By: Daniel Malcolm in C++ Tutorials on 2007-09-14
Every variable has an address. Even without knowing the specific address of a given variable, you can store that address in a pointer.
For example, suppose that howOld is an integer. To declare a pointer called pAge to hold its address, you would write
int *pAge = 0;
This declares pAge to be a pointer to int. That is, pAge is declared to hold the address of an int.
Note that pAge is a variable like any of the variables. When you declare an integer variable (type int), it is set up to hold an integer. When you declare a pointer variable like pAge, it is set up to hold an address. pAge is just a different type of variable.
In this example, pAge is initialized to zero. A pointer whose value is zero is called a null pointer. All pointers, when they are created, should be initialized to something. If you don't know what you want to assign to the pointer, assign 0. A pointer that is not initialized is called a wild pointer. Wild pointers are very dangerous.
NOTE: Practice safe computing: Initialize your pointers!
If you do initialize the pointer to 0, you must specifically assign the address of howOld to pAge. Here's an example that shows how to do that:
unsigned short int howOld = 50; // make a variable unsigned short int * pAge = 0; // make a pointer pAge = &howOld; // put howOld's address in pAge
The first line creates a variable--howOld, whose type is unsigned short int--and initializes it with the value 50. The second line declares pAge to be a pointer to type unsigned short int and initializes it to zero. You know that pAge is a pointer because of the asterisk (*) after the variable type and before the variable name.
The third and final line assigns the address of howOld to the pointer pAge. You can tell that the address of howOld is being assigned because of the address of operator (&). If the address of operator had not been used, the value of howOld would have been assigned. That might, or might not, have been a valid address.
At this point, pAge has as its value the address of howOld. howOld, in turn, has the value 50. You could have accomplished this with one less step, as in
unsigned short int howOld = 50; // make a variable unsigned short int * pAge = &howOld; // make pointer to howOld
pAge is a pointer that now contains the address of the howOld variable. Using pAge, you can actually determine the value of howOld, which in this case is 50. Accessing howOld by using the pointer pAge is called indirection because you are indirectly accessing howOld by means of pAge. Later today you will see how to use indirection to access a variable's value.
New Term: Indirection means accessing the value at the address held by a pointer. The pointer provides an indirect way to get the value held at that address.
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