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pointers

Christine S. WolfeOhio University Lancaster2008-Aug-01

This lesson introduces the use of pointers:

Vocabulary:

addresspointeepointer

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

The chart to the right represents computer memory.

• Each cell represents a single byte of memory.

• The addresses are written in hexadecimal to the left of the chart.• The first cell in the top

row is located at address 0000h

• The last cell in the top row is located at address 0003h

• The last cell in the bottom is located at address 001Bh

The first few slides explain the diagrams that will be used in this slideshow.

Don't fret about hexadecimal - you don't actually have to convert them. Just recognize that they are addresses in these diagrams.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

Age

When an int, float, char, or double is declared, the cell(s) will be green and the name of the variable will be written in very tiny letters in the lower left corner.

Click for Example

int Age;

ClickTip

The green rectangle is 2 bytes wide because declaring an int reserves 2 bytes.

char reserves 1 byte.float reserves 4 bytes.double reserves 8 bytes.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

Age

When a value is assigned to a variable, the value will be centered in the cell in a larger font than the name of the variable

Click for Example

Age = 23;

Age 23

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

Click for Example

int *pAge;

Age 23

When pointer is declared, the cell will be blue and the name of the pointer will be written in very tiny letters in the lower left corner.

pAge

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

Age

When an address is assigned to a pointer, the value will be centered in the cell in a larger font than the name of the pointer.

Click for Example

pAge = &Age;

Age 23

pAge000CpAge

A pointer is a memory location that stores the address of a different memory location.The destination is called the pointee.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

pAge

Age

000CpAge

The pointer “points” to the pointee.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

pAge

Age

000CpAge

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

int *pAge;

int Age;

pAge

Age

When declaring a pointer, the data type given must be the data type of the pointee. So, if I’m defining a pointer that will point to an int, then I declare the pointer as int.

When declaring a pointer, an asterisk, *, must be included either immediately after the data type or immediately before the name of the pointer.

Notice that neither Age nor pAge contain a value at this point. They have been declared but have not been assigned a value.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

int *pAge;

int Age;

pAge = &Age;

pAge

Age

000CpAge

To store a value in pAge, assign it the address of a variable.Remember that the address of a scalar variable is expressed by placing an ampersand, &, in front of the variable name.

Notice that I don't include the * when I assign an address to the pointer.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

int *pAge;

int Age;

pAge = &Age;

pAge

Age

000CpAge

There are 2 ways to store a value in Age.

1: The most common way is by using the assignment operator on the variable to make a direct assignment.

Age = 23;

Age 23

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

int *pAge;

int Age;

pAge = &Age;

pAge

Age

000CpAge

There are 2 ways to store a value in Age.

2: An alternate method is to way is by using the assignment operator and the dereference operator on the pointer to make an indirect assignment.

Age = 23;

Age 30*pAge = 30;

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

int *pAge;

int Age;

pAge = &Age;

pAge000CpAge

When assigning a value to a variable via the pointer, the computer first looks up the address in the pointer and then moves to that address to store the value.

Age = 23;

*pAge = 30;Age 23

Step 1: Find pAge.

Step 3: Go to the address.

Step 2: Read the address in pAge.

Step 4: Write the value into that memory address.

Age 30

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

*pSalary = 54000.00;2130

float *pSalary;

float Salary;pSalary = &Salary;

Another example. In this case, we will work with a float so the actual float variable will take up 4 bytes.

pSalary0014

pSalary

Age 30

Salary

000CpAge

54000.00Salary

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

*pSalary = 54000.00;2130

float *pSalary;

float Salary;pSalary = &Salary;

pSalary0014

pSalary

Age 30

Salary

000CpAge

54000.00Salary

Notice: The * (dereference operator) is used when declaring the pointer and when I assigning a value in the pointee.

The * (dereference operator) is NOT used when assigning an address to the pointer.

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

*pSalary = 54000.00;2130

float *pSalary;

float Salary;pSalary = &Salary;

pSalary0014

pSalary

Age 30

Salary

000CpAge

54000.00Salary

Another way to remember when to use the dereference operator (*)

If storing a value (address) here, don't use the *. You don't want to redirect the storage to a different location. You want the address stored in the pointer location itself!

If storing a value (real content) here, use the *. You do want to redirect (dereference) the storage to a location other than the cell that contains the pointer.

WHY BOTHER

????

You are probably asking a question....

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

000Ch 0014h

2130

54000.00

One of the most common uses of pointers is to pass the location of local variables between functions.

void Fort(void);

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

000Ch 0014h

2130

54000.00

When processing is outside of the local function, the variables local to the function cannot be referenced by name.

void Fort(void);Oh, heck. I can't remember the

variables

0000-0003h

0004-0007h

0008-000Bh

000C-000Fh

0010-0013h

0014-0017h

0018-001Bh

000Ch 0014h

2130

54000.00

BUT...the memory location can be accessed if the address is known!

void Fort(&Age);

Store 18 in 000Ch