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

Hello! I recently started learning C++11 and came across the `constexpr` keyword. I understand that `const` means the value of a variable cannot be changed after initialization. But `constexpr` also seems to make a variable constant. What's the difference then? For example, I can write: ```cpp const int x = 5; constexpr int y = 5; ``` Both work. But I've heard that `constexpr` can be used with functions. I tried: ```cpp constexpr int square(int n) { return n * n; } int main() { const int a = square(5); // Works? constexpr int b = square(5); // Also works? int arr[ square(3) ]; // Is this possible? } ``` This compiles, but I don't understand the real advantage. When should I use `constexpr` instead of `const`? And p