Using enum class values in constexpr branches

Question

the code below prints val2 on both f() calls. What would be a proper way to execute specific branch in f() based on enum value ?

enum class E
{
    val1,
    val2
};

using val1_t = std::integral_constant<E, E::val1>;
using val2_t = std::integral_constant<E, E::val2>;
    

template <typename T>
void f(T t)
{
    if constexpr (std::is_same_v<T, val1_t>)
    {
        std::cerr << "val1\n";
    }
    else
    {
        std::cerr << "val2\n";
    }
}


int main()
{
    f(E::val1);
    f(E::val2);
}

std::is_same_v is comparing types, not values. E::val1 and E::val2 have the same type (E), which isn't the same type as std::integral_constant<...> — Kevin
– Kevin, Commented Sep 13, 2021 at 18:09

NathanOliver · Accepted Answer · 2021-09-13 18:07:38Z

10

If you move the enum into the template parameter, then you could use

template <E val>
void f()
{
    if constexpr (val == E::val1)
    {
        std::cerr << "val1\n";
    }
    else
    {
        std::cerr << "val2\n";
    }
}

And you would use it like

int main()
{
    f<E::val1>();
    f<E::val2>();
}

answered Sep 13, 2021 at 18:07

NathanOliver

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7 Comments

psb Over a year ago

Thank you. So there is no way to somehow infer std::integral_constant-generated-type from passed enum value ? Maybe not with std::is_same_v, but somehow else ? I would still like to have something like f(E::val1) at call site.

NathanOliver Over a year ago

@psb You could do something like this

NathanOliver Over a year ago

@psb The issue with function parameters is they are never compile time constants. That means you can't use them where a compile time constant is required.

NathanOliver Over a year ago

@psb That first example was a little to complicated. Here is a reduced version: coliru.stacked-crooked.com/a/c6ba1f71a2d9f76f

psb Over a year ago

Yeah, I more or less understand that, but I've been thinking that since all enum values are known at compile time, there may be a way to deduce a type, based on enum value, from enum value at compile time.

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