> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/c++-notes/02.-constant.md).

# 02. Constant

## 02. `const`

`const` makes a variable **read-only** after it's initialized. Once set, you can't change it — the compiler will throw an error if you try. Use it for values that should never change.

```cpp
#include <iostream>

int main() {
    const double PI = 3.14159;
    const int LIGHT_SPEED = 299;

    double radius = 42;
    double circumference = 2 * PI * radius;

    std::cout << circumference << " Cm" << std::endl;

    return 0;
}
```

***

### Why bother with `const`?

* **Prevents bugs** — you can't accidentally overwrite a value that should be fixed.
* **Documents intent** — anyone reading the code instantly knows "this never changes."
* **Compiler-enforced** — it's not just a convention, breaking it is a compile error, not a warning.

```cpp
const int MAX_USERS = 100;
MAX_USERS = 200; // ❌ compile error: assignment of read-only variable
```

***

### Naming convention

Constants are often written in `UPPER_SNAKE_CASE`. This is **not required** by the compiler — it's a human convention so constants stand out at a glance.

```cpp
const double GRAVITY = 9.81;   // clearly a constant
double gravity = 9.81;         // works too, but harder to spot
```

***

### More cool examples

#### 1. Circle calculations (area + circumference)

```cpp
#include <iostream>

int main() {
    const double PI = 3.14159;
    double radius = 7;

    double area = PI * radius * radius;
    double circumference = 2 * PI * radius;

    std::cout << "Area: " << area << '\n';
    std::cout << "Circumference: " << circumference << '\n';

    return 0;
}
```

#### 2. A simple discount calculator

```cpp
#include <iostream>

int main() {
    const double DISCOUNT_RATE = 0.20; // 20% off, never changes
    double price = 1500;

    double finalPrice = price - (price * DISCOUNT_RATE);

    std::cout << "Original price: " << price << '\n';
    std::cout << "You save: " << price * DISCOUNT_RATE << '\n';
    std::cout << "Final price: " << finalPrice << '\n';

    return 0;
}
```

#### 3. `const` with characters and strings

```cpp
#include <iostream>

int main() {
    const char GRADE_A = 'A';
    const std::string APP_NAME = "PixelForge";

    std::cout << APP_NAME << " gives you a grade: " << GRADE_A << '\n';

    return 0;
}
```

#### 4. Physics constants (multiple `const`s together)

```cpp
#include <iostream>

int main() {
    const double SPEED_OF_LIGHT = 299792458; // meters per second
    const double GRAVITY = 9.81;             // meters per second^2
    const int ABSOLUTE_ZERO_C = -273;        // Celsius

    std::cout << "Light speed: " << SPEED_OF_LIGHT << " m/s\n";
    std::cout << "Gravity: " << GRAVITY << " m/s^2\n";
    std::cout << "Absolute zero: " << ABSOLUTE_ZERO_C << " C\n";

    return 0;
}
```

#### 5. What happens if you try to break it (the error you'll see)

```cpp
#include <iostream>

int main() {
    const int LIVES = 3;
    LIVES = LIVES - 1; // ❌ error: assignment of read-only variable 'LIVES'

    return 0;
}
```

Compiling this gives something like:

```
error: assignment of read-only variable 'LIVES'
```

That error is `const` doing its job — protecting the value from accidental changes.

***

### Quick Recap

* `const TYPE name = value;` — locks the value at declaration.
* Trying to reassign it is a **compile-time error**, not a runtime bug — you catch it immediately.
* Convention: name constants in `UPPER_SNAKE_CASE`.
* Works with any type — `int`, `double`, `char`, `std::string`, etc.


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