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ProgrammingAbstractions-Lecture
Instructor (Julie Zelenski) :Okay. Well welcome, welcome. We got a couple parents who have braved the other activities of Parents Weekend to come here, so thank you for coming to see what it is that we do here in CS106B.
We are still in the process of grading the midterm. In fact, most of our grading, we’re planning on taking care of tomorrow, no, Sunday, in some big, massive grading sessions. So with any luck, if it all goes well, we’ll actually have them ready to go back on Monday, but if somewhere it turns into some disaster, it may take us a little longer, but hopefully. That’s the plan. So you get to have your weekend without worrying about it. Just put it aside and have some doing some stuff.
What I’m gonna talk about today is I’m gonna briefly go through scanner, really fast. Actually, what I really want to talk about is vectors, so maybe I won’t even bother with the scanner. We’ll just talk about vector, which is, the reading going on with this, Chapter 10: Implementing the Class Template.
So what we’re gonna do here for the next two weeks is we’re gonna take and put all those templates we did a lot of stuff with. Right? We saw vector, we saw stack, we saw a queue, we saw map, we saw set. And now it’s time to say, “Well, how do they work? How do they manage to do the things they do, efficiently, safely, robustly; what is it like to implement those things?”
And so it’s – we’ve had a good run of being the client, which hopefully has impressed you with like why you want these things around. And now, with some knowledge about linked lists and pointers and Big O, how can we make some make those operations run efficiently and make them do cool things? What does it take to be the backend side?
So that’s kind of gonna be our role for pretty much the next two weeks straight. And then at the end, we’ll kinda come back and try to join both sides together and get a vision of where we’re at. But it is time to be implementer.
We’ll be doing the Terman Café today after class, so hopefully those of you whose parents aren’t here, or your parents can come with us too, we’ll – anybody who has time is definitely welcome to come over and have a nice latte with me and hang out. And tell me about their midterm success or their midterm troubles, or just things that are on your mind. So hopefully some of you will be able to join me.
So I’m gonna finish up a couple slides from last time, and then I’m gonna mostly go on to do some code with you together. But I just wanted to try to give you some perspective on the value of abstraction and the idea of an abstract data type, or an ADT, and why this is such a powerful and important concept in management of complexity.
So we saw this, for example, in those first couple assignments that the client uses classes as an abstraction. You have the need to manage something that has a queue-like behavior:
first in, first out. So what you want is something that you can put things in that will enforce that: put things at the end of the line, always return to the head of the line. And that how it managed what it did and what went on behind the scenes wasn’t something we had to worry about or even concern ourselves with.
And in fact, even if we wanted to, we couldn’t muck around in there. Like it wasn’t our business, it was maintained privately. And that is a real advantage for both sides, right? That the client doesn’t have to worry about it and can’t even get in the way of us, that we can work independently and get our things done.
And so one sort of piece of terminology we often use here, we talk about this wall of abstraction. That there is kind of a real block that prevents the two of us from interfering with each other’s process, as part of, you know, combining to build a program together. And there’s a little tiny chink in that wall that we call the interface. And that’s the way that you speak to the stack and ask it to do things on your behalf, and it listens to your requests and performs them.
And so if you think about the lexicon class, which we used in the Boggle and in the recursion assignment that managed a word list, the abstraction is, yeah, you say, “Is this word in there? Add this word to it. Load the words from a file.” How does it store them? How does it do stuff? Did anybody go open up the lexicon.cpp file just to see? Anybody who was curious? And what did you find out?
Student: I just – I think it ended up in there [inaudible].
Instructor (Julie Zelenski) :You ended up in there and when you got in there, what did you decide to do?
Student: Leave.
Instructor (Julie Zelenski) :Leave. Yeah. Did anybody else open it up and have the same sort of reaction? Over here, what did you think?
Student: I didn’t really understand.
Instructor (Julie Zelenski) :Yeah. And what did you see?
Student: It was a mess.
Instructor (Julie Zelenski) :It was a mess. Who wrote that code? My, gosh, she should be fired.
It’s scary. It does something kind of scary. We’ll talk about it. Actually, at the end, we’ll come back here because I think it’s actually a really neat class to study. But in fact, like you open it up and you’re like, “I don’t want to be here. I want to use a word list. Let me
So that makes it easy for me to write some piece and give it to you in a form that’s easy for you to incorporate. We can work on our things without stepping on each other. As you get to larger and larger projects beyond this class, you’ll need ways of making it so three people can work together without stepping on each other’s toes the whole time. And classes provide a really good way of managing those boundaries to keep each other out of each other’s hair.
