C++ const & constexpr

const & constexpr

• const: meaning roughly ‘‘I promise not to change this value’’.
• constexpr: meaning roughly ‘‘to be evaluated at compile time’’.

example

    const int dmv = 17; // dmv is a named constant
    int var = 17; // var is not a constant
    constexpr double max1 = 1.4 * square(dmv); // OK if square(17) is a constant expression
    //constexpr double max2 = 1.4 * square(var); // error : var is not a constant expression
    const double max3 = 1.4 * square(var); // OK, may be evaluated at run time