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This PR fixes #4424. It was unfortunately not possible to just implement this with template <typename T> T& format_as(std::reference_wrapper<T>); in the general case so I've just guarded the formatter specialization for format_as.

Happy to adjust the changes here if needed!

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vitaut commented May 3, 2025

It was unfortunately not possible to just implement this with template T& format_as(std::reference_wrapper);`

Could you elaborate?

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When I attempted the approach of

template <typename T>
T& format_as(std::reference_wrapper<T> ref) { return ref.get(); }

I got the following error for the test line EXPECT_EQ("35", fmt::format("{}", std::ref(num)));

/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/base.h: In instantiation of ‘constexpr fmt::v11::detail::value<Context>::value(const T&, fmt::v11::detail::custom_tag) [with T = std::reference_wrapper<int>; typename std::enable_if<(! has_formatter<T, typename Context::char_type>()), int>::type <anonymous> = 0; Context = fmt::v11::context]’:
/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/base.h:2235:65:   required from ‘fmt::v11::detail::value<Context>::value(T&) [with T = std::reference_wrapper<int>; typename std::enable_if<(std::integral_constant<bool, (((((((std::is_class<T>::value || std::is_enum<T>::value) || std::is_union<T>::value) || std::is_array<T>::value) && (! fmt::v11::detail::has_to_string_view<T, void>::value)) && (! fmt::v11::detail::is_named_arg<T>::value)) && (! fmt::v11::detail::use_format_as<T>::value)) && (! fmt::v11::detail::use_format_as_member<typename std::remove_const<T>::type, std::integral_constant<bool, true> >::value))>::value || (!1)), int>::type <anonymous> = 0; Context = fmt::v11::context]’
/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/format.h:4200:17:   required from ‘std::string fmt::v11::format(format_string<T ...>, T&& ...) [with T = {std::reference_wrapper<int>}; std::string = std::__cxx11::basic_string<char>; format_string<T ...> = fstring<std::reference_wrapper<int> >]’
/mnt/c/Users/rifkin/home/projects/open-source/fmt/test/std-test.cc:419:3:   required from here
/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/base.h:2262:45: error: ‘fmt::v11::detail::type_is_unformattable_for<std::reference_wrapper<int>, char> _’ has incomplete type
 2262 |     type_is_unformattable_for<T, char_type> _;
      |                                             ^

And similar for fmt::to_string.

Adding the line fmt::detail::format_as_result<std::reference_wrapper<int>> x; to debug

/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/base.h: In substitution of ‘template<class T> using fmt::v11::detail::format_as_result = fmt::v11::remove_cvref_t<decltype (format_as(declval<const T&>()))> [with T = std::reference_wrapper<int>]’:
/mnt/c/Users/rifkin/home/projects/open-source/fmt/test/std-test.cc:421:61:   required from here
/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/base.h:1115:38: error: ‘format_as’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
 1115 |     remove_cvref_t<decltype(format_as(std::declval<const T&>()))>;
      |                             ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/c/Users/rifkin/home/projects/open-source/fmt/include/fmt/std.h:750:4: note: ‘template<class T> T& format_as(std::reference_wrapper<_Tp>)’ declared here, later in the translation unit
  750 | T& format_as(std::reference_wrapper<T> ref) { return ref.get(); }
      |    ^~~~~~~~~

I figured it'd be easiest to just update the formatter to check for existing format_as / format_as_member ability

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I suggest using format_as but make it a member (since we can't inject it into the std namespace`).

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Adding the following to the formatter causes the most bizarre thing to happen

  static auto format_as(std::reference_wrapper<T> ref) -> T& {
    return ref.get();
  }

Suddenly the to_string here evaluates to "#"

  int num = 35;
  fmt::to_string(std::cref(num));

This seems to be because the chosen to_string is

template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value &&
                                    !detail::use_format_as<T>::value)>
FMT_NODISCARD auto to_string(const T& value) -> std::string {
  auto buffer = memory_buffer();
  detail::write<char>(appender(buffer), value);
  return {buffer.data(), buffer.size()};
}

which writes the int value as a char. Presumably this is because there is no to_string using member format_as, yet adding such an overload causes more issues.

Since any use of member format_as here will depend on the additional constraints for the reference_wrapper formatter specialization this PR adds and there appear to be more involved changes required to get member format_as working, could this PR be merged as-is with the other changes deferred to another PR?

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vitaut commented May 10, 2025

could this PR be merged as-is with the other changes deferred to another PR?

OK but please move has_format_as* to detail.

