[PATCH v2] libstdc++: mark integer std::to_(w)string constexpr
Jonathan Wakely
jwakely@redhat.com
Mon Jun 1 14:40:46 GMT 2026
On Fri, 15 May 2026 at 19:06, Nathan Myers <ncm@cantrip.org> wrote:
>
> On 5/15/26 3:03 AM, Tomasz Kamiński wrote:
> > From: Ivan Lazaric <ivan.lazaric1@gmail.com>
> >
> > In C++26 paper P3391, "constexpr formatting", has been adopted,
> > part of which marks std::to_string & std::to_wstring for integers
> > as constexpr. The __cpp_lib_constexpr_string FTM value is updated
> > per resolution of LWG4531, "Should there be a feature-test macro
> > update for constexpr std::to_(w)string?".
> >
> > Since pre-cxx11 copy-on-write string is not constexpr-enabled,
> > restricting this constexpr-ification to cxx11 ABI strings.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/version.def (constexpr_string): Bump to 202511.
> > * include/bits/versin.h: Regenerate.
> > * include/bits/basic_string.h (std::to_string, std::to_wstring)
> > [__glibcxx_constexpr_string >= 202511L]: Mark as constexpr.
> > * testsuite/21_strings/basic_string/numeric_conversions/char/to_string_constexpr.cc:
> > New test.
> > * testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_constexpr.cc:
> > New test.
> > * testsuite/21_strings/basic_string/cons/char/constexpr.cc: Update
> > __cpp_lib_constexpr_string check.
> > testsuite/21_strings/basic_string/cons/wchar_t/constexpr.c: Likewise.
> > * 21_strings/basic_string/version.cc: Add check for value of
> > __cpp_lib_constexpr_string in C++26.
> >
> > Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
> > ---
> > v2 changed:
> > - bumped constexpr_string per LWG4531, and used it for preprocessor
> > check in basic_string
> > - updated checks for __cpp_lib_constexpr_string in existing tests
> > - used require cxx11_abi target instead of if predefines in test files
> >
> > Tested on x86_64-linux. OK for trunk?
> >
> > libstdc++-v3/include/bits/basic_string.h | 36 +++++++++++
> > libstdc++-v3/include/bits/version.def | 8 +++
> > libstdc++-v3/include/bits/version.h | 7 +-
> > .../basic_string/cons/char/constexpr.cc | 4 +-
> > .../basic_string/cons/wchar_t/constexpr.cc | 4 +-
> > .../char/to_string_constexpr.cc | 64 +++++++++++++++++++
> > .../wchar_t/to_wstring_constexpr.cc | 64 +++++++++++++++++++
> > .../21_strings/basic_string/version.cc | 12 ++++
> > 8 files changed, 194 insertions(+), 5 deletions(-)
> > create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/char/to_string_constexpr.cc
> > create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_constexpr.cc
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index af4e5d9486f..39477157858 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -4581,6 +4581,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > // DR 1261. Insufficent overloads for to_string / to_wstring
>
> This seems like a place that merits a macro:
>
> #if __glibcxx_constexpr_string >= 202511L
> #define _GCLIBC_STRING_CONSTEXPR constexpr
> #else
> #define _GCLIBC_STRING_CONSTEXPR
> #endif
>
> Which is not to say I like macros, but #if is worse.
>
> Otherwise, LGTM.
I agree, I think it would be better to do:
#if __glibcxx_constexpr_string >= 202511L
#define _GLIBCXX_TO_STRING_CONSTEXPR constexpr
#else
#define _GLIBCXX_TO_STRING_CONSTEXPR inline
#endif
That can then replace the existing 'inline' keywords.
Then #undef it after it's no longer needed.
