On Thu, Jul 2, 2026 at 12:00 AM Jonathan Wakely <[email protected]> wrote:
>
> On Wed, 01 Jul 2026 at 23:11 +0800, Yuao Ma wrote:
> >On second thoughts, I think we can actually just remove the check
> >altogether. It delegates to the string_view overload, which ultimately
> >calls the overload without __pos anyway.
> >
> >On Wed, Jul 1, 2026 at 11:08 PM Jonathan Wakely <[email protected]> wrote:
> >>
> >> On Wed, 1 Jul 2026 at 15:47, Tomasz Kaminski <[email protected]> wrote:
> >> >
> >> > Are the following checks correct? I think we need string of at least
> >> > length __pos + __len.
> >>
> >> The macro is defined as:
> >> # define __glibcxx_requires_string_len(_String,_Len) \
> >> _GLIBCXX_DEBUG_PEDASSERT(_String != 0 || _Len == 0)
> >>
> >> So it says we can't have a null pointer unless n==0 is true. I think
> >> that's the right check here.
> >>
> >>
> >>
> >> > __glibcxx_requires_string_len(__s, __n);
> >> >
> >> >
> >> >
> >> >
> >> > On Wed, Jul 1, 2026 at 4:41 PM Yuao Ma <[email protected]> wrote:
> >> >>
> >> >> Hi!
> >> >>
> >> >> This patch implements LWG3662 for C++11 ABI and COW ABI of strings.
> >> >>
> >> >> Tested on x86_64 linux, ok for trunk?
> >> >>
> >> >> Thanks,
> >> >> Yuao
> >>
>
> >From abae92d920dcb27beab2720ac079f1e852fafb66 Mon Sep 17 00:00:00 2001
> >From: Yuao Ma <[email protected]>
> >Date: Wed, 1 Jul 2026 23:09:35 +0800
> >Subject: [PATCH] libstdc++: implement LWG3662
> > basic_string::append/assign(NTBS, pos, n) suboptimal
> >
> >This patch implements LWG3662 for both ABIs of std::string.
> >
> >libstdc++-v3/ChangeLog:
> >
> > * include/bits/basic_string.h: Add new append/assign API.
>
> Please follow the documented form for the ChangeLog and name the
> functions being added:
>
Fixed. Sometimes I forget to add the function name when
gcc-commit-mklog doesn't generate it automatically.
> * include/bits/basic_string.h (append, assign): Add new
> overloads.
>
> > * include/bits/cow_string.h: Ditto.
> > * testsuite/21_strings/basic_string/modifiers/append/char/2.cc: Test
> > new API.
>
> Please break this line and the next three lines after the colon.
>
> 'git log' indents the commit log by 4 spaces, which means the
> ChangeLog entry is indended by 12 spaces, so if your ChangeLog entry
> already uses 85 lines it's going to be much too long.
>
Fixed.
> > * testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc:
> > Ditto.
> > * testsuite/21_strings/basic_string/modifiers/assign/char/3.cc: Ditto.
> > * testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc:
> > Ditto.
> >---
> > libstdc++-v3/include/bits/basic_string.h | 28 +++++++++++++++++++
> > libstdc++-v3/include/bits/cow_string.h | 26 +++++++++++++++++
> > .../basic_string/modifiers/append/char/2.cc | 4 +++
> > .../modifiers/append/wchar_t/2.cc | 4 +++
> > .../basic_string/modifiers/assign/char/3.cc | 4 +++
> > .../modifiers/assign/wchar_t/3.cc | 4 +++
> > 6 files changed, 70 insertions(+)
> >
> >diff --git a/libstdc++-v3/include/bits/basic_string.h
> >b/libstdc++-v3/include/bits/basic_string.h
> >index 65d92ebfbcf..4f07b1c14f1 100644
> >--- a/libstdc++-v3/include/bits/basic_string.h
> >+++ b/libstdc++-v3/include/bits/basic_string.h
> >@@ -1639,6 +1639,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > return _M_append(__s, __n);
> > }
> >
>
> Please add a _GLIBCXX_RESOLVE_LIB_DEFECTS comment:
>
> // _GLIBCXX_RESOLVE_LIB_DEFECTS
> // 3662. basic_string::append/assign(NTBS, pos, n) suboptimal
>
Fixed all 4 cases.
> >+ /**
> >+ * @brief Append a C substring.
> >+ * @param __s The C string to append.
> >+ * @param __pos The position in the C string to append from.
> >+ * @param __n The number of characters to append.
> >+ * @return Reference to this string.
> >+ */
> >+ _GLIBCXX20_CONSTEXPR
> >+ basic_string&
> >+ append(const _CharT* __s, size_type __pos, size_type __n)
> >+ {
> >+ return append(__sv_type(__s).substr(__pos, __n));
> >+ }
>
> This should be one line:
>
> { return append(__sv_type(__s).substr(__pos, __n)); }
>
Fixed all 4 cases.
> >+
> > /**
> > * @brief Append multiple characters.
> > * @param __n The number of characters to append.
> >@@ -1902,6 +1916,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > traits_type::length(__s));
> > }
>
> _GLIBCXX_RESOLVE_LIB_DEFECTS comment.
>
> >+ /**
> >+ * @brief Set value to a C substring.
> >+ * @param __s The C string to use.
> >+ * @param __pos The position in the C string to assign from.
> >+ * @param __n Number of characters to use.
> >+ * @return Reference to this string.
> >+ */
> >+ _GLIBCXX20_CONSTEXPR
> >+ basic_string&
> >+ assign(const _CharT* __s, size_type __pos, size_type __n)
> >+ {
> >+ return assign(__sv_type(__s).substr(__pos, __n));
> >+ }
>
> Just one line.
