object’s dynamic type. When you call a virtual function, the program doesn’t call a fixed memory address. Instead, it looks up the correct function via the vptr. This mechanism is what allows dynamic dispatch: the program chooses the right function at runtime depending on the actual type of the object. We’ll see this in action in a bit and manually trigger dynamic dispatch.

So, memory for objects of type Base and Derived roughly looks like:

Figure 1: Memory layout of a Base object and a Derived object. The Derived layout includes the virtual function table pointer (vptr) inherited from Base, followed by the Base’s `b_data`, and then Derived’s `d_data`. This shows how Derived extends Base while maintaining the base subobject’s memory layout.

Let’s do an experiment to visualize this.

We’ll review a few modern features that will come in handy. This is meant to serve a cheat-sheet as we build our experiment, not a comprehensive explanation.

Concept	Intuitive Explanation
std::uintptr_t	A number type that can safely hold a pointer’s value.
std::byte	A special type that represents a single byte of raw memory.
std::span	A non-owning “view” of an array or vector (like a safe pointer + size).
reinterpret_cast	Reinterprets raw bits as another type (used for low-level memory manipulation).
std::as_bytes	Obtains a view to the object representation of the elements of a span.

Goal: visualize how an object is laid out in memory.

For each data member, we want to see three things:

• its offset from the start of the object,
• its size, and
• the actual bytes it occupies in memory.

Pointer Arithmetic Using std::uintptr_t

In standard C++, you can only do pointer arithmetic within the same array object. That’s why something like:

&derived.b_data - &derived

is undefined behavior because derived and derived.b_data are not elements of the same array. They’re separate subobjects inside a class.

Even if you cast to char*:

reinterpret_cast<char*>(&derived.b_data) - reinterpret_cast<char*>(&derived)

This usually works in practice (and is what people often do), but technically it’s only guaranteed to be valid if the two pointers point into the same object or one past it.

To stay fully defined, we can cast pointers to integers (std::uintptr_t) and do arithmetic there instead. We are basically doing integer math.

So, to get the offset of a member:

Derived derived;
std::uintptr_t derivedPtr { reinterpret_cast<std::uintptr_t>(&derived)};
offset = reinterpret_cast<std::uintptr_t>(&derived.b_data) - derivedPtr

Displaying Offset, Size and Raw Bytes

We’ll group the information we want into a simple structure:

struct Field {
std::size_t offset;
std::size_t size;
std::string_view name;
};

Then fill it in. The vptr (the virtual table pointer) isn’t visible directly, so we cheat a little and assume it starts at offset 0x00. Don’t worry, we’ll verify that this is the case by calling the functions through the vtable.

Field fields [] = {
{0x00, sizeof(void*), "vptr"},
{reinterpret_cast<std::uintptr_t>(&derived.b_data) - derivedPtr, sizeof(derived.b_data), "b_data"},
{reinterpret_cast<std::uintptr_t>(&derived.d_data) - derivedPtr, sizeof(derived.d_data), "d_data"}
};

To inspect the object’s raw bytes, we can use std::span and std::as_bytes:

auto dataBytes { std::as_bytes(std::span{&derived, 1}) };

std::as_bytes gives us a safe view of the object’s binary representation as std::bytes.

Finally, we can display everything nicely:

void dumpLabeled(std::span<const std::byte> dataBytes,
std::span<const Field> fields) {

std::println("Offset  Size  Field     Bytes");
std::println("------  ----  -----     ------");

for (const auto& field: fields) {
std::print("0x{:02X}      {}    {:<8} ", field.offset, field.size, field.name);

for (std::size_t i {}; i < field.size && (field.offset + i) < dataBytes.size(); i++) {
auto b { dataBytes[field.offset + i] };
std::print("{:02x} ", std::to_integer<unsigned>(b));
}
std::println();
}
}

This function simply walks over the object’s bytes and prints each field’s offset, size, and raw memory contents. The formatting inside std::print is for readability, don’t worry too much about that part.