>
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(int __val)
> > #if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_INT__) <= 32
> > @@ -4600,6 +4603,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(unsigned __val)
> > #if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_INT__) <= 32
> > @@ -4616,6 +4622,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(long __val)
> > #if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_LONG__) <= 32
> > @@ -4635,6 +4644,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(unsigned long __val)
> > #if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_LONG__) <= 32
> > @@ -4651,6 +4663,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(long long __val)
> > {
> > @@ -4668,6 +4683,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline string
> > to_string(unsigned long long __val)
> > {
> > @@ -4892,31 +4910,49 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > #pragma GCC diagnostic pop
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(int __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(unsigned __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(long __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(unsigned long __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(long long __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> >
> > _GLIBCXX_NODISCARD
> > +#if __glibcxx_constexpr_string >= 202511L
> > + constexpr
> > +#endif
> > inline wstring
> > to_wstring(unsigned long long __val)
> > { return std::__to_wstring_numeric(std::to_string(__val)); }
> > diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> > index 1f0d3a2670e..6d789987642 100644
> > --- a/libstdc++-v3/include/bits/version.def
> > +++ b/libstdc++-v3/include/bits/version.def
> > @@ -1398,6 +1398,14 @@ ftms = {
> >
> > ftms = {
> > name = constexpr_string;
> > + // 202511 LWG4531 Should there be a feature-test macro update for constexpr std::to_(w)string?
> > + // P3391R2 constexpr std::format
> > + values = {
> > + v = 202511;
> > + cxxmin = 26;
> > + hosted = yes;
> > + cxx11abi = yes;
> > + };
> > values = {
> > v = 201907;
> > cxxmin = 20;
> > diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> > index 66ac0ebef68..c44fe07ec60 100644
> > --- a/libstdc++-v3/include/bits/version.h
> > +++ b/libstdc++-v3/include/bits/version.h
> > @@ -1542,7 +1542,12 @@
> > #undef __glibcxx_want_constexpr_flat_set
> >
> > #if !defined(__cpp_lib_constexpr_string)
> > -# if (__cplusplus >= 202002L) && _GLIBCXX_USE_CXX11_ABI && _GLIBCXX_HOSTED && (defined(__glibcxx_is_constant_evaluated))
> > +# if (__cplusplus > 202302L) && _GLIBCXX_USE_CXX11_ABI && _GLIBCXX_HOSTED
> > +# define __glibcxx_constexpr_string 202511L
> > +# if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_string)
> > +# define __cpp_lib_constexpr_string 202511L
> > +# endif
> > +# elif (__cplusplus >= 202002L) && _GLIBCXX_USE_CXX11_ABI && _GLIBCXX_HOSTED && (defined(__glibcxx_is_constant_evaluated))
> > # define __glibcxx_constexpr_string 201907L
> > # if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_string)
> > # define __cpp_lib_constexpr_string 201907L
> > diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
> > index 7822c89497b..34abfe60bc5 100644
> > --- a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
> > +++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
> > @@ -6,8 +6,8 @@
> >
> > #ifndef __cpp_lib_constexpr_string
> > # error "Feature-test macro for constexpr std::string missing in <string>"
> > -#elif __cpp_lib_constexpr_string != 201907L
> > -# error "Feature-test macro for constexpr std::string has wrong value in <string>"
> > +#elif __cpp_lib_constexpr_string < 201907L
> > +# error "Feature-test macro for constexpr std::string has too small value in <string>"
> > #endif
> >
> > #include <testsuite_hooks.h>
> > diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
> > index 44c8391ebc2..59d5ee270cc 100644
> > --- a/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
> > +++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
> > @@ -6,8 +6,8 @@
> >
> > #ifndef __cpp_lib_constexpr_string
> > # error "Feature-test macro for constexpr std::string missing in <string>"
> > -#elif __cpp_lib_constexpr_string != 201907L
