What is the difference between const and constexpr?

Question

Accepted Answer

**const:** Value cannot be modified at runtime
```cpp
const int x = 10;  // x cannot be changed
const int* p = &x; // pointer to const int
int* const q = &y; // const pointer to int
```

**constexpr:** Evaluated at compile time
```cpp
constexpr int factorial(int n) {
    return n <= 1 ? 1 : n * factorial(n - 1);
}

constexpr int x = factorial(5); // computed at compile time
std::array<int, factorial(5)> arr; // size is compile-time constant
```

**Key differences:**
- `const` = runtime immutability
- `constexpr` = compile-time evaluation
- `constexpr` functions must have simple bodies
- `constexpr` variables can be used as template arguments

C++: What is the difference between const and constexpr?