Difference Between C++ Define Directive and Const Keyword

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

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.