The name "pointer" describes the job of the item; a pointer "points" to another variable or constant. Some tasks in C++ are easier to do with pointers; others would be utterly impossible without pointers. This article discusses how to create and work with pointers in C++. It is taken from chapter 11 of the book C++ Demystified, written by Jeff Kent (McGraw-Hill/Osborne, 2004; ISBN: 0072253703).
A pointer is a variable or constant whose value is the address of another variable or constant. Some C++ tasks are performed more easily with pointers, while other C++ tasks, such as dynamic memory allocation, cannot be performed without pointers.
Like any variable or constant, you must declare a pointer before you can work with it. The only difference between declaring a pointer and a variable or constant which stores a value instead of an address is that the pointer declaration includes an asterisk between the data type and the pointer name. However, the data type in the declaration of a pointer is not the data type of its value, as is the case with a variable or constant which stores a value instead of an address. The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. Rather, the data type in the declaration of a pointer refers to the data type of another variable (or constant) whose memory address is the value of the pointer. In other words, the value of an integer pointer variable must be the address of an integer variable or constant, the value of a float pointer variable must be the address of a float variable or constant, and so forth.
You should always explicitly assign a pointer a value before you use it; otherwise, you risk a runtime error or worse. When you are ready to assign a pointer the address of another variable or constant, you use the address operator with the target variable or constant. However, if it is too early in your code to know which address to assign to the pointer, you first assign the pointer NULL, which is a constant defined in several standard libraries, including iostream. The value of NULL, the memory address 0, signals that the pointer is not intended to point to an accessible memory location.
The indirection operator is used on a pointer to obtain the value of the variable or constant to which the pointer points. This operation is said to dereference the pointer.
A pointer may be a variable or a constant. The name of an array is a constant pointer, pointing to the base address of the array. A pointer variable, being a variable, may point to different variables or constants at different times in the program.
A pointer variable may be incremented. Incrementing a pointer variable is common when looping through consecutive indices of an array. Incrementing a pointer variable does not necessarily increase its value by 1. Instead, incrementing a pointer variable increases its value by the number of bytes of its data type.
Pointers may be passed as function arguments. This is called passing by address. Pointer notation usually is used to note that an argument is a pointer. The difference in syntax between passing by reference and passing by address is that, in the function prototype and header, you use an ampersand (&) for passing by reference, but an asterisk (*) for passing by address. Additionally, if the pointer argument is the name of a single variable as opposed to an array, there are two further differences in syntax between passing by reference and passing by address. First, when you call the function, you donít need the address operator (&) for passing by reference, but you do for passing by address. Second, in the body of the called function, you donít need to dereference the argument when you pass it by reference, but you do when you pass by address.
You can use a variable as a size declarator for an array if you use dynamic memory allocation because memory is allocated at runtime from a different placeóthe heapóthan where memory is allocated for variables declared at compile time on the stack. You need to use a pointer with the new operator to dynamically allocate memory, and the pointer must be of the same data type as the array which is to be allocated dynamically.
The lifetime of a dynamically allocated variable may be as long as that of the pro-gramís execution. However, if before the end of the program the pointer that points to a dynamically created variable goes out of scope, you no longer have any way of accessing the dynamically created memory. Therefore, the dynamically created variable still takes up memory, but is inaccessible. This is called a memory leak. To avoid memory leaks, you use the delete operator on the pointer that points to the dynamically allocated memory. This deallocates the dynamically allocated memory.
The return value of a function may be a pointer. If so, the pointer should point to either a static local variable or a dynamically created variable, not a local variable.
What is a pointer?
Name a C++ task that requires a pointer to be performed.
What is the difference between declaring an integer variable and declaring an integer pointer variable?
What is the meaning of the data type in the declaration of a pointer?
What is the meaning and purpose of NULL?
What operator do you use to assign a pointer the address of another variable or constant?
What is the purpose of the indirection operator?
May a pointer point to different memory addresses at different times in the program?
May more than one pointer point to the same memory address?
What is the effect of incrementing a pointer variable?
What are the purposes of the new and delete operators?