What does ampersand "&" do in front of pointers?

Question

When functions are being called I often see the ampersand in front of the pointer in the function parameter.

E.g.

int *ptr;
randomFunction(&ptr);

I have done some research and found that this means that the function uses pointers to pointers. Is the & sign in front of a pointer used just to indicate this or does it do something else?

Answer 1

It's a pointer to the pointer.

& is the reference operator, and can be read as address of. In your example, it will get another pointer, that is the address of the pointer given as it's argument, i.e. a pointer to the pointer.

Look at the following example:

int **ipp;
int i = 5, j = 6, k = 7;
int *ip1 = &i, *ip2 = &j;
ipp = &ip1;

You will get:

In the above example, ipp is a pointer to pointer. ipp stores the address of ip1 and ip1 stores the address of i.

You can check out Pointers to Pointers for more info.

Answer 2

Take a step back. The fundamental rules of pointer operators are:

• The * operator turns a value of type pointer to T into a variable of type T.
• The & operator turns a variable of type T into a value of type pointer to T.

So when you have

int *ptr;

ptr is a variable of type pointer to int. Therefore *ptr is a variable of type int -- the * turns a pointer into a variable. You can say *ptr = 123;.

Since ptr is a variable of type pointer to int, &ptr is a value -- not a variable -- of type pointer to pointer to int:

int **pp = &ptr;

&ptr is a value of type pointer to pointer to int. pp is a variable of type pointer to pointer to int. *pp is a variable of type pointer to int, and in fact is the same variable as ptr. The * is the inverse of the &.

Make sense?

Answer 3

It helps to think of "&" this way. int function_name ( &( whatever ) ); You are passing the address of ( whatever ). Whatever can be a number of things: an elementary variable. a function. a structure. a union. an array. You should mentally translate "&" to "take the address of". So your example means : pass a COPY of the address of the address of the variable ptr of type int!