Difference Between C++ Define Directive and Const Keyword

- April 03, 2018
In C++, the symbolic constant can be declared in the following two ways:
  1. Using #define
  2. Using const keyword

The #define Directive

The "define" is a preprocessor directive that is used to define a constant identifier known as constant macro. A constant macro is an identifier, which is assigned a particular constant value. Like other preprocessor directives, the ‘define' directive is also used at the beginning of the C/C++ program.
The general syntax of ‘define’ directive is as follows:
#define identifier expression
Where
identifier It indicates constant identifier name. This name can't be used again in program (i.e as function name or variable).
expression It indicates constant value for the identifier that may be a string or arithmetic expression.
For example, to assign value 3.1417 to PI, the statement will be as:
#define PI 3.1417

The const Keyword

In C++, the "const" keyword is used to declare constant identifier in a similar way as variable declared except that const keyword is used to specify a constant identifier. At the time of declaration, a value must be assigned to it. Once a value is assigned, then this value can't be changed during the program execution.

For example, to declare a variable PI and to assign value 3.1417 by using const keyword, the statement will be as:
const float PI  = 3.1417

Difference Between Define and Const

The following is the main difference between #define directive and const qualifier.
Main difference between #define & const qualifier
#define const
It's not terminated with semicolon ";" It is terminated with semicolon.
It is used as preprocessor directive. It is used as a statement.
Data type of constant identifier is not specified. Data type of constant identifier is specified.