It's a moment to remember when you finally realize that C++ is just C with one scoop of syntactic sugar and ten scoops of bureaucracy trying to fix that first scoop.

A long time back, I debugged a buffer overflow that was corrupting a vtable pointer. Oh boy, that was a mess. Basically, execution would jump seemingly randomly and I’d end up seeing a nonsensical and corrupted backtrace. It’s fun when your program crashes inside a function that you know for sure is never called.

Had to actually stop and think about why I love this particular programming channel so much and it hit me. You don't talk philosophy, just computing, and it's so unbelievably refreshing to hear someone not spend hours lecturing me on why I'm a terrible programmer and just get right down to business. Wonderfully informing as always, keep up the good work!

The fact that methods receive the invisible this pointer internally shows a concept which I started to understand by coding in Python, where you have to specify the self parameter in method heads explicitly. It also shows that C is not that far away from an object oriented programming language, you can do the basic stuff with structs and functions that take a struct as their first argument, just like the this pointer. Thanks for the great video!

"Wait, it's just pointers?" "Always has been" Funny you mention the alignment table at 5:07. I've actually been wondering this: On a CPU that has strict alignment rules (like 68000, ARM, or most RISC CPUs) can you get more bang for your buck by doing this with your structs: int foo; char a; char b; short y; int bar; instead of: char a; int foo; char b; short y; int bar;

Wrote a compiler for a little language when I was in university. The language was made specifically for the class, and changed as new ideas were taught in the class. All of this lines up pretty much how I expected. If you want to learn how memory is handled in something like C++ write a compiler by hand for a small language and it all becomes pretty clear. There are lots of books you can find to follow on this stuff, but i had the benefit of an awesome teacher.

I think that it makes perfect sense that an object-oriented paradigm would be implemented using the simplest and fasted code that gets the job done.

Heh. Reminds me when I was first tasked with modifying a consted variable in my introductory C++ class. People looked at me like I was a homicidal maniac when I presented my inline assembly solution and proclaimed, "well, if you want me to ignore the compiler, nothing better than a bit of assembly!" Apparently, I was only meant to const_cast that poor devil. 😂

Creel, it's a real pleasure to watch your videos and partake in your years of (especially low level) language and implementation details. Thank you.

The real world use case for code like this is job security.

I reached most of your older videos at least twice a couple of weeks ago. I was craving that Creel and I can finally scratch it with this brand new video!

Now THAT'S what I call Forbidden C++

I love this guy's videos. He is revealing all the magic computers have these days.

You never fail to amaze me Creel, thanks for your time.

I went to school for visual C++ over a decade ago. _Definitely a bit rusty_ The main thing that stuck with me is my teacher complaining about my loops not terminating. _I can modify code, but can't write from scratch anymore_ Never even messed with memory directly, I could imagine his face if I did. I'm of a mindset that I gotta break something to figure out how it works, and this seems a good starting point to get back into coding.

Circa 1990 I was writing 65816 assembly for the Apple IIGS and was really just getting interested in C++, which was just becoming mainstream. I was using MASM, which has a very robust macro facility and wrote a set of macros to create vtables and make virtual function calls so that I could do object oriented programming in assembly language. It was so much fun!

This has got to be the most Aussie programming channel on the internet. Incredible.

I really love the way you explain that stuff! Thanks for spreading the knowledge in such an accessible way! :)

This was absolutely amazing to watch:) thanks a gazillion times and keep up the great job :)

Thanks Creel, this is an extremely good and interesting video.

This is an excellent introduction! Would be really cool to see an advanced version of this that examines the implementation of multiple inheritance, virtual inheritance, and dynamic_cast in the presence of the other two.

Awesome video. I never knew the vtable pointer was placed at the beginning of the class, but thinking of it now it makes a lot of sense

im learning so much internally about programming as a whole. ive never really understood what happens in the background, only what ive been taught in school, all horseshit. thank you, you're a great teach

Thank you so much for making the video! I before was modding games, so I knew that before. But I never would have thought of creating custom VTables. Before I was modifying the addresses of the VTable, but this allows for even better stuff, like function swapping for only one instance (for ex. Localplayer). I would like to see a part 2 that covers virtual inheritance and dynamic casting :D

Very good video! It really shows how simple C++ needs to be to retain its speed compared to C and assembly.

I amaze myself that I understand this video perfectly having learned all this stuff over 25 years ago. (Although I haven't used it very much.) Great video. Well done. Gut gemacht.

I thought this was gonna be a boring old news video about C++ magic, but I really liked that demo where you created a function in assembly alongside the C++ and how you dug to find the address value. Nice stuff!

That was so awesome! Thanks for the video Chris. Cheers :)

I love your humour and the educational value you provide 😊😊

Super interesting video! Thank you for making those videos

A very entertaining, educational and overall fun video Many thanks for making it

i found this channel randomly not long ago and i cant figure out why i find his videos so comfy. I feel its to do with how its just like hes talking and having fun. What a cozy fella

currently in the process of researching how oo works so I can apply it to my own programming language, this video helped a ton, keep it up!

