One benefit of pointers is when you use them in function arguments, you don't need to copy large chunks of memory around, and you can also change the state by dereferencing the pointer.
For example, you may have a huge struct MyStruct, and you have a function a().
void a (struct MyStruct* b) {
// You didn't copy the whole `b` around, just passed a pointer.
}
One benefit of pointers is when you use them in function arguments, you don't need to copy large chunks of memory around, and you can also change the state by dereferencing the pointer.
For example, you may have a huge struct MyStruct, and you have a function a().
void a (struct MyStruct* b) {
// You didn't copy the whole `b` around, just passed a pointer.
}
Coming from Java, you'll have a slightly different perspective than what is presented in K&R (K&R doesn't assume that the reader knows any other modern programming language).
A pointer in C is like a slightly more capable version of a reference in Java. You can see this similarity through the Java exception named NullPointerException. One important aspect of pointers in C is that you can change what they point to by increment and decrement.
In C, you can store a bunch of things in memory in an array, and you know that they are sitting side by side each other in memory. If you have a pointer to one of them, you can make that pointer point to the "next" one by incrementing it. For example:
int a[5];
int *p = a; // make p point to a[0]
for (int i = 0; i < 5; i++) {
printf("element %d is %d\n", i, *p);
p++; // make `p` point to the next element
}
The above code uses the pointer p to point to each successive element in the array a in sequence, and prints them out.
(Note: The above code is an expository example only, and you wouldn't usually write a simple loop that way. It would be easier to access the elements of the array as a[i], and not use a pointer there.)
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Newbie here:
I know how pointers work, what do they do and how to deference them, but I don’t understand why would someone use pointers instead of the variable name. Can someone tell me what’s up?
Pointers are necessary for dynamic memory location, many data structures, and efficient handling of large amounts of data. Without pointers, you'd have to allocate all the program data globally or in functions or the equivalent, and you'd have no recourse if the amount of data grew beyond what you had originally allowed for. I hesitate to use absolutes here, but as far as I know all modern computer languages have pointers in some form or other.
In most languages that use pointers, there are certain sorts of references that are pointers, and perhaps certain sorts of references that aren't, and there is no further notational difference. A Lisp cons cell is a pair of pointers, although a fixnum is not a pointer. In Java, the variable used for the instance of a class is a pointer, but an int isn't. The language syntax doesn't reflect that.
C is unusual in that pointers are optional, explicit, and allow explicit pointer arithmetic. It is perfectly possible to write struct foo bar; struct foo * baz;, and once you've allocated memory for baz you can use both bar and baz to represent struct foos. Since pointers are optional, it is useful to have notational differences. (It's essential in C++ for smart pointers, as given boost::shared_ptr<foo> bar;, bar.reset() has one meaning and bar->reset() is likely to have a much different one.)
(Actually, explicit pointers were often used in other languages when C was originally being developed, such as ^ in Pascal. C is an older language than most in common use today, and it shows.)
One of C's design goals was to write Unix in, and therefore it needed to handle memory locations in a detailed manner. (C is actually one of a family of system implementation languages common when it was being designed, another example being Cybol for Control Data computers. C is the one that became a big hit.) Therefore, it is possible to manipulate C pointers directly, assigning memory addresses and calculating new ones. This also led to some design decisions in C. C arrays are based heavily on pointer arithmetic, and indeed an array decays into a pointer in very many situations. Passing variables to C functions by reference is done by pointer. There was no strong need for arrays and passing variables by reference in the form that other contemporary languages had, so C didn't get those.
So, the answer is that, in most languages nowadays, you use pointers constantly without being reminded of the fact. In C, and to a lesser extent C++, you use pointers either to do low-level things, or as accomplish higher-level things that there's no special notation for.
Complex data structures. You can't build something like a linked list or a binary tree without pointers.
There are no "pros" and "cons" of pointers. They are just a tool, like a hammer.
It depends on what you try to achieve:
- you can change the value of a variable inside a function
- you can pass a struct to a function without having to copy all its fields - think of a function that receives a struct.
- you can point to a specific variable/struct and point to it from other structs
and many other advantages (advantages is purpose dependant and it depends on whats your program is doing).
Pointers are quite basic C and there is a lot of material online you should get yourself familiar with them and the advantages will pop up themselves.
The reason is that pointers are used to bodge into C some vital features which are missing from the original language: arrays, strings, & writeable function parameters. They can also be used to optimize a program to run faster or use less memory that it would otherwise. A few tasks these days, such as programming microcontrollers, still need this.