Objects correspond to locations within the computer's memory where data may be stored. Every object has a name, type, and value and is stored in a certain amount of memory. Objects are "declared" within a program using declaration statements (also called deinitions in the text) and "assigned" values using assignment statements, as shown in the example below:
1 #include <iostream.h>
2 #include <stdlib.h>
3 main()
4 {
5 int num1 = 10;
6 int num2 = 5;
7 int sum;
8 sum = num1 + num2;
9 cout << num1 << " + " << num2 << " = " << sum << endl;
10 }
Lines 5, 6, and 7 are declaration statements. The words num1, num2, and sum are the names of objects. The declarations specify that the objects num1, num2, and sum are data of type int, which means that these objects will hold integer values (i.e: whole numbers such as 12, -33, 0, 32123). In C++, the keyword int is reserved to declare integer objects.
All objects must be declared with a name and data type before they can be used in a program. Several objects may be declared in one declaration statement, for example, lines 5, 6, and 7 could have been written as a single line as follows:
int num1 = 10, num2 = 5, sum;
Objects may also be declared and initialized to a known value as shown in the program above. Note that the object sum was not initialized so the value of the object is not known until it is assigned a value. Line 8 assigns the object sum the result of adding num1 and num2 together.
A object name is any valid identifier. An identifier is a series of characters consisting of letters, digits and underscores (_) that does not begin with a digit. C++ allows identifiers of any length, but your C++ environment may impose some restrictions on the length of identifiers. C++ is case sensitive, uppercase and lowercase letters are different, so the objects a1 and A1 are considered to be two different objects.
Declarations can be placed almost anywhere in a function. However, the declaration of a object must appear before the object is used in a program.
C++ supports a variety of data types used when declaring objects. The standard data types are shown in the table below:
| Type keyword(s) |
Description | Range and Precision |
| char | One byte integer, unsigned | 0 to 255 |
| short | Two-byte integer | -32,768 to 32,767 |
| unsigned short | Two-byte integer, unsigned | 0 to 65,535 |
| long | Four-byte integer | Approx. -2.147 billion to 2.127 billion |
| unsigned long | Four-byte integer, unsigned | 0 to Approx. 4.292 billion |
| int | Same as short or long depending on implementation |
|
| unsigned int or unsigned |
Same as unsigned short or unsigned long, depending on implementation |
|
| float | Four-byte floating point | Approx. plus or minus 3.4 times 10 to the 38th. Seven digits of precision. |
| double | Eight-byte floating point | Approx. plus or minus 1.7 times 10 to the 308th. 15 digits of precision. |
Note: Character strings in C++ are implemented as an array of char. We will discuss arrays and strings in a subsequent topic.
The program below declares a number of objects of different types and then displays the values of each:
#include <iostream.h>
#include <stdlib.h>
main()
{
signed char v1 = 'A';
unsigned char v2 = 'B';
char v3 = 67;
short v4 = 100;
unsigned short v5 = 200;
long v6 = 1000;
unsigned long v7 = 2000;
int v8 = 3000;
unsigned int v9 = 4000;
unsigned v10 = 5000;
float v11 = 3.141592;
double v12 = 1.1710394893;
cout << " v1 = " << v1 << endl;
cout << " v2 = " << v2 << endl;
cout << " v3 = " << v3 << endl;
cout << " v4 = " << v4 << endl;
cout << " v5 = " << v5 << endl;
cout << " v6 = " << v6 << endl;
cout << " v7 = " << v7 << endl;
cout << " v8 = " << v8 << endl;
cout << " v9 = " << v9 << endl;
cout << "v10 = " << v10 << endl;
cout << "v11 = " << v11 << endl;
cout << "v12 = " << v12 << endl;
return 0;
}
The const keyword lets you create a object while ensuring that its value will not change. This keyword modifies a declaration, informing the compiler that it must watch for and prevent all assignments to the item. Initialization, however, is allowed. Once a object is defined with const, the compiler flags all statements in which the object is the target of an assignment.
const int id = 12345; // This is valid; id may be initialized.
...
id = 1000; // ERROR! Attempt to assign a const item a new value.