And there’s a lot of flexibility given to us by this. And we’re gonna see this actually as we go forward, that we can talk about what a vector is. It keeps things in index order. Or a stack is, it does LIFO. But there is no guarantee there about how it internally is implemented, no guarantee expressed or implied, and that actually gives you a lot of flexibility as the implementer.
You can decide to do it one way today, and if upon later learning about some other new technique or some way to save memory or time, you can swap it out, replace an implementation with something better, and all the code that depends on it shouldn’t require any changes. That suddenly add word just runs twice as fast or ten times as fast, would be something everybody could appreciate without having to do anything in their own code to take advantage of that. So these are good things to know.
So what I’m gonna do actually today is I’m gonna just stop doing slides, because I’m sick of doing slides. We do way too many slides; I’m bored with slides. And what I’m gonna do is I’m gonna actually go through the process of developing vector from just the ground up. So my plan here is to say our goal is to make the vector abstraction real, so to get dirty, get behind the scenes and say we know what vector is. It acts like an array. It has these indexed slots. It’s this linear collection. It lets you put things anywhere you like in the vector.
We’re gonna go through the process of making that whole thing. And I’m gonna start at the – with a simple form that actually is not templatized and then I’m gonna change it to one that is templatized. So we’re gonna see kind of the whole process from start to end.
So all I have are empty files right now. I have myvector.h and myvector.cpp that really have sort of nothing other than sort of boilerplate stuff in them. So let me look at myvector.h to start with. So I’m calling this class myvector so that it doesn’t confuse with the existing vector class, so that we have that name there. And I’m gonna start by just putting in the simple parts of the interface, and then we’ll see how many other things we have time to kind of add into it. But I’m gonna get the kinda basic skeletal functions down and show how they get implemented, and then we’ll see what else we’ll try.
So having the size is probably pretty important, being able to say, “Well how many things will I put in there,” get that back out, being able to add an element. And I’m gonna assume that right now the vector’s gonna hold just scranks. Certainly that’s not what we’re gonna want in the long run, but I’m just gonna pick something for now so that we have something to practice with. And then I’m gonna have the get at, which give it an index and return something.
Okay, so I think these are the only ones I’m gonna have in the first version. And then the other ones, you remember, there’s like a remove at, there’s an insert at, there’s an overloaded bracket operator, and things like that I’m not gonna show right away.
Question?
Student: [Inaudible].
Instructor (Julie Zelenski) :Oh yeah, yeah. This is actually you know kind of just standard boilerplate for C or C++ header files. And you’ll see this again and again and again. I should really have pointed this out at some point along the way is that the compiler does not like to see two definitions of the same thing, ever. Even if those definitions exactly match, it just gets its total knickers in a twist about having seen a class myvector, another class myvector.
And so if you include the header file, myvector, one place and you include it some other place, it’s gonna end up thinking there’s two myvectors classes out there and it’s gonna have a problem. So this little bit of boilerplate is to tell the compiler, “If you haven’t already seen this header, now’s the time to go in there.” So this ifindex, if not defined, and then the name there is something I made up, some symbol name that’s unique to this file. When I say, “Define that symbol,” so it’s like saying, “I’ve seen it,” and then down here at the very bottom, there’s an end if that matches it.
And so, in the case where we have – this is the first time we’ve seen the file it’ll say, “If not to define the symbol it’s not.” It’ll say, “Define that symbol, see all this stuff, and then the end if.” The second time it gets here it’ll say, “If not define that symbol, say that symbol’s already defined,” so it’ll skip down to the end if. And so, every subsequent attempt to look at myvector will be passed over. If you don’t have it, you’ll get a lot of errors about, “I’ve seen this thing before. And it looks like the one I saw before but I don’t like it. You know smells suspicious to me.” So that is sort of standard boilerplate for any header file is to have this multiple include protection on it.
Anything else you want to ask about the – way in the back?
Student: Why would it look at it multiple times, though?
Instructor (Julie Zelenski) :Well because sometimes you include it and sometimes – for example, think about genlib. Like I might include genlib but then I include something else that includes genlib. So there’s these ways that you could accidentally get in there more than once, just because some other things depend on it, and the next thing you know. So it’s better to just have the header file never complain about that, never let that happen, than to make it somebody else’s responsibility to make sure it never happened through the includes.
So I’ve got five member functions here that I’m gonna have to implement. And now I need to think about what the private data, this guy’s gonna look like. So now, we are the
Say myvector V, but the process of constructing that vector V will cause the constructor to get called, will cause a ten-member string element array to get allocated out in the heap that’s pointed to by arr, and then it will set those two integers to know that there’s ten of them and zero of them are used. So just to kind of all as part of the machinery of declaring, the constructor is just wired into that so we get setup, ready to go, with some empty space set aside.
So to go with that, I’m gonna go ahead and add the destructor right along side of it, which is I need to be in charge of cleaning up my dynamic allocation. I allocated that with the new bracket, the new that allocates out in the heap that uses the bracket form has a matching delete bracket that says delete a whole array’s worth of data, so not just one string out there but a sequence of them.
We don’t have to tell it the size; it actually does – as much as I said, it doesn’t its size. Well somewhere in the internals, it does know the size but it never exposes it to us. So in fact, once I delete [inaudible] array, it knows how much space is there to cleanup.
Yeah?
Student: Are you just temporarily setting it up so the vector only works on strings?
Instructor (Julie Zelenski) :Yes, we are.
Student: Okay.
Instructor (Julie Zelenski) :Yes. We’re gonna come back and fix that, but I think it’s easier maybe to see it one time on a fixed type and then say, “Well, what happens when you go to template? What things change?” And we’ll see all the places that we have to make modifications.
So I have myvector size. Which variable’s the one that tells us about size? I got none used. I got none allocated. Which is the right one?
Student: Num used.
Instructor (Julie Zelenski) :Num used, that is exactly right. So num allocated turned out to be something we only will use internally. That’s not gonna – no one’s gonna see that or know about it, but it is – the num used tracks how many elements have actually been put in there.
Then I write myvector add. So I’m gonna write one line of code, then I’m gonna come back and think about it for a second. I could say arr num used ++ = S, so tight little line that has a lot of stuff wrapped up in it.
Using the brackets on the dynamic array here are saying, “Take the array and right to the num used slot, post-incrementing it so it’s a slide effect.” So if num used is zero to start
with, this has the effect of saying array of, and then the num used ++ returns the value before incrementing.
So it evaluates to zero, but as a slide effect the next use of num used will now be one. So that’s exactly what we want, we want to write the slot zero and then have num used be one in subsequent usage. And then we assign that to be S, so it’ll put it in the right spot of the array. So once num used is five, so that means the zero through four slots are used. It’ll write to the fifth slot and then up the num used to be six, so we’ll know our size is now up by one.
What else does add need to do? Is it good?
Student: Needs to make sure that we have that [inaudible].
Instructor (Julie Zelenski) :It’d better make sure we have some space. So I’m gonna do this right now. I’m gonna say if num used is equal to num allocated, I’m gonna raise an error. I’m gonna come back to this but I’m gonna say – or for this first pass, we’re gonna make it so it just doesn’t grow. Picked some arbitrary size, it got that big, and then it ran out of space.
Question?
Student: So when the – for the vector zero, first time it gets called it’s actually gonna be placed in index one of the array?
Instructor (Julie Zelenski) :So it’s in place of index zero. So num used ++, any variable ++ returns the – it evaluates to the value before it increments, and then as a side effect, kind of in the next step, it will update the value. So there is a difference between the ++ num used and the num ++ form. Sometimes it’s a little bit of an obscure detail we don’t spend a lot of time on, but ++ num says first increment it and then tell me what the newly incremented value is. Num ++ says tell me what the value is right now and then, as a side effect, increment it for later use.
Student: So the num used gets changed in that?
Instructor (Julie Zelenski) :It does get changed in that expression, but the expression happened to evaluate to the value before it did that change.
Question?
Student: And is it really necessary to have myvector:: or is –
Instructor (Julie Zelenski) :Oh yeah, yeah, yeah, yeah.
Student: – there any way for –
So I feel okay about this. Not too much code to get kind of a little bit of something up and running here. Let’s go over and write something to test. Add Jason, and here we go. Okay, so I put some things in there and I’m gonna see if it’ll let me get them back out.
