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Fix reference_wrapper ambiguity with format_as #4434
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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 /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 Adding the line /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 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 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? |
OK but please move |
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Thanks, taken care of |
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Thank you! |
- 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

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]>
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!