On Wed, 11 Feb 2026 at 13:56, Ivan Lazaric <[email protected]> wrote:
>
> This patch implements formatting for std::filesystem::path from P2845R8,
> and defines the feature test macro __cpp_lib_format_path to 202403L,
> provided only in <filesystem>.
>
> libstdc++-v3/ChangeLog:
> * include/bits/fs_path.h: Include bits/formatfwd.h.
> (__format::__formatter_fs_path<_CharT>)
> (formatter<filesystem::path, _CharT>): Define.
> * include/bits/version.def: Add format_path.
> * include/bits/version.h: Regenerate.
> * include/std/filesystem: Expose __cpp_lib_format_path.
> * testsuite/std/format/fs_path.cc: New test.
>
> Signed-off-by: Ivan Lazaric <[email protected]>
> Co-authored-by: Jonathan Wakely <[email protected]>
> ---
> libstdc++-v3/include/bits/fs_path.h | 136 +++++++++++++++++++
> libstdc++-v3/include/bits/version.def | 10 ++
> libstdc++-v3/include/bits/version.h | 10 ++
> libstdc++-v3/include/std/filesystem | 1 +
> libstdc++-v3/testsuite/std/format/fs_path.cc | 88 ++++++++++++
> 5 files changed, 245 insertions(+)
> create mode 100644 libstdc++-v3/testsuite/std/format/fs_path.cc
>
> diff --git a/libstdc++-v3/include/bits/fs_path.h
> b/libstdc++-v3/include/bits/fs_path.h
> index 07b74de6cbe..1ca4942235e 100644
> --- a/libstdc++-v3/include/bits/fs_path.h
> +++ b/libstdc++-v3/include/bits/fs_path.h
> @@ -50,6 +50,10 @@
> # include <compare>
> #endif
>
> +#ifdef __cpp_lib_format_path // C++ >= 26 && HOSTED
I should have noticed this earlier, but this should really be testing
__glibcxx_format_path instead.
In this case it doesn't matter, because fs_path.h is only included by
<filesystem> which defines __cpp_lib_format_path. But if we included
fs_path.h elsewhere in the library without the rest of <filesystem>
then __cpp_lib_format_path would not be defined, whereas
__glibcxx_format_path would be.
tl;dr most internal uses of feature test macros should use
__glibcxx_xxx rather than __cpp_lib_xxx.
> +# include <bits/formatfwd.h>
> +#endif
> +
> #if defined(_WIN32) && !defined(__CYGWIN__)
> # define _GLIBCXX_FILESYSTEM_IS_WINDOWS 1
> #endif
> @@ -1451,6 +1455,138 @@ template<>
> { return filesystem::hash_value(__p); }
> };
>
> +#ifdef __cpp_lib_format_path // C++ >= 26 && HOSTED
> +namespace __format
> +{
> + template<__char _CharT>
> + struct __formatter_fs_path
> + {
> + __formatter_fs_path() = default;
> +
> + constexpr
> + __formatter_fs_path(_Spec<_CharT> __spec) noexcept
> + : _M_spec(__spec)
> + { }
> +
> + constexpr typename basic_format_parse_context<_CharT>::iterator
> + parse(basic_format_parse_context<_CharT>& __pc)
> + {
> + auto __first = __pc.begin();
> + const auto __last = __pc.end();
> + _Spec<_CharT> __spec{};
> +
> + auto __finalize = [this, &__spec] {
> + _M_spec = __spec;
> + };
> +
> + auto __finished = [&] {
> + if (__first == __last || *__first == '}')
> + {
> + __finalize();
> + return true;
> + }
> + return false;
> + };
> +
> + if (__finished())
> + return __first;
> +
> + __first = __spec._M_parse_fill_and_align(__first, __last);
> + if (__finished())
> + return __first;
> +
> + __first = __spec._M_parse_width(__first, __last, __pc);
> + if (__finished())
> + return __first;
> +
> + if (*__first == '?')
> + {
> + __spec._M_debug = true;
> + ++__first;
> + }
> + if (__finished())
> + return __first;
> +
> + if (*__first == 'g')
> + {
> + __spec._M_type = _Pres_g;
> + ++__first;
> + }
> + if (__finished())
> + return __first;
> +
> + __format::__failed_to_parse_format_spec();
> + }
> +
> + template<typename _Out>
> + _Out
> + format(const filesystem::path& __p,
> + basic_format_context<_Out, _CharT>& __fc) const
> + {
> + using _ValueT = filesystem::path::value_type;
> + using _FmtStrT = __formatter_str<_CharT>;
> +
> + filesystem::path::string_type __s;
> + if (_M_spec._M_type == _Pres_g)
> + __s = __p.generic_string<_ValueT>();
> + else
> + __s = __p.native();
> +
> + auto __spec = _M_spec;
> + // 'g' should not be passed along.