And before I get any further, I might as well test the code I have. Right? This is – one of the things about designing a class is it’s pretty hard to write any one piece and test it by itself because often there’s a relationship: the constructor and then some adding and then some getting stuff back out. So it’s a little bit more code than I typically would like to write and not have a chance to test, having all five of these member functions kind of at first.
So if I run this test and it doesn’t work, it’s like well which part what was wrong? Was the constructor wrong? Was the add wrong? Was the size wrong? You know there’s a bunch of places to look, but unfortunately, they’re kind of all interrelated in a way that makes it a little hard to have them independently worked out. But then subsequent thing, hopefully I can add the like the insert at method without having to add a lot more before I test.
Okay. So I run this guy and it says Jason, Illia, Nathan. Feel good, I feel good, I feel smart. Put them in, in that order. See me get them out in that order. I might try some things that I’m hoping will cause my thing to blow up. Why don’t I get at ten? Let’s see, I like to be sure, so I want you to tell me what’s at the tenth slot in that vector. And it’s, ooh, out of bounds, just like I was hoping for. Oh, I like to see that. Right?
What happens if I put in enough strings that I run out of memory? And we talked about a thing – it’s set it to be ten right now. Why don’t I just make it smaller, that’ll way it’ll make it easier for me. So I say, “Oh how about this?” I’m only gonna make space for two things. And it just: error, out of space. Right? That happened before it got to printing anything. It tried to add the first one. It added the second one. Then it went to add that third one and it said, “Oh okay, we run out of space. We only had space for two set aside, you asked me to put a third in. I had no room.”
So at least the kind of simple behaviors that I put in here seem to kind of show evidence that we’ve got a little part of this up and running. What I’m gonna fix first is this out of space thing. So it would be pretty bogus and pretty unusual for a collection class like this to have some sort of fixed limit. Right? It wouldn’t – you know it’d be very unusual to say well it’s always gonna hold exactly 10 things or 100 things or even a 1,000 things. Right?
You know one way you might design it is you could imagine adding an argument to the constructor that said, “Well, how many things do you plan on putting in there? And then I’ll allocate it to that. And then when we run out of space, you know you’re hosed.” But certainly a better strategy that kind of solves more general case problems would be like, “Oh let’s grow. When we run out of space, let’s make some more space.”
Okay, let’s think about what it takes to make more space in using pointers. So what a vector really looks like is it has three fields, the arr, the num used, and the num allocated. When I declare one, the way it’s being setup right now, it’s over here and it’s allocating space for some number, let’s say it’s ten again, and then marking it as zero. Then as it fills these things up, it puts strings in each of these slots and it starts updating this number, eventually it gets to where all of them are filled. That when num used equals num allocated, it means that however much space I set aside, every one of those slots is now in use.
So when that happens, it’s gonna be time to make a bigger array. There is not a mechanism in C++ that says take my array and just make it bigger where it is. That the way we’ll have to do this is, we’ll have to make a bigger array, copy over what we have, and then, you know, have it add it on by making a bigger array full of space.
So what we’ll do is we’ll make something that’s like twice as big, I’m just gonna draw it this way since I’m running out of board space, and it’s got ten slots and then ten more. And then I will copy over all these guys that I have up to the end, and then I have these new spaces at the end.
And so I will have to reset my pointer to point to there, update my num allocated, let’s say to be twice as big or something, and then delete this old region that I’m no longer using. So we’re gonna see an allocate, a copy, a delete, and then kind of resetting our fields to know what we just did. So I’m gonna make a private helper to do all this, and I’ll just call that enlarge capacity here.
Question?
Student: Is this like [inaudible]?
Instructor (Julie Zelenski) :Well it is – it can be, is the truth. So you’re getting a little bit ahead of us. But in the sense that like, you know, if the array has a thousand elements and now we got to put that thousand-first thing in, it’s gonna take all thousand and copy them over and enlarge the space. So in effect what is typically an O of one operation, just tacking something on the end, every now and then is gonna take a whole hit of an end operation when it does the copy.
But one way to think about that is that is every now it’s really expensive but kind of if you think of it across the whole space, that you got a – let’s say you started at 1,000 and then you doubled to 2,000, that the first 1,000 never paid that cost. And then all of a sudden one of them paid it but then now you don’t pay it again for another 1,000. But if you kind of divided that cost, sort of amortized it across all those adds, that it was a – it didn’t change the overall constant running time.
