Introduction to Objects and Variables

What are Objects and Variables?

In C++, an object is a piece of memory that can store values. A variable is a named object - it gives the object an identity we can use in our code.

Declaring Variables

To use a variable, you must first declare it by specifying its type and name:

int age;        // Declares a variable named 'age' of type int
double price;   // Declares a variable named 'price' of type double
char grade;     // Declares a variable named 'grade' of type char

Note: all variables must have a data type in C++.

Common Data Types

Here's a table of the most commonly used data types in C++:

Data Type Size (bytes) Range/Values Usage Example
int 4 -2,147,483,648 to 2,147,483,647 Whole numbers int age = 25;
short 2 -32,768 to 32,767 Small whole numbers short year = 2024;
long 8 Very large whole numbers Large integers long population = 8000000000;
float 4 ±3.4 × 10^38 (6-7 digits precision) Decimal numbers float price = 19.99f;
double 8 ±1.7 × 10^308 (15-16 digits precision) High-precision decimals double pi = 3.14159265359;
char 1 Single character (ASCII) Individual characters char grade = 'A';
bool 1 true or false Boolean logic bool isValid = true;
string Variable Text of any length Text and words string name = "John";

Key Notes:

  • Integers: Use int for most whole numbers, long for very large values
  • Decimals: Use double for most decimal calculations (better precision than float)
  • Characters: Use char for single characters, string for text
  • Boolean: Use bool for true/false conditions
  • Size: Actual sizes may vary by system, but these are typical values

Integer Types

  • int: Whole numbers (-2147483648 to 2147483647)
  • short: Smaller integers (-32768 to 32767)
  • long: Larger integers

Floating-Point Types

  • float: Decimal numbers (6-7 decimal digits precision)
  • double: Double precision decimals (15-16 decimal digits precision)

Note: double has more precision than float. We will discuss precision in later chapters.

Other Types

  • char: Single characters ('A', '5', '$')
  • bool: True/false values
  • string: Text (requires #include )

Variable Declaration vs Definition

  • Declaration: Tells the compiler about the variable's type and name
  • Definition: Actually creates the variable and allocates memory
int x;      // Declaration and definition
x = 10;     // Assignment

Multiple Variables

You can declare multiple variables of the same type:

int a, b, c;           // Three integer variables
double x, y, z;        // Three double variables

Best Practices

  • Choose descriptive names: studentAge not a
  • Initialize variables when possible
  • Declare variables close to where you use them