Constants

Sometimes, we want a value that never changes while the program is running. That’s what constants are for!

Constants help prevent mistakes (like changing a number by accident) and make the code easier to understand.

Example: Regular variable vs constant

#include <iostream>
using namespace std;

int main() {
    int age = 15;
    age = 16; // we can change it

    const double PI = 3.14159;
    // PI = 3.14;  Error! cannot change a constant

    cout << "PI is " << PI << endl;
    return 0;
}

Output:

PI is 3.14159

Why use constants?

  • To protect important values from being changed by accident.
  • To make code more readable — PI is clearer than 3.14159.
  • To update easily — if PI ever changes (it won’t, but you get the point), you just change it in one place.

Declaring constants

You make a constant by adding the keyword const before the type:

const int DAYS_IN_WEEK = 7;
const double GRAVITY = 9.81;
const char GRADE = 'A';

Constants must always be given a value at the moment they are declared.

Literal constants

Literal constants are values that appear directly in the code, like:

int x = 5;         // 5 is an integer literal
double pi = 3.14;  // 3.14 is a double literal
char letter = 'A'; // 'A' is a character literal
string word = "Hi"; // "Hi" is a string literal

These are just raw values written in your code.

Common literal suffixes

You can tell C++ what type a number literal should be using suffixes:

Literal Type Meaning
5 int default integer
5U unsigned int no negative values
5L long long integer
5LL long long very large integer
3.14f float single-precision decimal
3.14 double default for decimals
3.14L long double high precision decimal

Example:

float a = 3.14f;
long long b = 10000000000LL;
long double c = 3.1415926535L;

If you omit the suffix, C++ will assume int for whole numbers and double for decimals.

Constants with #define (old way)

Before const existed, people used #define to create constants:

#define PI 3.14159
#define GREETING "Hello"

It still works, but const is safer and preferred in modern C++.

Summary table

Method Example Can change? When to use
const const int x = 5; No Always for fixed values
#define #define Z 15 No Old style, avoid in modern C++

Quick recap

  • Use const for values that should never change.
  • Use suffixes like LL or f to pick the right numeric type.
  • Avoid #define for constants unless you are reading old code.

With constants, your code becomes safer, clearer, and easier to maintain.