Pointers 1 slides

Pointer fundamentals, syntax, and operations.¹⁾
Mithat Konar
October 19, 2021

• Pointers found in many programming languages.
• Important in C++ because a close relationship between pointers and arrays, C strings, and references.

• Variables are stored in blocks of computer memory.
• base address: the memory address of the first byte of this block.
• pointer variable: a variable that stores the base address of some other variable.
• Usefulness will be seen after understanding how to use them.

• Pointers hardware model
• Pointer visual model

• Assume an int named count storing the value 7 and ints occupy 4 bytes.

• The address of count is the base address 58000.

• A pointer variable set to point to count would store the value 58000.

• The value of a pointer variable is the base address of another variable.

Given the count variable used above: a pointer variable countPtr pointing to count would be represented as:

The value of countPtr is the box the arrow points to (i.e., the integer count).

Changing the value of countPtr to point to num:

• * character is used to indicate pointer variables in declarations and function parameter lists.
• Type of data being pointed to must also be indicated.

int *myPtr;     // declare a pointer to an int
bool *yourPtr;  // declare a pointer to a bool

• Location of the * character is flexible:

int *myPtr;
int* myPtr;
int * myPtr;

• You can declare more than one pointer variable at a time:

int *myPtr, *anotherOne;   // declare two pointers to an int

• But be careful::

int* myPtr, anotherOne;   // a pointer and an int

• To set the value of a pointer, you need the address of something.
• address operator & returns the base address of a variable:

int num = 42;
cout << num << endl;   // prints value held in variable num
cout << &num << endl ; // prints the base address of variable num

• Most environments show base addresses as hexadecimal numbers.

• Declare an integer variable y and a pointer variable myPtr set to store the address of (“point to”) y.

int y = 5;   // declare an integer variable y
int *myPtr;  // declare a pointer to int
myPtr = &y;  // myPtr gets address of ("points to") y

• Declare an integer variable y and a pointer variable myPtr set to store the address of (“point to”) y.

int y = 5;   // declare an integer variable y
int *myPtr;  // declare a pointer to int
myPtr = &y;  // myPtr gets address of ("points to") y

• Changing the value of pointer variables:

double z = 3.33;
double x = 42.0;
double *myPtr;
 
myPtr = &z;   // myPtr gets address of z
myPtr = &x;   // myPtr now has address of x

• Local pointer variables in C++ are not automatically initialized.
• Uninitialized pointers point to arbitrary memory. Very dangerous.
• You can initialize when declared:

int y = 5;
int *myPtr = &y;  // myPtr gets address of y

• You can set a pointer to a value that indicates that it is pointing to nothing: nullptr (C++11) or NULL (earlier versions).
• Otherwise uninitialized pointers should be set to nullptr (or NULL).

int *yourPtr = nullptr; // yourPtr points to nothing
int y = 5;
yourPtr = &y;           // yourPtr gets address of y

• nullptr and NULL have an integer value of 0, so sometimes programmers use 0 or '\0'.

• address operator & (already covered).
• indirection or dereferencing operator * (covered next).

• indirection or dereferencing operator, *, accesses the value of what its operand points to.

int y = -1;       // declare y and initialize its value
int *myPtr = &y;  // declare pointer and set it to point to y
 
cout << *myPtr;   // defererence myPtr (prints -1)

• Dereferencing can be used to assign a value to a location in memory:

int y = -1;       // declare y and initialize its value
int *myPtr = &y;  // declare pointer and set it to point to y
 
*myPtr = 7;       // change value in y to 7
cout << y;        // prints 7

• You can think of the indirection/dereferencing operator as meaning, “the_thing_at_the_end_of_”, as in:

the_thing_at_the_end_of_myPtr = 7;

• * and & operators complement each other.
• The following expressions all evaluate as true:

*&y == y
&*myPtr == myPtr
*&myPtr == myPtr

pointer-example.cpp

/** Demonstrate basic pointer usage. */
#include <iostream>
using namespace std;
int main()
{
    int a;                  // a is an integer
    int *aPtr = nullptr;    // aPtr is a pointer to an integer
 
    a = 7;      // give a a value
    aPtr = &a;  // set aPtr to the address of a
 
    cout << "The value of a is:     " << a << endl
         << "The address of a is:   " << &a << endl
         << "The value of aPtr is:  " << aPtr << endl;
    cout << endl;
 
    cout << "The value of a is:     " << a << endl
         << "The value of *aPtr is: " << *aPtr << endl;
    cout << endl;
 
    cout << "Showing that * and & are inverses of each other:"
         << endl
         << "&*aPtr = " << &*aPtr << endl
         << "*&aPtr = " << *&aPtr << endl;
 
    return 0;
}