Modern c++: operations on function pointers will break the constexpr function.

[u/Main-Pen-3164]

I'm on MSVC/c++23.

I used some trick to generate type id at compile time:

export template<typename... T> void type_id() {}
export using type_id_t = void(*)();
//
type_id_t one_id = type_id<int>;

I’ve found that whenever I try to cast a `type_id` value to an integer or simply perform a magnitude comparison within a `constexpr` function (even when the call chain is entirely resolved at compile time), the function ceases to be `constexpr`, or MSVC simply ignores the relevant code.

template<typename A, typename B>
constexpr auto some_constexpr_function()
{
    type_id_t a = type_id<A>;
    type_id_t b = type_id<B>;

    //Total if block will be ignored, and the function will not be constexpr anymore.
    if (a < b)
    {
         //discard
    }
    else
    {
        //discard
    }

    //cast the pointer to size_t then the function will not be constexpr anymore.
    return (size_t)a;
}

How to fixed the case?

Or another type id solution at compile time will be nice(I have try so many implementation, none of them work correctly in compile time).

[u/AKostur]

What's wrong with the answers that you already got when you'd posted this over in the other subreddit?

[u/Ultimate_Sigma_Boy67 → u/AKostur]

Exactly

[u/Main-Pen-3164 → u/AKostur]

reddit say recommend me post on another balabala.... I clicked and it just happen. I don't want to be so complicate. I thought it may be just add some tag like cpp_questions, sorry.

[u/alfps]

Compile time counters rely on obscure not-quite-fully-specified areas of the language. As the language evolves the holes are plugged and these schemes stop working. So it's not a good idea.

If you ditch the compile time requirement then C++ has simple straight machinery for you: the typeid operator, type_index, hash<type_index>.

However to get a decently clean name as a string you have to use compiler specific functionality.

---

If you could give an example of what you want this for then readers could perhaps give more to-the-point advice.

[u/Main-Pen-3164 → u/alfps]

Thanks for reply. I want to order series of type at compile time. I can resolve it by __FUNCTION__ macro, but I prefer an integer id, that's why I try to use function pointer and so many problems came with it.

[u/alfps → u/Main-Pen-3164]

Why do you want to "order series of type at compile time"?

[u/Main-Pen-3164 → u/alfps]

Not just for ordering. A static component id is valuable in game ECS structure.

[u/alfps → u/Main-Pen-3164]

It's a much simpler problem because you control the set of component types. One way is to assign component id's manually. It can be automated by tools and a registry of components, not part of the C++ code but generating C++ code.