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Thanks, taken care of

@vitaut vitaut merged commit 4999416 into fmtlib:master May 10, 2025
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vitaut commented May 10, 2025

Thank you!

mtremer pushed a commit to ipfire/ipfire-2.x that referenced this pull request Nov 6, 2025
- Update from version 11.2.0 to 12.1.0
- Update of rootfile
- so-bump so mpd requires shipping
- Changelog
    12.1.0
	- Optimized `buffer::append`, resulting in up to ~16% improvement on spdlog
	  benchmarks (fmtlib/fmt#4541). Thanks @fyrsta7.
	- Worked around an ABI incompatibility in `std::locale_ref` between clang and
	  gcc (fmtlib/fmt#4573).
	- Made `std::variant` and `std::expected` formatters work with `format_as`
	  (fmtlib/fmt#4574,
	  fmtlib/fmt#4575). Thanks @phprus.
	- Made `fmt::join<string_view>` work with C++ modules
	  (fmtlib/fmt#4379,
	  fmtlib/fmt#4577). Thanks @Arghnews.
	- Exported `fmt::is_compiled_string` and `operator""_cf` from the module
	  (fmtlib/fmt#4544). Thanks @CrackedMatter.
	- Fixed a compatibility issue with C++ modules in clang
	  (fmtlib/fmt#4548). Thanks @tsarn.
	- Added support for cv-qualified types to the `std::optional` formatter
	  (fmtlib/fmt#4561,
	  fmtlib/fmt#4562). Thanks @OleksandrKvl.
	- Added demangling support (used in exception and `std::type_info` formatters)
	  for libc++ and clang-cl
	  (fmtlib/fmt#4542,
	  fmtlib/fmt#4560,
	  fmtlib/fmt#4568,
	  fmtlib/fmt#4571).
	  Thanks @FatihBAKIR and @rohitsutreja.
	- Switched to global `malloc`/`free` to enable allocator customization
	  (fmtlib/fmt#4569,
	  fmtlib/fmt#4570). Thanks @rohitsutreja.
	- Made the `FMT_USE_CONSTEVAL` macro configurable by users
	  (fmtlib/fmt#4546). Thanks @SnapperTT.
	- Fixed compilation with locales disabled in the header-only mode
	  (fmtlib/fmt#4550).
	- Fixed compilation with clang 21 and `-std=c++20`
	  (fmtlib/fmt#4552).
	- Fixed a dynamic linking issue with clang-cl
	  (fmtlib/fmt#4576,
	  fmtlib/fmt#4584). Thanks @FatihBAKIR.
	- Fixed a warning suppression leakage on gcc
	  (fmtlib/fmt#4588). Thanks @ZedThree.
	- Made more internal color APIs `constexpr`
	  (fmtlib/fmt#4581). Thanks @ishani.
	- Fixed compatibility with clang as a host compiler for NVCC
	  (fmtlib/fmt#4564). Thanks @valgur.
	- Fixed various warnings and lint issues
	  (fmtlib/fmt#4565,
	  fmtlib/fmt#4572,
	  fmtlib/fmt#4557).
	  Thanks @LiangHuDream and @teruyamato0731.
	- Improved documentation
	  (fmtlib/fmt#4549,
	  fmtlib/fmt#4551,
	  fmtlib/fmt#4566,
	  fmtlib/fmt#4567,
	  fmtlib/fmt#4578,).
	  Thanks @teruyamato0731, @petersteneteg and @zimmerman-dev.
    12.0.0
	- Optimized the default floating point formatting
	  (fmtlib/fmt#3675,
	  fmtlib/fmt#4516). In particular, formatting a
	  `double` with format string compilation into a stack allocated buffer is
	  more than 60% faster in version 12.0 compared to 11.2 according to
	  [dtoa-benchmark](https://github.com/fmtlib/dtoa-benchmark):
	  ```
	  Function  Time (ns)  Speedup
	  fmt11        34.471    1.00x
	  fmt12        21.000    1.64x
	  ```
	  <img width="766" height="609" src="https://github.com/user-attachments/assets/d7d768ad-7543-468c-b0bb-449abf73b31b" />
	- Added `constexpr` support to `fmt::format`. For example:
	  ```c++
	  #include <fmt/compile.h>
	  using namespace fmt::literals;
	  std::string s = fmt::format(""_cf, 42);
	  ```
	  now works at compile time provided that `std::string` supports `constexpr`
	  (fmtlib/fmt#3403,
	  fmtlib/fmt#4456). Thanks @msvetkin.
	- Added `FMT_STATIC_FORMAT` that allows formatting into a string of the exact
	  required size at compile time.
	  For example:
	  ```c++
	  #include <fmt/compile.h>
	  constexpr auto s = FMT_STATIC_FORMAT("{}", 42);
	  ```
	  compiles to just
	  ```s
	  __ZL1s:
	        .asciiz "42"
	  ```
	  It can be accessed as a C string with `s.c_str()` or as a string view with
	  `s.str()`.
	- Improved C++20 module support
	  (fmtlib/fmt#4451,
	  fmtlib/fmt#4459,
	  fmtlib/fmt#4476,
