Classes part 18 - Understanding the vtable (Popular interview question)

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Classes part 18 - Understanding the vtable (Popular interview question) | Modern Cpp Series Ep. 54

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2 жыл бұрын

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►Lesson Description: In this lesson I show you what exactly the vtable is in C++. The vtable is the data structure which holds information about which member functions to call for an instance of class when the presence of 'virtual' exists in inheritance. Understanding a little bit about this data structure, will also give you some intuition about a small performance cost (i.e. storage for the pointer, and following a pointer to call a function) about virtual functions as well.
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Пікірлер: 37

@dhanushs1802 2 жыл бұрын

Great video. Very neatly explained as always. Thank you.

@jugalshah8199 2 жыл бұрын

Thank you i waited for this topic for long its crystal clear now.

@MikeShah 2 жыл бұрын

You are most welcome!

@nevis2769 2 жыл бұрын

Great video! Thanks Mike.

@MikeShah 2 жыл бұрын

You are most welcome!

@shivampanjiyara8288 3 ай бұрын

thanx bro . love y0ou=

@heretoinfinity9300 7 ай бұрын

What's the link with the multiple inheritance vtables mentioned at the end?

@MikeShah 7 ай бұрын

I'm not 100% sure on implementation, but you could have multiple 'vtables' for a derived object pointing to different member functions in the derived class (that's likely how this is implemented at least in langauges that allow for multiple interface inheritance) -- perhaps a more clever implementation of multiple inheritance builds a single table with the derived member functions however.

@tejasbachhav3921 10 ай бұрын

kudos for your hardwork. But it's difficult to understand, can you explain again with good diagrams?

@MikeShah 10 ай бұрын

Cheers! See around 2 minute mark for some pictures

@grenadier4702 Жыл бұрын

If I don't override a base function, is vtable still created for my derived class? Or only the pointer is inherited and points straight to the base vtable? For instance: class Base { public: virtual void swim() {} } class Derived: public Base {} Derived derived;

@MikeShah Жыл бұрын

As soon as you introduce 'virtual' or 'override' in a class a vtable (or some mechanism to achieve polymorphism) is created. In this instance, If you make a call to 'swim' from the derived class, then we'll be looking into Base's vtable to call the appropriate swim. So while it's probably implementation defined (i.e. up to the compiler), I would assume this example generates a vtable.

@grenadier4702 Жыл бұрын

@@MikeShah Thanks! :)

@MikeShah Жыл бұрын

@@grenadier4702 Cheers!

@MikeShah 11 ай бұрын

@@AlberTesla1024 I believe that is true

@mouseminer2978 8 ай бұрын

Hi, I am strugling with this vtable error. Please help Below is the arduino code for ePaper display. #include #include //2.13" E-paper Module (White/Red/Black) #include #include #include GxIO_Class io(SPI, SS, 17, 16); GxEPD_Class display(io, 16, 4); void setup() { } void loop() { } Here is the error below undefined reference to `vtable for GxIO' collect2.exe: error: ld returned 1 exit status exit status 1 Compilation error: exit status 1

@el_joker_2243 11 ай бұрын

thanks alot

@MikeShah 11 ай бұрын

Cheers!

@writecompiler Жыл бұрын

nice

@MikeShah Жыл бұрын

Cheers!

@antonfernando8409 Жыл бұрын

why destuctors have to be virtual and constructors can't be virtual, I was asked this once in an interviews, still not sure I have realize an answer.

@MikeShah Жыл бұрын

The way I would probably answer this is that we need an object to be fully constructed (including its vtable for later redirection) before we can do anything with it. Probably best to hear it from Bjarne himself www.stroustrup.com/bs_faq2.html#virtual-ctor

@antonfernando8409 Жыл бұрын

@@MikeShah cool thanks.

@capetzproductions8690 Жыл бұрын

Virtual functions work based on the vtable mechanism, which is set up after the constructor finishes executing.Moreover, there's no need for constructors to be virtual. When an object is created, the constructor of the specific class is known at compile time, and there's no ambiguity about which constructor to call.

@antonfernando8409 Жыл бұрын

cool

@MikeShah Жыл бұрын

Cheers!