I'm just starting out with pointers, and I'm slightly confused. I know & means the address of a variable and that * can be used in front of a pointer variable to get the value of the object that is pointed to by the pointer. But things work differently when you're working with arrays, strings or when you're calling functions with a pointer copy of a variable. It's difficult to see a pattern of logic inside all of this.
When should I use & and *?
Yeah that can be quite complicated since the * is used for many different purposes in C/C++.
If * appears in front of an already declared variable/function, it means either that:
* gives access to the value of that variable (if the type of that variable is a pointer type, or overloaded the * operator).* has the meaning of the multiply operator, in that case, there has to be another variable to the left of the *If * appears in a variable or function declaration it means that that variable is a pointer:
int int_value = 1;
int * int_ptr; //can point to another int variable
int int_array1[10]; //can contain up to 10 int values, basically int_array1 is an pointer as well which points to the first int of the array
//int int_array2[]; //illegal, without initializer list..
int int_array3[] = {1,2,3,4,5}; // these two
int int_array4[5] = {1,2,3,4,5}; // are identical
void func_takes_int_ptr1(int *int_ptr){} // these two are identical
void func_takes_int_ptr2(int int_ptr[]){}// and legal
If & appears in a variable or function declaration, it generally means that that variable is a reference to a variable of that type.
If & appears in front of an already declared variable, it returns the address of that variable
Additionally you should know, that when passing an array to a function, you will always have to pass the array size of that array as well, except when the array is something like a 0-terminated cstring (char array).
Actually, you have it down pat, there's nothing more you need to know :-)
I would just add the following bits:
& takes a variable and gives you the address, * takes an address and gives you the variable (or contents).char **p means that p is a pointer to a pointer to a char.As to things working differently, not really:
\0) character.Put simply
& means the address-of, you will see that in placeholders for functions to modify the parameter variable as in C, parameter variables are passed by value, using the ampersand means to pass by reference.* means the dereference of a pointer variable, meaning to get the value of that pointer variable.int foo(int *x){
*x++;
}
int main(int argc, char **argv){
int y = 5;
foo(&y); // Now y is incremented and in scope here
printf("value of y = %d\n", y); // output is 6
/* ... */
}
The above example illustrates how to call a function foo by using pass-by-reference, compare with this
int foo(int x){
x++;
}
int main(int argc, char **argv){
int y = 5;
foo(y); // Now y is still 5
printf("value of y = %d\n", y); // output is 5
/* ... */
}
Here's an illustration of using a dereference
int main(int argc, char **argv){
int y = 5;
int *p = NULL;
p = &y;
printf("value of *p = %d\n", *p); // output is 5
}
The above illustrates how we got the address-of y and assigned it to the pointer variable p. Then we dereference p by attaching the * to the front of it to obtain the value of p, i.e. *p.
When you are declaring a pointer variable or function parameter, use the *:
int *x = NULL;
int *y = malloc(sizeof(int)), *z = NULL;
int* f(int *x) {
...
}
NB: each declared variable needs its own *.
When you want to take the address of a value, use &. When you want to read or write the value in a pointer, use *.
int a;
int *b;
b = f(&a);
a = *b;
a = *f(&a);
Arrays are usually just treated like pointers. When you declare an array parameter in a function, you can just as easily declare it is a pointer (it means the same thing). When you pass an array to a function, you are actually passing a pointer to the first element.
Function pointers are the only things that don't quite follow the rules. You can take the address of a function without using &, and you can call a function pointer without using *.
There is a pattern when dealing with arrays and functions; it's just a little hard to see at first.
When dealing with arrays, it's useful to remember the following: when an array expression appears in most contexts, the type of the expression is implicitly converted from "N-element array of T" to "pointer to T", and its value is set to point to the first element in the array. The exceptions to this rule are when the array expression appears as an operand of either the & or sizeof operators, or when it is a string literal being used as an initializer in a declaration.
Thus, when you call a function with an array expression as an argument, the function will receive a pointer, not an array:
int arr[10];
...
foo(arr);
...
void foo(int *arr) { ... }
This is why you don't use the & operator for arguments corresponding to "%s" in scanf():
char str[STRING_LENGTH];
...
scanf("%s", str);
Because of the implicit conversion, scanf() receives a char * value that points to the beginning of the str array. This holds true for any function called with an array expression as an argument (just about any of the str* functions, *scanf and *printf functions, etc.).
In practice, you will probably never call a function with an array expression using the & operator, as in:
int arr[N];
...
foo(&arr);
void foo(int (*p)[N]) {...}
Such code is not very common; you have to know the size of the array in the function declaration, and the function only works with pointers to arrays of specific sizes (a pointer to a 10-element array of T is a different type than a pointer to a 11-element array of T).
