C =++ supports two notations of immutability:

1. const - which implies that the value will not change. First of all, this is used to specify interfaces, for data that is passed to functions and methods so that they are not afraid that they will be changed. The compiler monitors the presence of the const qualifier;
2. constexp - which involves calculating the constant at compile time. Used to store data in memory where they will not be damaged, and to improve performance.

For example:

const int dmv = 17;                      // Constant with the name dmv
int var = 17;                            // Var is not a constant
constexpr double max1 = 1.4∗square(dmv); // OK, since square (17) is a constant expression
constexpr double max2 = 1.4∗square(var); // Error, because var is not a constant
const double max3 = 1.4∗square(var);     // OK, because the expression can be evaluated in runtime
double sum(const vector<double>&);       // sum will not modify its arguments
vector<double> v {1.2, 3.4, 4.5};        // v is not a constant
const double s1 = sum(v);                // OK, it will be calculated in runtime
constexpr double s2 = sum(v);            // Error, sum (v) is not a constant expression.