So you have to – you kind of think of it maybe in the picture. It does mean every now and then though one of them is gonna surprise you with how slow it is, but hopefully that’s few and far between, if we’ve chosen our allocation strategy well.
Instructor (Julie Zelenski) :Well if you did it –
Student: It has to be a constant.
Instructor (Julie Zelenski) :So like if you used this form of an array, you know, where you declared it like this. Did you see what I just did?
Student: Yeah.
Instructor (Julie Zelenski) :Yeah. That way won’t work, right? That way is fixed size, nothing you can do about it. So I’m usually totally the dynamic array at all times, so that everything –
Student: So you do it when you’re declaring it on a heap?
Instructor (Julie Zelenski) :Yes.
Student: Okay.
Instructor (Julie Zelenski) :Yes, exactly. All I have in the – stored in the vector itself is a pointer to an array elsewhere. And that array in the heap gives me the flexibility to make it as big as I want, as small as I want, to change its size, to change where it points to, you know all the – the dynamic arrays typically just give you a lot more flexibility than a static array.
Student: That’s really stack heap.
Instructor (Julie Zelenski) :It is. Array is a stack heap thing. When you put on the stack, you had to say how big it is at compile time and you can’t change it. The heap let’s you say, “I need it bigger, I need it smaller, I need to move it,” in a way that the stack just doesn’t give you that at all.
So when I go back to myvector.cpp, the place I want to put in my double capacity here is when num used is equal to num allocated, but what I want to do is call double capacity there. After I’ve done that, num allocated should’ve gone up by a factor of two. Space will be there at that point. I know that num used is a fine slot to now use to assign that next thing. So whenever we’re out of space, we’ll make some more space.
And so I’m gonna – right now, I think my allocated is still set at two. That’s a fine place. I’d like it to be kind of small because I’d like to test kind of the – some of the initial allocations. So I’ll go ahead and add a couple more people so I can see that I know that – at that point, if I’ve gotten to five, I’m gonna have to double once and then again to get there. And let’s, I’ll take out my error case here, see that I’ve managed to allocate and move stuff around. Got my five names back out without running into anything crazy, so that makes me feel good about what I got there.
So I could go on to show you what insert and remove do; I think I’m probably gonna skip that because I’d rather talk about the template thing. But I could just tell you – I could sketch the [inaudible]: what does insert at do, what does remove at do? Basically, that they’re doing the string – the array shuffling for you. If you say insert at some position, it has to move everything down by one and then put in there. Whereas at is actually just tacking it onto the end of the existing ones. The insert and remove have to do the shuffle to either close up the space or open up a space.
They’ll probably both need to look at the capacity as well. That the – if you’re inserting and you’re already at capacity, you better double before you start. And then the remove at, also may actually want to have a shrink capacity. Where when we realize we previously were allocated much larger and we’ve gotten a lot smaller, should we take away some of that space.
A lot of times the implementations don’t bother with that case. They figure, “Ah, it’s already allocated, just keep it around. You might need it again later.” So it may be that actually we just leave it over-allocated, even when we’ve deleted a lot of elements, but if we were being tidy we could take an effort there.
What I want to do is switch it to a template. So if you have questions about the code I have right here, now would be a really good time to ask before I start mucking it up. Way in the back?
Student: [Inaudible].
Instructor (Julie Zelenski) :I will. You know that’s a really good idea because if I – at this point, I’ll start to change it and then it’s gonna be something else before we’re all done. So let me take a snapshot of what we have here so that I – before I destroy it.
Question?
Student: When does the deconstructor get called?
Instructor (Julie Zelenski) :Okay, so the destructor gets called in the most – there are two cases it gets called. One is when the – so the constructor gets called when you declare it, and then destructor gets called when it goes out of scope. So at the opening brace of the block where you declared it in, is when the constructor’s happening, and then when you leave that. So in the case of this program, it’s at the end of May, but and if it were in some called function or in the body of a for loop or something, it would get called when you enter the loop and then called as – destroyed as it left.
For things that were allocated out of the heap, so if I had a myvector new myvector, it would explicitly when I called delete that thing when I was done with it. So especially when the variable that holds it is going away, right? Either because you’re deleting it out of the heap or because it’s going out of scope on the stack.
