Fix formatting of types with custom addressof operator

[PeterFeicht]

I have a type with a custom operator& that I want to format. That currently doesn't work, because fmt uses operator& to get a void* to a formatted object.
This changes fmt::detail::value to use std::addressof instead, so types with an overloaded operator& will work.

[vitaut]

Thanks for the PR.

[vitaut]

fmt/core.h is designed to have minimal includes and <memory> is not free (https://artificial-mind.net/projects/compile-health/). Can we avoid the include by using a cast (see e.g. https://en.cppreference.com/w/cpp/memory/addressof)?

[PeterFeicht]

Sure, I'll add a function to the detail namespace. It won't be constexpr though, but since it's not used for parsing I guess that won't matter.

[phprus]

The function that calls addressof is constexpr for C++14+.

[PeterFeicht]

Dammit, didn't notice that. I'm pretty sure that constexpr addressof requires compiler support...
In Boost they detect the compiler and use a builtin function, would that be an option here too?

[phprus]

But std::addressof is constexpr only in C++17...

[thomthom]

@PeterFeicht did you find a way to format user types with custom operator&? (I also have a wrapper type, running into the same scenario.)

[PeterFeicht]

Oh, I completely forgot about that. We didn't yet have fmt v9 at that point so there was no format_as. I'll do a PR when I have some time, then format_as should work just fine for that purpose.

[thomthom]

Thank you Peter. Can you tag me in the PR if you remember?

Btw, when I look at the latest source it appear that this conditional change to be is_enum or is_class:

fmt/include/fmt/core.h

Line 1322 in 552c43a

FMT_ENABLE_IF(std::is_enum<U>::value || std::is_class<U>::value)>

Compared to 9.10 that was is_enum and is_integral.

fmt/include/fmt/core.h

Line 1335 in a337011

FMT_ENABLE_IF(std::is_enum<U>::value&& std::is_integral<V>::value)>

I updated to 10.0 yesterday, that includes that change. But that wasn't enough to allow a wrapper class with custom address-or. From this thread it appear that std::addressof cannot be used due to compatibility?

(I'm curious to what the fix would be.)

[PeterFeicht]

Oh nice, didn't notice that. Now it should be enough to provide an overload of format_as that maps to a wrapper type for formatting (which you would then provide a formatter specialization for).

[thomthom]

Having another look at this. Does this mean there is no need for any changes to fmt, but that one can already implement formatting for wrapper types? I tried to add format_as that return a string (for my string wrapping type) and then I tried to add a fmt::formatter specialisation. But I got is not an entity that can be explicitly specialized. (I'm not sure I fully understand how to correctly implement a specialisation.)