The What, How, and Why of Void Pointers in C and C++?

today i want to talk about pointers specifically pointers that don't have type information are also known as void pointers [Music] hey everybody welcome back today's video is for the beginners for fairly beginner programmers out there who think that you've finally wrapped your head around pointers but then you see something like this and you're not really sure what to make of it because this causes a serious moment of pause for a lot of beginners so i thought today we would talk about it talk about what a void pointer is why you might want to use one and we'll also talk about some examples where you may have already used a void pointer and you just didn't realize it also at the end i'm going to talk a little bit about a subtle difference between c and c plus plus might save you some time but first a big thank you to all of you who support this channel through patreon buying merch and online classes also for those of you that are curious about where you can get example code from my videos you can get that through patreon and you can get access to my monthly office hour so now back to void pointers if you look long enough into the void the void begins to look back through you i don't know if nietzsche was talking about new programmers but sometimes the concept of void takes a second to get a grip on now the first time you see void is probably when you are looking at a void function maybe something like this and we make a quick function and let's just say it's a simple function that prints out the word hello with a newline character nothing complicated about that so in this case the void simply says that this function does not have a result type it doesn't produce a result it doesn't give us a value when it's done so if i come down here in main i can definitely call say hello and this is going to work just fine now let's make sure just by coming down here and compiling with my make file i am using a make file you can check it out here nothing complicated just a really basic make file if you haven't seen make be sure to check it out on my other videos that deal with make and actually hold up really quick i just noticed i was editing my c plus example let's do this over in c and compile that one we'll come back to c plus plus later but if i run example here then you will see that it works just fine it prints out hello so no worries but note that i can't do something like this i can't say result equals say hello like this if i try to do this i'm going to get an error because it says hey look you're trying to assign to an integer a value void doesn't have any type information it's not a variable simply the compiler just doesn't know how to take nothing no variable no result and produce an int out of it so it gives us an error so hopefully that makes sense hopefully that's straightforward enough and i'll leave this up here but so what if what about if i come down here and say something like void star p we understand what it means for a function to be void mean it doesn't return anything but what does it mean to have a variable p which is a pointer that points to void which may be up till now we've thought of as nothing so is this a pointer to nothing okay so calm down calm down you see at least in this case it's probably more helpful to think of void as meaning no type rather than nothing so we can think of this as a pointer but we have no type information that is attached to that pointer so the compiler is storing an address for us in memory but it's not making any assumptions about what is stored at that location in memory but really what i want to emphasize is that it is still a pointer just like any other pointer it stores an address and just to make this clear what i want to do is to come down here and we'll just add a little bit to our program first thing i'm going to add an int pointer call this p2 or let's call this ip this might be the kind of pointer that you maybe were introduced to and let's come down here and add a couple printf statements we're going to say the size of a void pointer equals we'll do zu because we're using memory sizes good habit to get into and we'll do size of void pointer and then let's do the same thing with our int pointer so we'll do p right here i p right here and change this to be an int pointer okay so now we can compile it we can come down here and run it and you can see that both of these are the same size at least on my machine we're looking at eight bytes per pointer because they're both simply storing an address and on my machine an address is eight bytes and since these pointers are basically the same we can do something like i could say p equals ip right i can just assign one address value to another and just to see this in action let's come up here and we're gonna make let's make an integer we'll call it x and assign it to a value let's say it's speed beef and then i'm going to come down here and take my in pointer and assign it to the address of x now note that i could have just done the same thing with p i could say p equals the address of x that's going to work just fine too but let's not do that for right now we'll come to that later my point right now is that we have a value here and we should have an actual address being assigned to ip and then when we assign p to be ip we should end up with the same addresses so just to verify that let's come down here and get ourselves a another printf statement it's going to be p points to percent p if you haven't seen the percent p format specifier this is simply there for printing out addresses which is exactly what i want to do right now is i'm going to print out the address of p and the address of ip and now if we come down once again we compile it and if we run it you can see we get the same address for both pointers so i hope at this point it's making it clear that they're both pointers they're both storing an address in this case they're storing the exact same address so what's the difference well for one one difference is because we have type information around it i can come in here and i can say ip you know star iep i can de-reference it and say this equals something else like let's say coffee okay so this is going to store the integer coffee at the address that ip points to but i'm going to run into trouble if i try to do the same thing like so let's say i do star p equals and i give it its own value if i can type beef now this is going to give me a little bit of trouble if i come down here and try to compile it it's going to say hey wait a minute void is not assignable you can't assign something to void you have no type information here so the first thing you need to know about void pointers i guess is that you can't dereference them or at least you shouldn't de-reference them in general because you can't do much with void but of course this is only a minor inconvenience because as i mentioned in some previous videos c is not a type safe language so we can just add a cast in here and what i'm saying here with this cast is i'm saying i want you to treat p as though it were an in pointer and then dereference it and now with this added cast this line is now going to work just fine so again once more let's just double check let's make sure that this is actually working and we'll print out our contents we will add a percent x specifier because i'm going to print out in hexadecimal and we'll come down here and do star p and star ip once again i am going to need the cast because otherwise it would