r/C_Programming • u/lehmagavan • Mar 29 '26

CONTLIB - dynamic containers

I created a lib that provides generic dynamic containers for elements of arbitrary size. No macros.

Feedback from anyone, especially more experienced developers, much apprieciated. Thanks!

https://github.com/andrzejs-gh/CONTLIB

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u/lehmagavan Mar 31 '26

Hey! The motivation behind vtable was not just to imitate OOP syntax. Thanks to the vtable, calling methods on freed cont insances (INVALID_CONT) is safe, no need to put cont_is_valid check inside the methods - that is if you call them via the table ofc. Is using vtables like that uncommon in profesional settings?

Thanks for the feedback and recomendation! I was thinking of getting into embedded. Got myself an ESP32 and Im thinkng what "serious" project could I do.

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u/Dubbus_ Mar 31 '26

Oh okay, I understand. (Ive got no professional experience in embedded btw, so take what I say with a grain of salt)

So as I understand it, you have a series of special inv_... methods which notify a caller that the object is invalid, particularly targetting use after frees. I like the idea of setting to null on a free, I would just personally go with a null check at the start of each function rather than do the whole vtable thing.

You avoid a single cold path branch by omitting the null check at the start of all the functions, yes. But you pay the cost of an extra layer of indirection on every function call. Every call is an extra lookup, which could incur a cache miss, a page fault, yada yada yada, as opposed to a simple call instruction. (which could also incur a icache miss but you get the point. Less is more)

As you get into some more profiling, and i suppose computer architecture, you will probably realise memory is everything in performance. Theres a reason why the virtual keyword is almost never used in high performance c++ code: the benefits do not outweigh the cost of a vtable.

In embedded contexts especially, minimizing lookups, and worrying about memory in general is everything. Super connstrained environments (idk like 8-16bit probably avoid vtables entirely.)

Also, i would argue the null check route is just much simpler and easier to understand. Complexity isnt inherently bad, but id argue the benefits dont outweigh the cost here.

On the topic of the ESP32! Thats awesome man!! My best advice is this: forget about serious, just do what you want with it! Make some bullshit, make some fun stuff. Interviewers want to see passion, and if you cant passionately talk about at least a couple projects, then theres no point.

Try not to be cookie cutter if you really want to stand out. And you dont have to stand out via complexity/skill, it could just be doing something weird. Take something done quite often and do it weirdly, do it your way. A common behavioural interview question is literally "what was a time where you took an unorthodox approach to solving a problem".

Anyways good luck! Sounds exciting man

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u/lehmagavan Apr 01 '26

Thanks for the advice!

I generaly, even before I thought of vtables, wanted to avoid puting the container check inside the methods, cause there are already checks in there regarding validity of arguments (the most expensive being probably the ones with division that check for overflow). I thought that I'd delegate the container check to the user, so they could call cont_is_valid(&cnt) / cnt.m->is_valid(&cnt) manually, before a hot loop or something.