What is the difference between const and constexpr?

Question

Accepted Answer

**const** — runtime constant. **constexpr** — compile-time constant.

```cpp
// const — runtime
const int x = 10;
int arr[x];  // Error in C++ (VLA not standard)

// constexpr — compile-time
constexpr int y = 10;
int arr2[y];  // OK!

// constexpr function
constexpr int square(int n) {
    return n * n;
}

int arr3[square(5)];  // arr3[25]

// const with functions
class MyClass {
public:
    int getValue() const {  // const method
        return value;
    }

private:
    int value;
};
```

**Key differences:**
- **const** — runtime, can be initialized at runtime.
- **constexpr** — compile-time, must be constant expression.
- **const** — for variables and methods.
- **constexpr** — for variables, functions, constructors.

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