complain to me saying it doesn't know what to do with this void pointer and now we can come down and compile it and if we run it you can see that both of these memory addresses are pointing both of these addresses are pointing to the memory containing dead beef now let's pause here really quickly to make sure this all makes sense some of you may be wondering about what happened to coffee that i wrote up here in the location pointed to by ip and i did this on purpose because i wanted you to remember that these two pointers are at this time pointing to the same location in memory so the last write to the memory pointed to by p that overwrote the previous values in that memory and both pointers are pointing to the same thing so they only are showing the last thing that was written so just to illustrate this just to try to tease this apart let's now make them separate so instead of assigning p to be ip right here i'm going to take this out and let's come up here and make another int and y and i'm going to assign p to be the address of y so now with just that subtle change that very small change i can come down here and i can run it and now you can see that we have different addresses and different values because the two pointers are now pointing to two different locations okay so that's cool but why would we ever want to use a pointer with no type information it just seems like it makes more work because you have to keep casting things and it's just limiting right and the answer is that basically anytime you want to store generic memory addresses basically when you want to store an address but you don't really care what type it points to so one example that you're going to see a lot is when you use a memory allocator so let's say instead of assigning p up here to the address of why let's say we come in here and say malik's size of int so this is just saying hey i want you to dynamically allocate on the heap a block of memory that is integer sized and then we're going to assign it to p and then of course we should come down here at the end and free p once we're done with it and of course this would work with our ant pointer as well but let's just take a look before we get to all that let's just look at the man page for malik sorry and malek so this is showing us the malik and free man page we can see and of course calicryalic and realicarray but if you look closely here notice what malik returns when you ask for memory what you're doing is you're saying hey i want a certain size block of memory malik doesn't really care what you're going to put in that memory it just cares what size it is and it knows that you're not really going to care you just want the pointer back malik is trying to be generic so malik returns you avoid pointer and of course up here in our program we could use malik with a non-void pointer and we'll get to that in just a minute but but first before we finish with the man page if you look at free similarly when free when we're done with the memory and we're just trying to like give it up and let somebody else use it notice what we pass in we pass in a void pointer again because again free doesn't really care what types you put in that block it just wants to reclaim the block so just give me the address i don't care what's actually stored there another quick example just while we're looking at examples is mem copy which we looked at in a recent video that one involved an angry squirrel just in case you haven't checked it out but notice again here that mem copy is using void pointers and the reason is it doesn't care what you stored there it's just just give me an address i'm going to copy the memory i don't really care what it is i'm just going to copy bytes so again the point here is anytime that you're trying to work with an address a block of memory you want to store a pointer but you don't care what it actually points to then a void pointer is a really good option this will allow you to for example if i wanted an array of pointers and i wanted to have them each pointing to different things i could do that with void pointers so but now let's just tweak this a little bit let's get out of here and let's come in here and tweak things to make it work a little more how we usually do things in our programs because usually in a program if i knew that i wanted to have an int here i wouldn't have a void pointer assigned to the result from malik instead what i would do and let's just let's just swap these two so we'll have p be assigned to the address of x and we'll come down here and we will have the integer pointer now pointing to the or assigned to the result from malloc and i should come down here and free ip okay so now at this point i do have an unused variable we'll get rid of y really quick and if we come down now our example works just like it did before it doesn't really matter we we did change up the addresses so now ip is on the heap rather than p is on the heap but otherwise basically we got the same thing but the reason i bring this up is that this highlights a difference between c and c plus plus that i just want to point out so let's quickly just take this over to c plus plus and now if we compile it you notice that we have a problem you see when c saw me assign a void pointer coming from malik to an in pointer it was like cool people do that all the time that's what people do with malik but when c plus plus saw it it was like hey those aren't the same kind of pointer do you think they're aware of that you see c plus plus is a bit more of a worrier so in c plus plus we would have to come up here and add a cast to an integer pointer and now if we come down here it's going to compile just fine and in case you're wondering it will run just the same and i just want to make it clear here that i'm not saying that one of these approaches the c approach of the c plus approach is better than the other i do wish they would agree on one common way and do the same thing because it would mean a little bit less for us to keep track of in our heads but i really think this is just a reflection of the two languages individual personalities c over here is going to allow you to write slightly more concise code while c plus plus over here is going to force you to acknowledge when you assign a pointer of one type to a pointer of another type and that's not a bad thing because that's actually the cause that's the source of a lot of weird bugs the main thing is to be aware of it and if you're writing code that needs to be portable to both c and c plus plus then just include the cast right here that will create more portable code because c doesn't mind so he's happy with that cast but c plus requires it so thanks for looking into the void with me today i hope you learned something new please like subscribe go on a clicking spree just click on everything just to let the youtube algorithm know that there's something interesting going on here and until next week i'll see you later

Channel: Jacob Sorber

Views: 51,625

Keywords: void pointer, void c, void pointer c, void pointer in c, void pointer c++, void c++, void, programming, pointers in c, c programming, void pointers in c, void pointers, using void pointers in c, what is void pointer, what are void pointers, c (programming language), generic pointer, c programming tutorial, what are void pointers used for, c programming language basics, pointers in c programming, void pointer in c++, computer science

Id: Vd-4eGnBAK8

Length: 13min 12sec (792 seconds)

Published: Tue Jul 19 2022