COSC 2P91 Dynamic allocation Week 4a Brock University Brock - PowerPoint PPT Presentation

COSC 2P91 Dynamic allocation Week 4a Brock University Brock University (Week 4a) Dynamic allocation 1 / 22

Before we begin... Two quick addenda to last week... Brock University (Week 4a) Dynamic allocation 2 / 22

Pointers to local variables accessed outside of call... Remember how I said not to return a pointer to a local variable from a function? That was because the variable may no longer exist after the stack frame has been deallocated However, I forgot to mention one obvious case where it’ll still exist: ◮ A static member will obviously still exist ◮ e.g. to a static array (say to use as an intermediate buffer). It’s a little odd, but entirely legal Brock University (Week 4a) Dynamic allocation 3 / 22

On declaring arrays Particularly in parameters... If you’ll recall, you don’t always need to fully (explicitly) define the size of arrays. For example, when passing a 2D array into a function, only the number of columns needs to be specified However, in general, you might like to explicitly use a variable for a variable length array... Further to this, you could wish to declare this variable in a later parameter than the array... Only rely on this being in Gnu C, but you can always use a parameter forward declaration e.g. void doStuff(int r,c; int stuff[r][c], int r, int c) Brock University (Week 4a) Dynamic allocation 4 / 22

Dynamic allocation We’ve already looked at static declarations. We know how to create arrays, variables, etc. Basically, any time we know our precise storage requirements in advance, we’re good. And that’s why, in every other language, we only ever use arrays. For everything. ... right? Obviously not. We need some method of asking for additional memory when needed Sometimes we’ll be asking for an array of unknown dimensions ◮ Or we may simply want a ragged array Sometimes we’ll want to be able to create larger dynamic structures ◮ e.g. lists, trees, etc. Memory is dynamically allocated from the heap Brock University (Week 4a) Dynamic allocation 5 / 22

Memory allocation The basics We’ll look at the functions that provide memory allocation in just a moment. Let’s just make sure we have a rough understanding of the principle first... A memory allocation function sets aside some memory from the heap, and then returns a pointer to the first position of the allocated space ◮ Because the same function is used for allocation of any type, it returns a void pointer , which can then be (implicitly or explicitly) cast This should be self-evident by now, but be careful with pointers. Avoid unnecessary math, and always be careful whenever directly manipulating memory Memory can be allocated for an array, or a single variable ◮ If the latter seems unlikely, consider struct s (particularly linked together) Be really careful with memory safety! ◮ It can be easy to accidentally exceed the allocated bounds of an array ◮ Similarly, remember to definitely allocate, and not simply declare the pointer ⋆ (cough) Brock University (Week 4a) Dynamic allocation 6 / 22

Memory allocation malloc The basic memory allocation function is malloc . It simply allocates a block of some designated size, and returns a corresponding pointer. Like the other three functions, part of stdlib . i n t ∗ var ; // Pointer to a s i n g l e i n t e g e r ? f l o a t ∗ v a r r y ; // Pointer to a s i n g l e f l o a t var=malloc ( s i z e o f ( i n t ) ) ; // I m p l i c i t void ∗ − > i n t ∗ v a r r y=malloc ( s i z e o f ( i n t ) ) ; // I m p l i c i t void ∗ − > i n t . . . huh . ∗ Explicitly casting the pointer may at least give you a warning Brock University (Week 4a) Dynamic allocation 7 / 22

Memory allocation malloc (cont) To create an array, simply tell it that you need more space: i n t ∗ a r r =( i n t ∗ ) malloc ( s i z e o f ( i n t ) ∗ s i z e ) ; You can access the new space like any other array now It’s worth noting that malloc doesn’t initialize the memory, so you may have junk data until you clear it out yourself. If the requested memory isn’t available, a null pointer (0) is returned instead. Brock University (Week 4a) Dynamic allocation 8 / 22

