IndianTechnoEra: 02. Data Types & Variables in C++ | IndianTechnoEra

Data Types & Variables in C++

Built-in Data Types

There are four built-in data types in C++:-

1. Integer data type
2. Floating point data type
3. Void data type
4. Char data type

Integer Data Type

An integer is an integral whole number without a decimal point. These numbers are used for counting. For example 26, 272, -342 are valid integers. Normally an integer can hold numbers from -32768 to 32767.

Floating Data Type

A floating point number has a decimal point. Even if it has an integral value, it must include a decimal point at the end. Valid floating point examples are 45.0, 34.23, -234.34.

Void Data Type

• It is used for following purposes :
• It specifies the return type of a function when the function is not returning any value.
• It indicates an empty parameter list on a function when no arguments are passed.
• A void pointer can be assigned a pointer value of any basic data type.

Char Data Type

It is used to store character value in the identifier (variable/operand).

Derived Data Type

C++ also permits four types of derived data types. As the name suggests, derived data types are basically derived from built-in data types.

There are four derived derived data types.

1. Array
2. Function
3. Pointers
4. Reference

We will study these data types in further chapters.

User Define Data Type

C++ has four types of user-defined data types.

As the name suggests, user-defined data types are defined by the programmers(you) during the development of software.

These four user-defined data types are :

1. Structure
2. Union
3. Class
4. Enumerator

We will study these data types in further chapters.

Type Conversion

In C++ object-oriented language, a smaller memory data type variables can be converted to a larger data type by the compiler.

Type conversion can be done by two ways:

1. Automatic Type Conversion

When an expression consists of more than one type of data element, the C++ compiler converts the smaller data type elements into larger data type elements. This process is known as implicit or automatic conversion.

2. Typecasting

Below statement allows the programmer to convert one data type into another data type.

Syntax:

 aCharVar = static_cast<char>(an IntVar)

Here in the above syntax char variable will be converted into int variable after execution.

The C++ language has four typecast operators:

1. static_cast
2. reinterpret_cast
3. const_cast
4. dynamic_cast

Literals or Components

• An identifier that does not change its value during the execution of a program is known as a constant or a literal.
• Any attempt to change the value of a constant will result in an error message.
• A keyword const is added to the declaration of an identifier to make that identifier constant.

Syntax :

  const datatype variable = value;

Example :

  const float PI = 3.1415;
  const char ch = 'A';
  const int rate = 40;

Keywords

Below is the list of reserved keywords in C++. Since they are used by the language, these keywords are not available for re-definition or overloading.

Keywords	Description
and	Alternative to && operator
and_eq	Alternative to &= operator
asm	Insert an assembly instruction
auto	Declare a local variable, or we can let the compiler to deduce the type of the variable from the initialization
bitand	Alternative to bitwise & operator
bitor	Alternative to | operator
bool	Declare a boolean variable
break	Break out of a loop
case	A block of code in a switch statement
catch	Handles exceptions from throw
char	Declare a character variabl
class	Declare a class
compl	Alternative to ~ operator
const	Declare immutable data or functions that do not change data
const_cast	Cast from const variables
continue	Bypass iterations of a loop
default	Default handler in a case statement
#define	All header files should have #define guards to prevent multiple inclusion.
delete	Make dynamic memory available
do	Looping construct
double	Declare a double-precision floating-point variable
dynamic_cast	Perform runtime casts
else	Alternate case for an if statement
enum	Create enumeration types
exit()	Ending a process
explicit	Only use constructors when they exactly match
export	Allows template definitions to be separated from their declarations
extern	Declares a variable or function and specifies that it has external linkage
extern	"C" enables C function call from C++ by forcing C-linkage
false	A constant representing the boolean false value
float	Declare a floating-point variable
for	Looping construct
friend	Grant non-member function access to private data
goto	Jump to a different part of the program
if	Execute code based on the result of a test
inline	Optimize calls to short functions
int	Declare an integer variable
long	Declare a long integer variable
mutable	Override a const variable
namespace	Partition the global namespace by defining a scope
new	Allocate dynamic memory for a new variable
not	Alternative to ! operator
not_eq	Alternative to != operator
operator	Create overloaded operator function
or	Alternative to || operator
or_eq	Alternative to |= operator
private	Declare private members of a class
protected	Declare protected members of a class
public	Declare public members of a class
register	Request that a variable be optimized for speed
reinterpret_cast	Change the type of a variable
short	Declare a short integer variable
signed	Modify variable type declarations
sizeof	Return the size of a variable or type
static	Create permanent storage for a variable
static_cast	Perform a non-polymorphic cast
struct	Define a new structure
switch	Execute code based on different possible values for a variable
template	Create generic functions
this	A pointer to the current object
throw	Throws an exception
true	A constant representing the boolean true value
try	Execute code that can throw an exception
typedef	Create a new type name from an existing type
typeid	Describes an object
typename	Declare a class or undefined type
union	A structure that assigns multiple variables to the same memory location
unsigned	Declare an unsigned integer variable
using	Import complete or partial namespaces into the current scope
virtual	Create a function that can be overridden by a derived class
void	Declare functions or data with no associated data type
volatile	Warn the compiler about variables that can be modified unexpectedly
void	Declare functions or data with no associated data type
wchar_t	Declare a wide-character variable
while	Looping construct:
xor	Alternative to ^ operator
xor_eq	Alternative to ^= operator

Variables declaration in C++

Syntax :

 datatype variable_name;

Example :

    int a;
    float f;
    // bool is the new datatype, it can stores true or false value.
    bool a;
    char c;

There is a important difference between C and C++ regarding when local variable can be declared. In c, you must declare all local variables used within a block at the start of that block. You cannot declare a variable in a block after an "action" statement has occurred.

Example :

// Incorrect in C. Ok in C++
int function()
{
    int num1;
    num1 = 10;
    /*
     won't compile as a C program
     but compiling as C++ program its OK
    */
    int num2;
    num2 = num1 * 5;
    return num2;
}