Before dumping the object’s bytes, I decided to also print a few details such as the object’s address, its base pointer, and the sizes of the involved types, just to give some context. (Full source is on GitHub.)

sizeof(Base):    16
sizeof(Derived): 16

&derived (Derived*):          0x7fff31b5d5e0
&base (Base*):                0x7fff31b5d5d0

Base obj bytes (annotated):
Offset  Size  Field     Bytes
------  ----  -----     ------
0x00      8    vptr     d0 0b 3b 40 ad 55 00 00
0x08      4    b_data   01 00 00 00

Derived obj bytes (annotated):
Offset  Size  Field     Bytes
------  ----  -----     ------
0x00      8    vptr     b8 0b 3b 40 ad 55 00 00
0x08      4    b_data   02 00 00 00
0x0C      4    d_data   03 00 00 00

Offset of Base within Derived: 0 bytes

We can now see the object layout clearly:

• the first 8 bytes are the vptr,
• followed by the base class field b_data,
• and then the derived class field d_data.

For base, we see that the b_data is indeed 1, as we had initialized it. For derived, b_data is 2, while d_data is equal to 3. All looks good.

The Derived object takes up 16 bytes in memory. It’s what we’d expect: 8 bytes for the vptr and 4 bytes each for the two int members. But wait… shouldn’t Base be smaller, since it only has one int?

The reason for this is padding: extra bytes the compiler inserts to keep everything properly aligned in memory.

Padding

With a bit (no pun intended) more pointer arithmetic, we can actually peek at those padding bytes.

Base obj bytes (annotated with padding):
Offset  Size  Field     Bytes
------  ----  -----     ------
0x00      8    vptr     d0 0b 3b 40 ad 55 00 00
0x08      4    b_data   01 00 00 00
0x0C      4    padding  31 7f 00 00

Invoking Functions Directly via Vptr

Now for the fun part. I promised we would use the vptr, located at the start of each object, to access the virtual function and call them directly.

Let’s start out by making our functions print something so that we can recognize which function has been called. We don’t care about the data members for this part, so we can just get rid of them.

struct Base {
virtual void f() {
std::print("Base::f()");
}
virtual void g() {
std::print("Base::g()");
}
};

struct Derived : Base {
virtual void f() override {
std::print("Derived::f()");
}
virtual void h() {
std::print("Derived::h()");
}
};

Function Pointer Type For Member Functions

A vtable entry has the type roughly: (more on function pointers bellow)

using FnPtr = void(*)(void*);  // takes "this" pointer manually

Each virtual function is compiled to something like:

void Derived_f(Derived* this);
void Base_g(Base* this);

Here we can see that these functions take the this pointer manually. This is done automatically by the compiler, so we don’t have to worry about it when calling member functions.

So, if we reinterpret a vtable entry as a FnPtr, and pass it the address of our object, we can call it directly.

Accessing the Hidden Vptr

1. We first get the address of the derived object. It’s type is Derived*.

&derived

1. We cast this pointer to void** so we can treat the first word of the object as a pointer.

reinterpret_cast<void**>&derived

1. Next, we dereference that pointer to read the value stored at the start of the object:

*reinterpret_cast<void**>(&derived)

1. Finally, we store that value, the vtable address, into a void* variable:

void* vptr = *reinterpret_cast<void**>(&derived);

Reinterpreting the vptr as a Vtable Pointer

Right now, vptr is just a void*. That’s a pointer to “something,” but the compiler doesn’t know it points to an array of (function) pointers. By doing:

auto vtable = reinterpret_cast<void**>(vptr);

we now have a pointer to an array of void* (our vtable entries).

Function Pointer

Let’s review the syntax for a function pointer:

return_type (*FuncPtr) (parameter_type, ...);

So, we can define our own function pointer type:

using FnPtr = void(*)(void*);

• void -> The function returns nothing.
• (void*) -> The function takes a single argument, which is a void*.
• (*) -> This is the syntax for declaring a pointer to a function rather than a regular function.