> > -# error "Feature-test macro for constexpr std::string has wrong value in <string>"
> > +#elif __cpp_lib_constexpr_string < 201907L
> > +# error "Feature-test macro for constexpr std::string too small value in <string>"
> > #endif
> >
> > #include <testsuite_hooks.h>
> > diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/char/to_string_constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/char/to_string_constexpr.cc
> > new file mode 100644
> > index 00000000000..1b5a616df54
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/char/to_string_constexpr.cc
> > @@ -0,0 +1,64 @@
> > +// { dg-do compile { target c++26 } }
> > +// { dg-require-effective-target cxx11_abi }
> > +
> > +#include <string>
> > +#include <type_traits>
> > +#include <testsuite_hooks.h>
> > +
> > +template<typename T>
> > +constexpr void
> > +test()
> > +{
> > + using namespace std;
> > + string res;
> > + T value;
> > +
> > + value = 0;
> > + res = to_string(value);
> > + VERIFY( res == "0" );
> > +
> > + value = 1;
> > + res = to_string(value);
> > + VERIFY( res == "1" );
> > +
> > + value = 10;
> > + res = to_string(value);
> > + VERIFY( res == "10" );
> > +
> > + value = 3000;
> > + res = to_string(value);
> > + VERIFY( res == "3000" );
> > +
> > + value = 32767;
> > + res = to_string(value);
> > + VERIFY( res == "32767" );
> > +
> > + if (is_unsigned_v<T>)
> > + return;
> > +
> > + value = -1;
> > + res = to_string(value);
> > + VERIFY( res == "-1" );
> > +
> > + value = -40;
> > + res = to_string(value);
> > + VERIFY( res == "-40" );
> > +
> > + value = -32768;
> > + res = to_string(value);
> > + VERIFY( res == "-32768" );
> > +}
> > +
> > +constexpr bool
> > +test_all()
> > +{
> > + test<int>();
> > + test<unsigned int>();
> > + test<long>();
> > + test<unsigned long>();
> > + test<long long>();
> > + test<unsigned long long>();
> > + return true;
> > +}
> > +
> > +static_assert(test_all());
> > diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_constexpr.cc
> > new file mode 100644
> > index 00000000000..f884fc61ff5
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_constexpr.cc
> > @@ -0,0 +1,64 @@
> > +// { dg-do compile { target c++26 } }
> > +// { dg-require-effective-target cxx11_abi }
> > +
> > +#include <string>
> > +#include <type_traits>
> > +#include <testsuite_hooks.h>
> > +
> > +template<typename T>
> > +constexpr void
> > +test()
> > +{
> > + using namespace std;
> > + wstring res;
> > + T value;
> > +
> > + value = 0;
> > + res = to_wstring(value);
> > + VERIFY( res == L"0" );
> > +
> > + value = 1;
> > + res = to_wstring(value);
> > + VERIFY( res == L"1" );
> > +
> > + value = 10;
> > + res = to_wstring(value);
> > + VERIFY( res == L"10" );
> > +
> > + value = 3000;
> > + res = to_wstring(value);
> > + VERIFY( res == L"3000" );
> > +
> > + value = 32767;
> > + res = to_wstring(value);
> > + VERIFY( res == L"32767" );
> > +
> > + if (is_unsigned_v<T>)
> > + return;
> > +
> > + value = -1;
> > + res = to_wstring(value);
> > + VERIFY( res == L"-1" );
> > +
> > + value = -40;
> > + res = to_wstring(value);
> > + VERIFY( res == L"-40" );
> > +
> > + value = -32768;
> > + res = to_wstring(value);
> > + VERIFY( res == L"-32768" );
> > +}
> > +
> > +constexpr bool
> > +test_all()
> > +{
> > + test<int>();
> > + test<unsigned int>();
> > + test<long>();
> > + test<unsigned long>();
> > + test<long long>();
> > + test<unsigned long long>();
> > + return true;
> > +}
> > +
> > +static_assert(test_all());
> > diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/version.cc b/libstdc++-v3/testsuite/21_strings/basic_string/version.cc
> > index 71dd4dfb67f..b0cdf04274e 100644
> > --- a/libstdc++-v3/testsuite/21_strings/basic_string/version.cc
> > +++ b/libstdc++-v3/testsuite/21_strings/basic_string/version.cc
> > @@ -25,3 +25,15 @@
> > # endif
> > # endif
> > #endif
> > +
> > +#if __cplusplus > 202302L
> > +# if _GLIBCXX_USE_CXX11_ABI
> > +# if __cpp_lib_constexpr_string != 202511L
> > +# error "Feature-test macro for constexpr std::string has wrong value for C++20 in <version>"
> > +# endif
> > +# else // COW strings
> > +# if __cpp_lib_constexpr_string != 201811L
> > +# error "Feature-test macro for constexpr std::string has wrong value for C++20 in <version>"
> > +# endif
> > +# endif
> > +#endif
>
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