A named constant is an identifier that represents a permanent value.
The value of a variable may change during the execution of a program, but a named constant, or simply constant, represents permanent data that never changes. In our ComputeArea program, π is a constant. If you use it frequently, you don’t want to keep typing 3.14159; instead, you can declare a constant for π. Here is the syntax for declaring a constant:
const datatype CONSTANTNAME = value;
A constant must be declared and initialized in the same statement. The word const is a C++ keyword for declaring a constant. For example, you may declare π as a constant and rewrite Listing 2.2 as shown in Listing 2.4.
Listing 2.4 ComputeAreaWithConstant.cpp
1 #include <iostream>
2 using namespace std;
3
4 int main()
5 {
6 const double PI = 3.14159;
7
8 // Step 1: Read in radius
9 double radius;
10 cout << “Enter a radius: “;
11 cin >> radius;
12
13 // Step 2: Compute area
14 double area = radius * radius * PI;
15
16 // Step 3: Display the area
17 cout << “The area is “;
18 cout << area << endl;
19
20 return 0;
21 }
Source: Liang Y. Daniel (2013), Introduction to programming with C++, Pearson; 3rd edition.