	  fmtlib/fmt#4488,
	  fmtlib/fmt#4491,
	  fmtlib/fmt#4495).
	  Thanks @arBmind, @tkhyn, @Mishura4, @anonymouspc and @autoantwort.
	- Switched to using estimated display width in precision. For example:
	  ```c++
	  fmt::print("|{:.4}|\n|1234|\n", "🐱🐱🐱");
	  ```
	  prints
	  ![](https://github.com/user-attachments/assets/6c4446b3-13eb-43b9-b74a-b4543540ad6a)
	  because `🐱` has an estimated width of 2
	  (fmtlib/fmt#4272,
	  fmtlib/fmt#4443,
	  fmtlib/fmt#4475).
	  Thanks @nikhilreddydev and @localspook.
	- Fix interaction between debug presentation, precision, and width for strings
	  (fmtlib/fmt#4478). Thanks @localspook.
	- Implemented allocator propagation on `basic_memory_buffer` move
	  (fmtlib/fmt#4487,
	  fmtlib/fmt#4490). Thanks @toprakmurat.
	- Fixed an ambiguity between `std::reference_wrapper<T>` and `format_as`
	  formatters (fmtlib/fmt#4424,
	  fmtlib/fmt#4434). Thanks @jeremy-rifkin.
	- Removed the following deprecated APIs:
	  - `has_formatter`: use `is_formattable` instead,
	  - `basic_format_args::parse_context_type`,
	    `basic_format_args::formatter_type` and similar aliases in context types,
	  - wide stream overload of `fmt::printf`,
	  - wide stream overloads of `fmt::print` that take text styles,
	  - `is_*char` traits,
	  - `fmt::localtime`.
	- Deprecated wide overloads of `fmt::fprintf` and `fmt::sprintf`.
	- Improved diagnostics for the incorrect usage of `fmt::ptr`
	  (fmtlib/fmt#4453). Thanks @TobiSchluter.
	- Made handling of ANSI escape sequences more efficient
	  (fmtlib/fmt#4511,
	  fmtlib/fmt#4528).
	  Thanks @localspook and @Anas-Hamdane.
	- Fixed a buffer overflow on all emphasis flags set
	  (fmtlib/fmt#4498). Thanks @dominicpoeschko.
	- Fixed an integer overflow for precision close to the max `int` value.
	- Fixed compatibility with WASI (fmtlib/fmt#4496,
	  fmtlib/fmt#4497). Thanks @whitequark.
	- Fixed `back_insert_iterator` detection, preventing a fallback on slower path
	  that handles arbitrary iterators (fmtlib/fmt#4454).
	- Fixed handling of invalid glibc `FILE` buffers
	  (fmtlib/fmt#4469).
	- Added `wchar_t` support to the `std::byte` formatter
	  (fmtlib/fmt#4479,
	  fmtlib/fmt#4480). Thanks @phprus.
	- Changed component prefix from `fmt-` to `fmt_` for compatibility with
	  NSIS/CPack on Windows, e.g. `fmt-doc` changed to `fmt_doc`
	  (fmtlib/fmt#4441,
	  fmtlib/fmt#4442). Thanks @n-stein.
	- Added the `FMT_CUSTOM_ASSERT_FAIL` macro to simplify providing a custom
	  `fmt::assert_fail` implementation (fmtlib/fmt#4505).
	  Thanks @HazardyKnusperkeks.
	- Switched to `FMT_THROW` on reporting format errors so that it can be
	  overriden by users when exceptions are disabled
	  (fmtlib/fmt#4521). Thanks @HazardyKnusperkeks.
	- Improved master project detection and disabled install targets when using
	  {fmt} as a subproject by default (fmtlib/fmt#4536).
	  Thanks @crueter.
	- Made various code improvements
	  (fmtlib/fmt#4445,
	  fmtlib/fmt#4448,
	  fmtlib/fmt#4473,
	  fmtlib/fmt#4522).
	  Thanks @localspook, @tchaikov and @way4sahil.
	- Added Conan instructions to the docs
	  (fmtlib/fmt#4537). Thanks @uilianries.
	- Removed Bazel files to avoid issues with downstream packaging
	  (fmtlib/fmt#4530). Thanks @mering.
	- Added more entries for generated files to `.gitignore`
	  (fmtlib/fmt#4355,
	  fmtlib/fmt#4512).
	  Thanks @dinomight and @localspook.
	- Fixed various warnings and compilation issues
	  (fmtlib/fmt#4447,
	  fmtlib/fmt#4470,
	  fmtlib/fmt#4474,
	  fmtlib/fmt#4477,
	  fmtlib/fmt#4471,
	  fmtlib/fmt#4483,
	  fmtlib/fmt#4515,
	  fmtlib/fmt#4533,
	  fmtlib/fmt#4534).
	  Thanks @dodomorandi, @localspook, @remyjette, @Tomek-Stolarczyk, @Mishura4,
	  @mattiasljungstrom and @FatihBAKIR.

Signed-off-by: Adolf Belka <[email protected]>
Signed-off-by: Michael Tremer <[email protected]>
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Formatter for std::reference_wrapper<T> results in an ambiguous template when T can be used with format_as

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