So let’s say that I’m gonna use a num allocated of two, so this allocates two. So when I construct it, it makes a block that holds two things and num used is zero. So I do two adds: I add A, it increments num used to one; I had B, it increments num used to two. I try to add C. It says, “Oh, well num used equals num allocated. We’re gonna go to double capacity now.”
So double capacity has this little local variable called bigger. And it says bigger is gonna be something that is four strings worth in an array, so it gets four out there. It does a full loop to copy the contents of the old array on top of the initial part of this array; so it copies over the A and the B, into there. And then it goes, “Okay, I’m done with this old part. So let me go ahead and delete that.”
And then it resets the arr to point to this new one down here, where bigger was. So now, we got to aliases of the same location. And then it sets my num allocated to say and now what you’ve got there is something that holds four slots. And then that used call here says, “Okay and now writer the C into the slot at three.”
So the process here is the only way to enlarge an array in C++ is to make a bigger one, copy what you had, and then by virtue of you having made a bigger array to start with, you have some more slack that you didn’t have before.
Daniel?
Student: How does it delete arr with a star?
Instructor (Julie Zelenski) :You know it has to do with just delete takes a pointer. It does it – so a star arr is a string, arr is a pointer to a string. So both forms of delete, delete and delete bracket –
Student: So conceptually –
Instructor (Julie Zelenski) :– a pointer.
Student: – there is a start there because it’s delete –
Instructor (Julie Zelenski) :Well effectively, yeah. It’s delete the thing at the other end of the pointer, really. But it’s funny. Delete says take this address and reclaim its contents. And so it doesn’t really operate on a string, per se, it operates on the storage where that string is. And so I don’t know whether you want to call that is there an implicit star there or not, it really is about the pointer though rather than the contents. So saying that address has some memory associated with it, reclaim that memory.
Student: If I could raise –
Instructor (Julie Zelenski) :Uh-huh.
Student: So when you’re first declaring or when you’re making a pointer like string bigger, string star bigger, you have to declare it with the star notion. But then later on, you don’t ever have to use that again?
Instructor (Julie Zelenski) :You pretty much won’t see that star used again. Right? It’s interesting that things like bigger sub I and erase sub I implicitly have a D reference in them. And that can be misleading. You think, “Well how come I’m never actually using that star again on that thing to get back to the strings that are out there?”
And it has to do with the fact that the bracket notation kind of implicitly D references in it. If I did a star bigger, it would actually have the effect of giving me bigger sub zero, it turns out. You can use that notation but it’s not that common to need to.
Student: And so down on the last, one line up from the bottom, it says array equals bigger. You don’t have to –
Instructor (Julie Zelenski) :Yeah, if you did that, if I did say –
Student: If you said array –
Instructor (Julie Zelenski) :Star arr equals star bigger, I would not be getting what I want. Right? What it would be doing is it would say follow bigger and see what’s at the other end, so that would follow bigger and get that string A. And then it would say follow ARR and overwrite it with that A, so it would actually have the effect of only copying the first string from bigger on top of the first string of array. But array would still point to where it was, bigger would still point to where it was, and they would – we would’ve not have updated our, the pointer we really wanted to point to the new array.
So there is a difference. Without the star, we’re talking about the changing the pointers; with the star, we’re talking about the strings at the other end. And so we’re – this is a string assignment. It says assign one string to another. Without the star on it, it’s like assign one pointer to another; make two pointers point to the same place. When you’re done with this, bigger and arr will be aliases for the same location. That’s a very important question though to get kind of what that star’s doing for you.
Here?
Student: After arr is bigger, can you delete bigger after that?
Instructor (Julie Zelenski) :If I deleted bigger, at that point arr is pointing to the same place. And so remember that having two or three or ten pointers all at the same place, if you delete one of them, they actually effectively are deleted. The delete really deletes the storage out here. And then if I did that, it would cause arr to then be pointing to this piece of memory, and not a good scene will come from that. It means that when it later goes back in there and starts trying to read and write to that contents at any moment it could kind of shift underneath you. You don’t own it any more; it’s not reserved for your use.
bunch of cleanup on them.” But it doesn’t ever expose that information back to you. It doesn’t let you depend on it, so it’s up to you to maintain that information redundantly with it.
