if constexpr: Metaprogramming Made Easy

Problem

Documentation

• constexpr if

• is_even() evaluation in constexpr context is only guaranteed when assigning to a constexpr variable

• How to use in if?

code/constexpr-if-problem.cpp

#include <iostream>

constexpr bool is_even(unsigned num)
{
    return num % 2 == 0;
}

int main()
{
    constexpr bool even = is_even(6);                  // <-- workaround
    if (even)                                          // <-- intelligent compiler not guaranteed
        std::cout << "even\n";
    else
        std::cout << "odd\n";
    return 0;
}

• This is clumsy

• Still no guarantee because compilers are not required to treat constant conditions specially

Solution

code/constexpr-if-solution.cpp

#include <iostream>

constexpr bool is_even(unsigned num)
{
    return num % 2 == 0;
}

int main()
{
    if constexpr (is_even(6))                          // <-- better
        std::cout << "even\n";
    else
        std::cout << "odd\n";
    return 0;
}