Memory allocation calloc calloc differs from malloc in two fundamental ways: It initializes allocated space to all zeroes It takes two parameters: ◮ One for the number of elements ◮ One for the size of each element Brock University (Week 4a) Dynamic allocation 9 / 22

Memory allocation realloc Suppose you allocated for 10 elements, and now realize you really need space for 11. realloc lets you reallocate memory to expand (or shrink) the dynamic array. Note that, particularly if expanding the array, it may be necessary to move the location of the allocated space. As such, realloc returns a pointer to the new address of the reallocated space ◮ If you had more than one pointer to the array, then they can very easily fall out of sync ◮ This is one of the reasons why you may wish to try to avoid realloc altogether realloc will, of course, preserve those values that are common to both sizes Brock University (Week 4a) Dynamic allocation 10 / 22

Memory allocation We good? Do we need any other examples? Possibly returning a dynamic array from a function? Brock University (Week 4a) Dynamic allocation 11 / 22

Memory de allocation So, what do you do when you’re done with the requested memory? Of course, the memory will be reclaimed after the program exits... ◮ After all, if we had enough memory to allocate it, and if it was ever necessary for the program, then surely we don’t have to worry about memory constraints, right? Obviously, we’re going to need to be able to reclaim the memory sooner. For that, we use free Simply provide free with the same pointer you’ve been using ◮ Try not to putz around with this one (particularly when using pointer arithmetic) Note: free is not recursive! Brock University (Week 4a) Dynamic allocation 12 / 22

Importance of free Because we can’t all get away with being Mozilla... Why is it important to free ? Why does it matter that free isn’t recursive? Do you know what a memory leak is? ◮ Example time! Brock University (Week 4a) Dynamic allocation 13 / 22

Multidimensional arrays We’ve already discussed 2D arrays a bit. Have we talked about the difference between int dealie[][] and int *dealie[] ? Technically, arrays of pointers are slightly different, even if they’re generally used the same. Brock University (Week 4a) Dynamic allocation 14 / 22

Multidimensional arrays Ragged arrays Taking advantage of this knowledge, we can easily creat ragged arrays . Were I to say it was essentially the same as the Java approach, would we still need an example? Brock University (Week 4a) Dynamic allocation 15 / 22

Function pointers We’ve discussed having pointers to variables, to structs, and to arrays, but we’ve left something out: pointers to functions It may not initially seem to be of obvious benefit, but function pointers can have several uses: Invoking a single block of code on data, but allowing it to have a different behaviour/outcome ◮ e.g. comparators, operators, intersections, etc. Providing callbacks to a library Some more... creative uses (next slide) Usage is pretty simple: returnType ( ∗ localName )( paramTypeA , paramTypeB ) This type of declaration can be used for parameters or variables; you can even embed the pointers inside struct s! Brock University (Week 4a) Dynamic allocation 16 / 22

Function pointers Fancier uses As mentioned, you can include function pointers inside struct s. Why would you do this? Gee, if only one could think of a reason for a record to have not only per-instance variables, but also associated procedures... ◮ Besides that, one could combine this concept with what we saw in our code example to simulate basic notions of polymorphism and inheritance (and such) ◮ If you want a struct to be able to operate on itself, via its own functions, you should probably include a reference to itself as one of the parameters Brock University (Week 4a) Dynamic allocation 17 / 22

Basic data structures We should already know enough by now to be able to simulate most of the simpler data structures we’ve learned in other courses. Brock University (Week 4a) Dynamic allocation 18 / 22

Basic data structures Stacks How could we implement a stack? Actually, didn’t we already look at some code that seemed a bit familiar... Depending on why you’re creating it, you may wish to: ◮ Simply create code for a single type of stack — e.g. int ◮ Write separate stack code for multiple types ◮ Employ void pointers since the data itself doens’t affect the push/pop behaviour ⋆ Of course, this puts the burden of type safety onto the client Brock University (Week 4a) Dynamic allocation 19 / 22