Enable constexpr support for fmt::format (fmtlib#3403) #4456
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vitaut
requested changes
May 27, 2025
include/fmt/base.h
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| #endif | ||
|
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||
| // Detect constexpr std::string. | ||
| #if !FMT_USE_CONSTEVAL |
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Why does it depend on consteval?
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FMT_USE_CONSTEVAL enables FMT_CONSTEXPR20 which is required.
I could add comment or introduce something FMT_USE_CONSTEXPR20
test/compile-test.cc
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| constexpr auto result = []() { | ||
| return fmt::format(FMT_COMPILE("{}"), true) == "true"; | ||
| }(); |
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Yes, unfortunately we do need it. The issue is that all allocated memory at compile time should be deallocated.
That means we are not able to write like this:
constexpr auto s = fmt::format(FMT_COMPILE("{}"), true)); // leaks allocated memory at compile-time to runtime.
EXPECT_EQ("true", s);
Alternatively we could use static_assert:
static_assert(fmt::format(FMT_COMPILE("{}"), true) == "true");
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Let's keep the lambda for now but I don't think we need to test different arguments since this is already tested elsewhere. What we need to test here is that fmt::format can be called at compile time so I suggest picking on type of an argument (e.g. int) and testing it.
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I have replaced boolean test cases with int and drop test case for a type with user provided formatter.
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Merged, thanks! |
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# 12.0.0 - 2025-09-17 - 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 - 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 - Fix interaction between debug presentation, precision, and width for strings - Implemented allocator propagation on `basic_memory_buffer` move - Fixed an ambiguity between `std::reference_wrapper<T>` and `format_as` formatters - 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` - Made handling of ANSI escape sequences more efficient - Fixed a buffer overflow on all emphasis flags set - Fixed an integer overflow for precision close to the max `int` value. - Fixed compatibility with WASI (fmtlib/fmt#4496, - Fixed `back_insert_iterator` detection, preventing a fallback on slower path that handles arbitrary iterators - Fixed handling of invalid glibc `FILE` buffers - Added `wchar_t` support to the `std::byte` formatter - Changed component prefix from `fmt-` to `fmt_` for compatibility with NSIS/CPack on Windows, e.g. `fmt-doc` changed to `fmt_doc` - Added the `FMT_CUSTOM_ASSERT_FAIL` macro to simplify providing a custom `fmt::assert_fail` implementation - Switched to `FMT_THROW` on reporting format errors so that it can be overriden by users when exceptions are disabled - Improved master project detection and disabled install targets when using {fmt} as a subproject by default - Made various code improvements - Added Conan instructions to the docs - Removed Bazel files to avoid issues with downstream packaging - Added more entries for generated files to `.gitignore` - Fixed various warnings and compilation issues
mtremer
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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
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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This PR allows to use fmt::format at compile-time by: