Constants

A constant is a variable whose value cannot change once it is set. Why would you want that? Because some values should never change — the value of PI, the number of months in a year, the tax rate. Using constants makes your code safer and easier to read. If someone tries to change a constant later, the compiler will stop them.

The const Keyword

The most common way to create a constant in C++ is with the const keyword. Just put const before the type when you declare the variable. You must give it a value right away because you cannot change it later.

The #define Directive

There is also an older way using #define. It is a preprocessor directive that replaces every occurrence of a name with a value before compilation. It does not have a type and does not follow the same rules as const. These days, const is the preferred approach, but you will still see #define in older code.

#include <iostream>
using namespace std;

const double PI = 3.14159;
#define DAYS_IN_WEEK 7

int main() {
  double radius = 5.0;
  double area = PI * radius * radius;

  cout << "Radius: " << radius << endl;
  cout << "Area: " << area << endl;
  cout << "Days in a week: " << DAYS_IN_WEEK;

  // PI = 3.0; // This would cause an error!
  return 0;
}

Notice there is no semicolon after #define — it is a preprocessor directive, not a statement. And unlike const, #define does not create an actual variable. It is more like a find-and-replace that happens before your code is compiled.