C++ constexpr

Constexpr is a keyword that declares that the value of a variable or a function is declared at compile time. Similar to the const keyword a compiler error is raised when any code attempts to modify the value of a constexpr. While the value of a const variable can be initialised at run time time the constexpr needs to be initialised at compile time. Const expression is especially useful for template arguments and array declarations.

#include<iostream>

int main()
{
    constexpr int a = 10;
    int b = 0;
    constexpr int c = b + 1; // error as b is not a constexper
                             // or a const
    constexpr int d = a + 1; // no error as a is constexpr
}

A constexpr function is a function whose return value is computed at compile if the code code requires it. It can also function a normal function, that is evaluated at runtime, if the code does not require it.

#include<iostream>

constexpr int factorial(int n)
{
    if (n == 0)
        return 1;
    else
        return n * factorial(n - 1);
}

int main()
{
    // evaluated at compile time
    char arr[factorial(4)]; 

    // evaluated at run time
    std::cout<< "Factorial(10) " << factorial(10) <<std::endl; 
}