what's the difference between const and constexpr in C++?

Both const and constexpr are used to define constants in C++, but they have different meanings and use cases. const is used to declare a variable as constant, meaning its value cannot be changed after initialization. const can be used with any data type, including user-defined types, and can be initialized at runtime or compile-time. For example:

const int x = 10;      // x is a constant integer with value 10
const MyClass obj;     // obj is a constant object of MyClass

constexpr is used to define a constant expression that can be evaluated at compile-time. constexpr can only be used with literal types, which include arithmetic types, pointers, and certain user-defined types with constexpr constructors. For example:

constexpr int y = 20;  // y is a constant integer with value 20, evaluated at compile-time
constexpr MyClass obj2; // obj2 is a constant object of MyClass with a constexpr constructor

The main difference between const and constexpr is that const is evaluated at runtime, while constexpr is evaluated at compile-time. This means that constexpr can be used in situations where a constant value is required at compile-time, such as template parameters, array sizes, and switch statements. In contrast, const is more flexible and can be used in situations where a variable needs to be declared as constant, but its value is not known until runtime.