Good to actually see why classes were design for: let the compiler deal with the Vtable. Thanks!

Amazing video, interesting topics to watch as always.

Great video! For me, it always helps to understand how the language works under the hood.

Another amazing video Chris man, love messing under the hood with oop for funzies, another quick way I use for getting those variable addresses, open up memory viewer in debug and type &variable_name in address bar that will throw up the memory address of var and it's contents. Great content as always mate, noice one.

Your teaching ways are really awesome !

Great stuff! No bs and to the point, nice. I think the higher level the language is, more tricks it uses - objective c is also very interesting in this manner (class/instance structure, dynamic linking)

Awesome video, thanks for sharing! Regarding getting the address of a variable that you discuss around 10:30, you can use the "immediate" window and type "&a", that will give you the address and data. Another trick is using the watch window and enter an expression like "&b,bb", which will show the address of b in binary format.

Cool vid! I'd love to see more of this.

All good examples, why C and C++ are great. Thanks for the overview. ;-)

Thanks for this video. In the firs tpart of the video, it shown me that I was thinking right about struct in C. The fact that you should be able to cast a struct and change it's attributes as you will. :) Even though it's C++ with OO, struct are kind of OO but in C. It works pretty the same.

I've programmed in C, C++, Java, a couple assembly languages, Visual Basic, C#, Racket, some other languages, and Python. It wasn't until I got to Python and saw the "self" parameter at the start of all member functions that I had an epiphany about how OOP is done behind the scenes. This video did 2 things in addition to that epiphany. 1: It confirmed my epiphany. 2: When you showed polymorphism, it showed me that the designers who came up with OOP must have been geniuses to purposely design in polymorphism behind the scenes.

What a superb reference to a Norwegian masterpiece :) That's put a smile on my face for the rest of day. The content was excellent too btw :D

I am a complete hobbyist gamedev so I learned enough of programming just so I can work with the engines and being able to read people's code but years ago I had a thirst to really learn how computers actually work so I went on a journey to learn about architecture, assembly language, C language, C++, C# etc. And I had quite a bit of "Aha!" moments one of which was "Hey its all about pointers and data and hoping around data?"Also another aha was realizing you actually working in tandem with compiler, that's the main thing in programming, that's what a language is actually, compiler puts it together in a set way the designers of that language intended and it gets translated to machine code in the end. Absolutely loved how C was just enough step up from assembly that it didn't obscure true nature of programming. As I learned about the community I never could understand how people favored some languages over the others, still cannot to this day hear somebody say "This is better than that..." Like wtf do you even know it all works?? Its all the same under the hood but of course you are going to use the one its supported in whatever framework you work in. One time I watched this one programmer video how they would hire people coming with CS degrees but no work experience and some didn't know the difference between stack and heap... Sure developing some non demanding android app you don't need to know anything about that even how OOP works but if you are developing anything that requires above average performance you will need to know how it actually all works under the hood.

It's good to see how much you're enjoying MacGyvering the language mechanisms!

Nice to see you again, man

Vtables were explained to me before but now I understand that the vtables are per-class and not per-instance, I couldn't understand that before, so thank you for putting that in and insisting on it!

Could you clear up some confusion I had? At 17:13 it states "in the same order as defined in the class" is this actually supposed to say defined or should it be "declared" instead?

The fact that the first parameter of a member function is the this pointer is useful to know if one wants to pass stateful free functions. You use bind_front on that member function and its instance, thereby creating a free function with a state that you can pass along to other functions who'd be agnostic to the fact there's a class behind it.

Amazing channel, you produce awesome quality content

Visual Studio C++ compiler allows you to see the class memory layout at compile time using the compiler switch /d1reportAllClassLayout. The real mess starts when you have multiple layers of inheritance with non pure virtual classes. You get a nice interleave of function pointers and data that may or may not be aligned depending on user #pragma. It's crucial to understand this for reverse engineering applications from compiled binaries.

Great video, thanks a lot!

@21:00 Would Function1 have access to private members of *this_ptr?

Mindblowing video thanks

This is a fantastic video!

The Creel has returned... nice!

You just keep blowing my mind

Creel's throwing down the gauntlet! Nah, great stuff as always man, cheers!

Hello Creel, very interesting video. Please, can you share how did you get syntax highlight for the assembly code in Visual Studio?

14:50 "Dog Fish" made my day! :-) I really enjoyed this under the hood look at things, it makes things make more sense. I can follow pointer manipulation and intent far easier than posh words. :-)

Awesome video!

I had always thought of C as a strictly typed language until I read the book Expert C Programming: Deep C Secrets and it pointed out it was loosely typed and having more experience I realized, that yeah, you can always get to the bytes and do what you want.

About the vtable content being hardware-protected, does this apply specifically to msvc or is it something which is defined by the language standard?

Great video!