> + __spec._M_type = _Pres_none;
> +
> + if constexpr (is_same_v<_CharT, _ValueT>)
> + return _FmtStrT(__spec).format(__s, __fc);
> + else
> + {
> + if (__spec._M_debug)
> + {
> + _Str_sink<_ValueT> __sink;
> + basic_string_view<_ValueT> __sv(__s);
> + __format::__write_escaped(__sink.out(), __sv, _Term_quote);
> + __s = std::move(__sink).get();
> + __spec._M_debug = 0;
> + }
> + basic_string<_CharT> __out_str;
> + using _View = basic_string_view<_ValueT>;
> + __out_str.assign_range(__unicode::_Utf_view<_CharT, _View>(__s));
> + return _FmtStrT(__spec).format(__out_str, __fc);
> + }
> + }
> +
> + constexpr void
> + set_debug_format() noexcept
> + { _M_spec._M_debug = true; }
> +
> + private:
> + _Spec<_CharT> _M_spec{};
> + };
> +} // namespace __format
> +
> + template<__format::__char _CharT>
> + struct formatter<filesystem::path, _CharT>
> + {
> + formatter() = default;
> +
> + [[__gnu__::__always_inline__]]
> + constexpr typename basic_format_parse_context<_CharT>::iterator
> + parse(basic_format_parse_context<_CharT>& __pc)
> + { return _M_f.parse(__pc); }
> +
> + template<typename _Out>
> + typename basic_format_context<_Out, _CharT>::iterator
> + format(const filesystem::path& __p,
> + basic_format_context<_Out, _CharT>& __fc) const
> + { return _M_f.format(__p, __fc); }
> +
> + constexpr void set_debug_format() noexcept { _M_f.set_debug_format(); }
> +
> + private:
> + __format::__formatter_fs_path<_CharT> _M_f;
> + };
> +#endif // __cpp_lib_format_path
> +
> _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace std
>
> diff --git a/libstdc++-v3/include/bits/version.def
> b/libstdc++-v3/include/bits/version.def
> index 4b8e9d43ec2..c7709ba3a07 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1561,6 +1561,16 @@ ftms = {
> };
> };
>
> +ftms = {
> + name = format_path;
> + // 202403 P2845R8 Formatting of std::filesystem::path
> + values = {
> + v = 202403;
> + cxxmin = 26;
> + hosted = yes;
> + };
> +};
> +
> ftms = {
> name = freestanding_algorithm;
> values = {
> diff --git a/libstdc++-v3/include/bits/version.h
> b/libstdc++-v3/include/bits/version.h
> index 7602225cb6d..c72cda506f1 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1721,6 +1721,16 @@
> #endif /* !defined(__cpp_lib_format_ranges) */
> #undef __glibcxx_want_format_ranges
>
> +#if !defined(__cpp_lib_format_path)
> +# if (__cplusplus > 202302L) && _GLIBCXX_HOSTED
> +# define __glibcxx_format_path 202403L
> +# if defined(__glibcxx_want_all) || defined(__glibcxx_want_format_path)
> +# define __cpp_lib_format_path 202403L
> +# endif
> +# endif
> +#endif /* !defined(__cpp_lib_format_path) */
> +#undef __glibcxx_want_format_path
> +
> #if !defined(__cpp_lib_freestanding_algorithm)
> # if (__cplusplus >= 202100L)
> # define __glibcxx_freestanding_algorithm 202311L
> diff --git a/libstdc++-v3/include/std/filesystem
> b/libstdc++-v3/include/std/filesystem
> index f902c6feb77..b9900f49c33 100644
> --- a/libstdc++-v3/include/std/filesystem
> +++ b/libstdc++-v3/include/std/filesystem
> @@ -37,6 +37,7 @@
> #include <bits/requires_hosted.h>
>
> #define __glibcxx_want_filesystem
> +#define __glibcxx_want_format_path
> #include <bits/version.h>
>
> #ifdef __cpp_lib_filesystem // C++ >= 17 && HOSTED