Inspecting a Vtable and Manually Triggering Virtual Dispatch

We are finally ready to iterate over the vtable entries and invoke each of the virtual functions:

for (size_t i {}; i < 3; i++) {
std::print("[{}] {:p}  ", i, vtable[i]);

if (vtable[i] != nullptr) {
auto fn = reinterpret_cast(vtable[i]);
fn(obj);
}
std::println();
}

The output, where we can see our beautiful functions being called.

vtable pointer (vptr): 0x55b010347bb8

i     vtable[i]     calling funtion
---  -------------  ----------------
[0] 0x55b010323370  Derived::f()
[1] 0x55b010323318  Base::g()
[2] 0x55b0103233c8  Derived::h()

Let’s pause and recap what we’ve done so far. We accessed the first member of our polymorphic object, the vptr which points to the vtable. Using a few careful casts, we were able to reach the vtable itself. From there, we iterated over its entries and even called the functions directly, seeing virtual dispatch in action and proving that the first member of the object is in fact the vptr.

Difference Between Deleting and Complete Object Destructors

The next logical thing to try is to add a virtual destructor:

struct Base {
virtual ~Base() {
std::print(" Base::~Base()");
}
virtual void f() {
std::print("Base::f()");
}
virtual void g() {
std::print("Base::g()");
}
};

struct Derived : Base {
~Derived() {
std::print(" Derived::~Derived()");
}
virtual void f() override {
std::print("Derived::f()");
}
virtual void h() {
std::print("Derived::h()");
}
};

Let’s print out the first vtable element:

vtable pointer (vptr): 0x55e2d423db78

i     vtable[i]     calling funtion
---- -------------  ----------------
[ 0] 0x55e2d421849c  Derived::~Derived() Base::~Base()

Derived::~Derived() Base::~Base()

Now that we’ve confirmed the first entry is the destructor, we can safely skip it and start iterating from index 1. This is the tricky part. I’d assumed the first element of the vtable would be the destructor, and it is. However, the second element is also a destructor!

This is what happens when we try calling that second element on vtable:

vtable pointer (vptr): 0x56489b9b3b78

i     vtable[i]     calling funtion
---- -------------  ----------------
free(): invalid pointer
zsh: IOT instruction (core dumped)  ./a.out

So, I did some digging and I’m happy to share my findings.

Destructors are split into two variants:

Vtable Entry	Destructor	When used	What it does
[0]	Complete object destructor	When the object is destroyed but not freed (e.g. stack object, or called internally by delete)	Destroys the full object (calls base destructors, releases resources).
[1]	Deleting destructor	When deleting through a base pointer (e.g. delete basePtr;)	Calls the complete destructor, then invokes operator delete.

Basically what we did earlier was to call the deleting destructor, which will free the memory of the object. However, our object is stack-allocated. That’s why this results in free(): invalid pointer.

Visualizing The Deleting and Complete Object Destructors

This whole thing about two different destructors got me intrigued. I’ve always seen in diagrams of the vtable only one entry for the destructor, never two. Or at least I don’t remember hearing anything about two destructors, maybe I just missed it. What I’m trying to say is that this came as a surprise for me, so naturally I wanted to see these two destructors.

Let’s disassemble the binary:

objdump -dC a.out | grep -A20 "Derived::~Derived"

We can see that the compiler emits two destructor symbols:

00000000000064f4 <Derived::~Derived()>:
# this destructor destroys the object but does NOT free memory
# only prints the message and updates the vptr (ABI housekeeping)
64f4:       55                      push   %rbp
64f5:       48 89 e5                mov    %rsp,%rbp
64f8:       48 83 ec 30             sub    $0x30,%rsp
64fc:       48 89 7d d8             mov    %rdi,-0x28(%rbp)
6500:       64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
6507:       00 00
6509:       48 89 45 f8             mov    %rax,-0x8(%rbp)
650d:       31 c0                   xor    %eax,%eax
# computes address of vtable
650f:       48 8d 15 62 56 02 00    lea    0x25662(%rip),%rdx        # 2bb78 <vtable for Derived+0x10>
# loads object pointer
6516:       48 8b 45 d8             mov    -0x28(%rbp),%rax
# writes vtable pointer into object (vptr)
651a:       48 89 10                mov    %rdx,(%rax)
651d:       48 c7 45 e0 13 00 00    movq   $0x13,-0x20(%rbp)
6524:       00
6525:       48 8d 05 73 8d 01 00    lea    0x18d73(%rip),%rax        # 1f29f <std::ranges::_Cpo::data+0x297>
652c:       48 89 45 e8             mov    %rax,-0x18(%rbp)
6530:       48 8b 55 e0             mov    -0x20(%rbp),%rdx
6534:       48 8b 45 e8             mov    -0x18(%rbp),%rax
6538:       48 89 d7                mov    %rdx,%rdi
653b:       48 89 c6                mov    %rax,%rsi
# calls our print function
653e:       e8 ce 33 00 00          call   9911 <void std::print<>(std::basic_format_string<char>)>
--
655d:       74 05                   je     6564 <Derived::~Derived()+0x70>
655f:       e8 0c dd ff ff          call   4270 <__stack_chk_fail@plt>
6564:       c9                      leave
6565:       c3                      ret