I like the content; I might have to watch this again. I have a version of assembler that I have been creating for a few years now and I added some basic OOP to it this past winter. I didn't know how everyone else was doing it so I just made it up. And it turns out that I created a block of instance data variables and stored them in a section within the exe separate from the static data. Then when a new instance gets created the default instance data is copied into the upper memory instance block. But I did not do any vtable logic whatsoever in my version. The function calls are all statically linked to the code space where the function rests. Yes - it works at the simple level. But without vtables - i doubt that it can do any of that fancy polymorphic inheritance. Maybe next winter I will add the vtable concept and expand it. But this video gives me a great indicator that I am on the right track and also lays out what I may do next. Thanks for the under the hood examination!

Hey Creel, thanks for the excellent video! Can I ask at 11:03, why the first parameter of the function is at rcx register ? And what happens if the number of parameters of a function is greater than the number of registers of the CPU ? Does it do loops to load parameters onto the registers in several batches ?

This brings me so back to the 90s :)

Can you use the vtable pointer change for a self programming code? Like get one or two tables with default functions. At runtime write your acustomed vtable in RAM, point the original table to it and get a custom behavior?

well damn, that just shattered most courses where I was told that those OOP practices helped with security inside a program lol, pointers seem obvious but I really thought at least some form of protection from outside writes was implemented

Thank you for video.

Another fun point not mentioned is that single inheritance of member variables is implemented like struct DerivedStruct{BaseStruct base; int derivedVar;}; Thus any DerivedStruct pointer simultaneously points to a BaseStruct, and can be passed to functions that take a BaseStruct pointer. And if BaseStruct has a vtable pointer as its first member, then so does DerivedStruct. You just point it to DerivedStruct's vtable, and then virtual calls from inside BaseStruct functions will actually jump to DerivedStruct functions. Very easy to do in regular C. C++ just automates a lot of it.

This was very interesting! Although I knew about this "magic" I didn't know how exactly it looks in memory etc. Maybe some deep dive into the templates next?

Awesome video! Really demystifies OOP. Does this mean that upcasting is type punning? Can you use type punning to downcast?? (assuming classes are the same size)

Great video 👍

That vtable override trick is what i had to do for overriding some methods in precompiled proprietary DLL to make it interract with data on network rather than just accessing file localy.

Very interesting Video!

There some extra fun with multiple inheritance of class with a vtable that end up with 2 vtableptr. And in case of virtual inheritance, the offset to the subtype being stored in the vtable. Lot of fun edge case that are not meant to be messed with and could blow up when playing with them. ^^

Can't we use mprotect to change the VTable? I am using it to change my functions on runtime.

19:10 Shouldn't these three functions have a single dummy parameter which functions as 'this'? Or is this only needed in x86 programming?

Very interesting!

This is fascinating

Waiting for this man once coming out as a teacher in my university. That are what I wanted to hear and study on our lectures. (KPI Ukraine)

Your illustration skills are impressive 24:48

"There's better ways to call three functions." Ha! This was interesting and good fun, thanks!

great video. thank you (:

Long time Chris, hope you are keeping well.Good stuff as usual. Pity the sound on that lecture is a bit bad. I do have difficulty visualising objects in memory, since all memory is linear.

Dude you are brilliant

In whichs langages we can fusionne/merge / [inherit from] 2 uppers/parents classes ? Because in most OOP, you can inherit from only one.

I had Dr. Brady for Microprocessor Systems. He's a huge fan of talking about weather if I do say so myself, and also hating on Windows 😂

Love this video, such a good illustration. Question: at 20:50, what about adding an extra parameter to Function1, that is not in the class function? We read from the stack then?

14:14 That's the cool thing about it. You can f.e. store an array of base class objects and call a function that is overridden in each unique child class without that child class. Or you can also easily override certain methods, knowing that your custom class will do special things at common events. In Unreal Engine f.e. there is an Actor base class and you can inherit from it and override events like BeginPlay, BeginDestroy, Tick and so on.

So the title should actually be: How to Break C++ OOP with Dirty Rotten Low Down tricks? Great video though, loved it!

that's great, thanks. I think you can drive the point of the mangled C++ function home a little bit further, by actually writing the function in C with another name and renaming or aliasing its name to the mangled name in ASM or with a linker script.

this video is a must watch if you are into game hacking and also help you reverse engineer routine and i guess, with this knowledge, we can implement those oop feature in C if we wanted to

I just found your channel. I am also a Creel and do software dev, cool!

It's still better than no typing at all or optional typing. Languages like Python and JS are nightmares to code in when compared with typed languages. I didn't realize that until I started using C# and C to program things. Having the peace of mind of knowing that if something compiles, it'll run, unless you did something careless with memory management is priceless as a more advanced programmer. Hopefully, the game dev industry moves towards Rust at some point so that us game devs can be even more at ease while coding.

This is fascinating, and incredibly evil! Theoretically you can use this to build add-on or plugin packages that have control over functions and data that it has no business interacting with. Not authorized to touch core functionality at your business? Who cares! Video game allows for mods but no modifications to the engine? Sorry UE4, we now have an unstable and horribly awful workaround! Want to call a class function without a member? No problem! Basically: "Thanks, I hate it!"

The awesome and frightening power of C pointers.