constexpr function params for not known at compile time booleans C++

Question

I need to run a function with N boolean variables, I want to make them constexpr in order to exterminate comparisons and save the code from branch prediction failure.

What I mean is:

templateFunc<b1, b2, b3, b4 ...>(args...);

as the b1..bn variables are just boolean variables and may have only 2 states, I could write something like this:

if (b1 && b2)
  templateFunc<true, true>(args...);
else if (b1 && !b2)
  templateFunc<true, false>(args...);
else if (!b1 && b2)
  templateFunc<false, true>(args...);
else
  templateFunc<false, false>(args...);

The problem is obvious, I'd need 64 calls for 5 variables.. Any solution?

Answer 1

With std::variant (C++17), you might do the dynamic dispatch via std::visit:

// helper
std::variant<std::false_type, std::true_type> to_boolean_type(bool b)
{
    if (b) return std::true_type{};
    return std::false_type{};
}

and then

std::visit([&](auto... bs){templateFunc<bs...>(args...);},
           to_boolean_type(b1), to_boolean_type(b2));

Demo