All right, let me see if I can make it a template. I probably can’t do this actually fast enough to get it all done today, but we can at least get started on it. So then, I introduce a template header and I make up the name that I want here, so same class header now other than typing elem type. Then I look through my interface and I see places where I previously had said it’s strings, it’s strings, it’s storing strings. And I say it’s not actually storing strings; it’s gonna store elem type things, it’s gonna return elem type things and it’s going to have an array of elem type things.
So I think that’s everything that happened to the interface. Let me see if I see any other places that I – so the interface part is kind of small. There’s one other change I’m gonna have to make to it but I’m gonna come back to it. I’m gonna look at the code at the other side for a second. And I say, “Okay, well that wasn’t so bad.”
Now it turns out that it gets a little bit goopier over here because that template type name has to go on every one of these: introduce them to the template type and elem type. And now there’s another place where it needs to show up. So the full syntax for this is now saying this is a template function, depending on elem type, and it’s actually for the myvector who is being – we are writing the myvector constructor for something whose name is myvector angle bracket elem type.
So there’s gonna be a lot of this goo. Every one of these is kinda change its form, from just looking like the ordinary myvector class scope doesn’t really exist any more. Myvector is now a template for which there’s a lot of different class scopes, one for each kind of thing being stored. So myvector int is different than myvector string. So we say, “Well, if you were building the myvector constructor for myvector string, it looks like this.” Or you know having filled an elem type with those strings.
So everywhere I was using string, I got to change to elem type in the body as well. And then I kind of take this guy and use it in a bunch of places. I’m gonna use it here and then I’m gonna have to do it down here, on that side, do it here, and it’s gonna return something of elem type, here. It’s a little bit of a mess to do this, and the code definitely gets a little bit goopier as a result of this. It doesn’t look quite as pretty as it did when it wasn’t a template, but it becomes a lot more useful.
Okay. Then I need to look for places that I used a string. And every place where I was using string, assuming that’s what I was storing, it now actually turns into elem type. So my pointers and the kind of array I’m allocating is actually now made into elem type. The rest of the code actually didn’t say anything specific about what’s its doing, just copying things from one array to another. And now, depending on what the arrays are, it’s copying ints or strings or doubles.
And then other places in the interface where I’m doing add or I’m going get at, I have to be describing the things that are coming in and out as elem type so that they can be matched to whatever the client’s using. I think the rest of it looks okay.
Student: Why do you have to write template type name, and elem type above every –
Instructor (Julie Zelenski) :Because you just have to, because it’s C++. Because the thing is, that piece of code is, itself, a template, so these are like little mini-templates. So that I had the interface, which said here’s the template pattern for the interface, and each of these says when you’re ready to make the size member function for a vector of int, it comes off this template. So this template describes what the size member function looks like for any of the myvectors you might instantiate. And it describes the template because, in fact, we need to build a new size for ints versus doubles versus strings.
It’s even funny because you think of my size like, “Well size doesn’t even use anything related to the elem type.” But in fact, each of the member functions is kinda specific. It’s not just a myvector size; it’s the myvector int size, the myvector string size. And that for some of the member functions it’s quite obvious why you need a distinct copy. Get at returns an int in some cases and a double in others; but even though ones that don’t appear to have any dependence on the elem type, actually are separated into their own individual versions.
So I think I got all of that fixed, and then I’m gonna have to do one thing that’s gonna seem really quirky. And it is very quirky but it is C++. Let me show you what I’m gonna do. Is I’m going [inaudible] out of the project. Okay, stop compiling that. And I’m gonna change how it is that myvector gets compiled by doing this.
Okay. Take a deep breath. This is really just an oddity of C++. So the situation is this: that templates aren’t really compiled ahead of time, templates are just patterns. You know? They like describe a recipe for how you would build a myvector class. But you can’t just compile myvector and be done with it because until the client uses it, you don’t know what kind of myvectors you’re building. Are they myvectors of ints or strings or pseudo structures?
So it turns out that the myvector really needs to get compiled at the usage, at the instantiation. When you’re ready to make a myvector of students, it then needs to see all the code for myvector so it can go build you a myvector for students on the fly. In order to see that code, it actually has to be present in a different way than most code.
Most code is compiled, instead of .cpp, it just gets compiled once and once for all. The random library, random integer doesn’t change for anybody usage, there’s a random.cpp. It compiled the function. You’re done. So the template code does not get compiled ahead of time. It doesn’t get listed in the project. What happens is the .h typically has not only the interface, but actually all the code.