> diff --git a/libstdc++-v3/testsuite/std/format/fs_path.cc
> b/libstdc++-v3/testsuite/std/format/fs_path.cc
> new file mode 100644
> index 00000000000..5105e73f611
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/format/fs_path.cc
> @@ -0,0 +1,88 @@
> +// { dg-do run { target c++26 } }
> +
> +#include <filesystem>
> +#include <format>
> +#include <testsuite_hooks.h>
> +
> +using std::filesystem::path;
> +
> +template<typename... Args>
> +bool
> +is_format_string_for(const char* str, Args&&... args)
> +{
> + try {
> + (void) std::vformat(str, std::make_format_args(args...));
> + return true;
> + } catch (const std::format_error&) {
> + return false;
> + }
> +}
> +
> +template<typename... Args>
> +bool
> +is_format_string_for(const wchar_t* str, Args&&... args)
> +{
> + try {
> + (void) std::vformat(str, std::make_wformat_args(args...));
> + return true;
> + } catch (const std::format_error&) {
> + return false;
> + }
> +}
> +
> +void
> +test_format_spec()
> +{
> + // [fs.path.fmtr.funcs]
> + // \nontermdef{path-format-spec}\br
> + // \opt{fill-and-align} \opt{width} \opt{\terminal{?}}
> \opt{\terminal{g}}
> + path p;
> + VERIFY( is_format_string_for("{}", p) );
> + VERIFY( is_format_string_for("{:}", p) );
> + VERIFY( is_format_string_for("{:?}", p) );
> + VERIFY( is_format_string_for("{:g}", p) );
> + VERIFY( is_format_string_for("{:?g}", p) );
> + VERIFY( is_format_string_for("{:?g}", p) );
> + VERIFY( is_format_string_for("{:F^32?g}", p) );
> + VERIFY( is_format_string_for("{:G<{}?g}", p, 32) );
> + VERIFY( is_format_string_for(L"{:G<{}?g}", p, 32) );
> +
> + VERIFY( ! is_format_string_for("{:g?}", p) );
> +}
> +
> +void
> +test_format_path()
> +{
> + VERIFY( std::format("{}", path("/usr/include")) == "/usr/include" );
> + VERIFY( std::format("{:.<10}", path("foo/bar")) == "foo/bar..." );
> + VERIFY( std::format("{}", path("foo///bar")) == "foo///bar" );
> + VERIFY( std::format("{:g}", path("foo///bar")) == "foo/bar" );
> + VERIFY( std::format("{}", path("/path/with/new\nline")) ==
> "/path/with/new\nline" );
> + VERIFY( std::format("{:?}", path("multi\nline")) == "\"multi\\nline\"" );
> + VERIFY( std::format("{:?g}", path("mu///lti\nli///ne")) ==
> "\"mu/lti\\nli/ne\"" );
> + VERIFY( std::format(L"{}",
> path(L"\u0428\u0447\u0443\u0447\u044B\u043D\u0448\u0447\u044B\u043D\u0430"))
> == L"\u0428\u0447\u0443\u0447\u044B\u043D\u0448\u0447\u044B\u043D\u0430"
> );
> +
> + if constexpr (path::preferred_separator == L'\\')
> + {
> + VERIFY( std::format("{}", path("C:\\foo\\bar")) == "C:\\foo\\bar" );
> + VERIFY( std::format("{:g}", path("C:\\foo\\bar")) == "C:/foo/bar" );
> +
> + path p(L"\xd800");
> + VERIFY( std::format("{}", p) == "\uFFFD" );
> + VERIFY( std::format("{:?}", p) == "\"\\x{d800}\"" );
> + VERIFY( std::format(L"{:?}", p) == L"\"\\x{d800}\"" );
> +
> + path p2(L"///\xd800");
> + VERIFY( std::format("{}", p2) == "///\uFFFD" );
> + VERIFY( std::format("{:g}", p2) == "/\uFFFD" );
> + VERIFY( std::format("{:?}", p2) == "\"///\\x{d800}\"" );
> + VERIFY( std::format("{:?g}", p2) == "\"/\\x{d800}\"" );
> + VERIFY( std::format("{:C>14?g}", p2) == "CCC\"/\\x{d800}\"" );
> + }
> +}
> +
> +int main()
> +{
> + test_format_spec();
> + test_format_path();
> +}
> --
> 2.43.0
>