0000000000006566 <Derived::~Derived()>:
# this destructor calls the complete destructor and then frees memory
6566:       55                      push   %rbp
6567:       48 89 e5                mov    %rsp,%rbp
656a:       48 83 ec 10             sub    $0x10,%rsp
656e:       48 89 7d f8             mov    %rdi,-0x8(%rbp)
6572:       48 8b 45 f8             mov    -0x8(%rbp),%rax
6576:       48 89 c7                mov    %rax,%rdi
# calls the complete destructor
6579:       e8 76 ff ff ff          call   64f4 <Derived::~Derived()>
657e:       48 8b 45 f8             mov    -0x8(%rbp),%rax
6582:       be 08 00 00 00          mov    $0x8,%esi
6587:       48 89 c7                mov    %rax,%rdi
# calls the delete operator
658a:       e8 a1 dc ff ff          call   4230 <operator delete(void*, unsigned long)@plt>
658f:       c9                      leave
6590:       c3                      ret
6591:       90                      nop

The reason why it’s not talked about much is because the C++ language only defines one virtual destructor per class. The fact that compilers (like GCC/Clang) emit two actual function symbols is an implementation detail of the Itanium C++ ABI. For understanding polymorphic calls, it’s enough to think of the first element as the destructor, without worrying about the internal distinction between “deleting” and “complete object” destructors.

Now, let’s try again, skipping the first two elements.

i     vtable[i]         meaning
---- -------------  ----------------
[ 0] 0x55cf265075a2  complete object destructor
[ 1] 0x55cf26507614  deleting destructor

i     vtable[i]     calling funtion
---- -------------  ----------------
[ 2] 0x55cf26507640  Derived::f()
[ 3] 0x55cf2650754a  Base::g()
[ 4] 0x55cf26507698  Derived::h()


Derived::~Derived() Base::~Base()

We get our beautiful vtable:

However, the story is not over yet. For those who like finding hidden treasure, there are two more to be found.

Vtable Entries [-1] and [-2]

Another of the compiler’s (not-so-)well-kept secrets is the [-1] and [-2] entries of the vtable.

Vtable Entry	Meaning	Purpose
[-2]	offset-to-top	If a derived object is part of a multiple inheritance hierarchy, the offset-to-top tells the runtime how far the current subobject is from the start of the complete object.
[-1]	points to the typeinfo object for the class	This is the RTTI (Run-Time Type Information) for Derived. It’s used by dynamic_cast and typeid.

Our hard work pays off and we finally have our complete vtable:

i     vtable[i]         meaning
---- -------------  ----------------
[-2] 0x0             offset-to-top
[-1] 0x55cf2652cd00  typeinfo (RTTI)
[ 0] 0x55cf265075a2  complete object destructor
[ 1] 0x55cf26507614  deleting destructor

i     vtable[i]     calling funtion
---- -------------  ----------------
[ 2] 0x55cf26507640  Derived::f()
[ 3] 0x55cf2650754a  Base::g()
[ 4] 0x55cf26507698  Derived::h()

Here’s one final diagram (I did say there would be plenty!) that illustrates how Derived and Base coexist. It’s a bit complex, but personally, visualizing these relationships really helped me solidify the concepts.

Thanks for reading until the end and hope you enjoyed!

Resources for Further Reading

• What does C++ Object Layout Look Like?: Nimrod’s Coding Lab

• Explains memory layouts of C++ objects with diagrams and examples.

• Memory Layout of C++ Object: Vishal Chovatiya

• Offers insights into how different members and inheritance affect object memory.