C++ Constants

The const Keyword

Use const to declare a variable whose value cannot be changed after initialization. Constants make code safer and more readable.

#include <iostream>
using namespace std;

int main() {
    const double PI = 3.14159265;
    const int MAX_SIZE = 100;
    const string GREETING = "Hello";

    cout << "PI = " << PI << endl;
    cout << "Max Size = " << MAX_SIZE << endl;
    cout << GREETING << endl;

    // PI = 3.0; // ERROR: cannot assign to const variable
    return 0;
}

Output:

PI = 3.14159265
Max Size = 100
Hello

#define Preprocessor Macro

Before C++11, #define was the common way to create constants. It is a preprocessor directive — the compiler replaces every use of the name with the value before compiling.

#include <iostream>
#define MAX_SCORE 100
#define GAME_NAME "Space Shooter"
using namespace std;

int main() {
    cout << "Game: " << GAME_NAME << endl;
    cout << "Max Score: " << MAX_SCORE << endl;
    return 0;
}

constexpr (C++11)

constexpr means the value is computed at compile time, making programs faster:

constexpr int SQUARE(int x) { return x * x; }

int main() {
    constexpr int result = SQUARE(5);   // Computed at compile time
    cout << result;   // Output: 25
}

#include <iostream>
using namespace std;

int main() {
    const int DAYS_PER_WEEK = 7;
    const double GRAVITY = 9.8;
    const string APP_NAME = "MyApp";

    cout << "Days/Week: " << DAYS_PER_WEEK << endl;
    cout << "Gravity: " << GRAVITY << endl;
    cout << "App: " << APP_NAME << endl;
    return 0;
}

Summary

• const declares a variable that cannot be modified after initialization.
• #define creates preprocessor text-replacement macros (legacy approach).
• constexpr evaluates the value at compile time for performance.
• Prefer const or constexpr over #define in modern C++.