When an array expression appears as an operand to the & operator, the type of the resulting expression is "pointer to N-element array of T", or T (*)[N], which is different from an array of pointers (T *[N]) and a pointer to the base type (T *).
When dealing with functions and pointers, the rule to remember is: if you want to change the value of an argument and have it reflected in the calling code, you must pass a pointer to the thing you want to modify. Again, arrays throw a bit of a monkey wrench into the works, but we'll deal with the normal cases first.
Remember that C passes all function arguments by value; the formal parameter receives a copy of the value in the actual parameter, and any changes to the formal parameter are not reflected in the actual parameter. The common example is a swap function:
void swap(int x, int y) { int tmp = x; x = y; y = tmp; }
...
int a = 1, b = 2;
printf("before swap: a = %d, b = %d\n", a, b);
swap(a, b);
printf("after swap: a = %d, b = %d\n", a, b);
You'll get the following output:
before swap: a = 1, b = 2 after swap: a = 1, b = 2
The formal parameters x and y are distinct objects from a and b, so changes to x and y are not reflected in a and b. Since we want to modify the values of a and b, we must pass pointers to them to the swap function:
void swap(int *x, int *y) {int tmp = *x; *x = *y; *y = tmp; }
...
int a = 1, b = 2;
printf("before swap: a = %d, b = %d\n", a, b);
swap(&a, &b);
printf("after swap: a = %d, b = %d\n", a, b);
Now your output will be
before swap: a = 1, b = 2 after swap: a = 2, b = 1
Note that, in the swap function, we don't change the values of x and y, but the values of what x and y point to. Writing to *x is different from writing to x; we're not updating the value in x itself, we get a location from x and update the value in that location.
This is equally true if we want to modify a pointer value; if we write
int myFopen(FILE *stream) {stream = fopen("myfile.dat", "r"); }
...
FILE *in;
myFopen(in);
then we're modifying the value of the input parameter stream, not what stream points to, so changing stream has no effect on the value of in; in order for this to work, we must pass in a pointer to the pointer:
int myFopen(FILE **stream) {*stream = fopen("myFile.dat", "r"); }
...
FILE *in;
myFopen(&in);
Again, arrays throw a bit of a monkey wrench into the works. When you pass an array expression to a function, what the function receives is a pointer. Because of how array subscripting is defined, you can use a subscript operator on a pointer the same way you can use it on an array:
int arr[N];
init(arr, N);
...
void init(int *arr, int N) {size_t i; for (i = 0; i < N; i++) arr[i] = i*i;}
Note that array objects may not be assigned; i.e., you can't do something like
int a[10], b[10];
...
a = b;
so you want to be careful when you're dealing with pointers to arrays; something like
void (int (*foo)[N])
{
...
*foo = ...;
}
won't work.
I think you are a bit confused. You should read a good tutorial/book on pointers.
This tutorial is very good for starters(clearly explains what & and * are). And yeah don't forget to read the book Pointers in C by Kenneth Reek.
The difference between & and * is very clear.
Example:
#include <stdio.h>
int main(){
int x, *p;
p = &x; /* initialise pointer(take the address of x) */
*p = 0; /* set x to zero */
printf("x is %d\n", x);
printf("*p is %d\n", *p);
*p += 1; /* increment what p points to i.e x */
printf("x is %d\n", x);
(*p)++; /* increment what p points to i.e x */
printf("x is %d\n", x);
return 0;
}
Ok, looks like your post got editted...
double foo[4];
double *bar_1 = &foo[0];
See how you can use the & to get the address of the beginning of the array structure? The following
Foo_1(double *bar, int size){ return bar[size-1]; }
Foo_2(double bar[], int size){ return bar[size-1]; }
will do the same thing.
I was looking through all the wordy explanations so instead turned to a video from University of New South Wales for rescue.Here is the simple explanation: if we have a cell that has address x and value 7, the indirect way to ask for address of value 7 is &7 and the indirect way to ask for value at address x is *x.So (cell: x , value: 7) == (cell: &7 , value: *x) .Another way to look into it: John sits at 7th seat.The *7th seat will point to John and &John will give address/location of the 7th seat. This simple explanation helped me and hope it will help others as well. Here is the link for the excellent video: click here.
Here is another example:
#include <stdio.h>
int main()
{
int x; /* A normal integer*/
int *p; /* A pointer to an integer ("*p" is an integer, so p
must be a pointer to an integer) */
p = &x; /* Read it, "assign the address of x to p" */
scanf( "%d", &x ); /* Put a value in x, we could also use p here */
printf( "%d\n", *p ); /* Note the use of the * to get the value */
getchar();
}
Add-on: Always initialize pointer before using them.If not, the pointer will point to anything, which might result in crashing the program because the operating system will prevent you from accessing the memory it knows you don't own.But simply putting p = &x;, we are assigning the pointer a specific location.
Source: Stackoverflow.com