>
> >+
> > /**
> > * @brief Set value to multiple characters.
> > * @param __n Length of the resulting string.
> >diff --git a/libstdc++-v3/include/bits/cow_string.h
> >b/libstdc++-v3/include/bits/cow_string.h
> >index 7c945f6b998..7708c2474ba 100644
> >--- a/libstdc++-v3/include/bits/cow_string.h
> >+++ b/libstdc++-v3/include/bits/cow_string.h
> >@@ -1345,6 +1345,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > return this->append(__s, traits_type::length(__s));
> > }
>
> _GLIBCXX_RESOLVE_LIB_DEFECTS comment.
>
> >+ /**
> >+ * @brief Append a C substring.
> >+ * @param __s The C string to append.
> >+ * @param __pos The position in the C string to append from.
> >+ * @param __n The number of characters to append.
> >+ * @return Reference to this string.
> >+ */
> >+ basic_string&
> >+ append(const _CharT* __s, size_type __pos, size_type __n)
> >+ {
> >+ return append(__sv_type(__s).substr(__pos, __n));
> >+ }
>
> One line.
>
> >+
> > /**
> > * @brief Append multiple characters.
> > * @param __n The number of characters to append.
> >@@ -1517,6 +1530,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > return this->assign(__s, traits_type::length(__s));
> > }
>
> _GLIBCXX_RESOLVE_LIB_DEFECTS comment.
>
> >+ /**
> >+ * @brief Set value to a C substring.
> >+ * @param __s The C string to use.
> >+ * @param __pos The position in the C string to assign from.
> >+ * @param __n Number of characters to use.
> >+ * @return Reference to this string.
> >+ */
> >+ basic_string&
> >+ assign(const _CharT* __s, size_type __pos, size_type __n)
> >+ {
> >+ return assign(__sv_type(__s).substr(__pos, __n));
> >+ }
>
> One line.
>
> >+
> > /**
> > * @brief Set value to multiple characters.
> > * @param __n Length of the resulting string.
> >diff --git
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
> >index d70fc190b45..5dd39347574 100644
> >---
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
> >+++
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
> >@@ -24,6 +24,7 @@
> >
> > // append(const _CharT* __s, size_type __n)
> > // append(const _CharT* __s)
> >+// append(const _CharT* __s, size_type __pos, size_type __n)
> > void
> > test02()
> > {
> >@@ -55,6 +56,9 @@ test02()
> >
> > two.append(two.c_str(), 3);
> > VERIFY( two == "Written in your eyeseyesWri" );
> >+
> >+ two.append(two.c_str(), 8, 2);
> >+ VERIFY( two == "Written in your eyeseyesWriin" );
> > }
>
> Do we have a test for the s.append("a\0b", 2, 1) example I gave, which
> should throw std::out_of_range? I think that would be good to have.
>
Added.
> >
> > int main()
> >diff --git
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
> >
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
> >index 1922874de7f..fcc1ef54f28 100644
> >---
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
> >+++
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
> >@@ -24,6 +24,7 @@
> >
> > // append(const _CharT* __s, size_type __n)
> > // append(const _CharT* __s)
> >+// append(const _CharT* __s, size_type __pos, size_type __n)
> > void
> > test02()
> > {
> >@@ -55,6 +56,9 @@ test02()
> >
> > two.append(two.c_str(), 3);
> > VERIFY( two == L"Written in your eyeseyesWri" );
> >+
> >+ two.append(two.c_str(), 8, 2);
> >+ VERIFY( two == L"Written in your eyeseyesWriin" );
> > }
> >
> > int main()
> >diff --git
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
> >index 1bd3deb30f4..a742c1ed3df 100644
> >---
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
> >+++
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
> >@@ -24,6 +24,7 @@
> >
> > // assign(const _CharT* __s, size_type __n)
> > // assign(const _CharT* __s)
> >+// assign(const _CharT* __s, size_type __pos, size_type __n)
> > void
> > test03()
> > {
> >@@ -47,6 +48,9 @@ test03()
> >
> > one.assign(one.c_str() + 8, 6);
> > VERIFY( one == "by the" );
> >+
> >+ one.assign(one.c_str(), 3, 3);
> >+ VERIFY( one == "the" );
> > }
> >
> > int main()
> >diff --git
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
> >
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
> >index e7fcb231dff..37ff7e1ca2b 100644
> >---
> >a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
> >+++
> >b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
> >@@ -24,6 +24,7 @@
> >
> > // assign(const _CharT* __s, size_type __n)
> > // assign(const _CharT* __s)
> >+// assign(const _CharT* __s, size_type __pos, size_type __n)
> > void
> > test03()
> > {
> >@@ -47,6 +48,9 @@ test03()
> >
> > one.assign(one.c_str() + 8, 6);
> > VERIFY( one == L"by the" );
> >+
> >+ one.assign(one.c_str(), 3, 3);
> >+ VERIFY( one == L"the" );
> > }
> >
> > int main()
> >--
> >2.54.0
> >
>
From d873e2e1470efe80470b577a4b95a5118df41081 Mon Sep 17 00:00:00 2001
From: Yuao Ma <[email protected]>
Date: Thu, 2 Jul 2026 00:47:48 +0800
Subject: [PATCH] libstdc++: implement LWG3662
basic_string::append/assign(NTBS, pos, n) suboptimal
This patch implements LWG3662 for both ABIs of strings.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h(append, assign): Add new
overloads.
* include/bits/cow_string.h(append, assign): Ditto.
* testsuite/21_strings/basic_string/modifiers/append/char/2.cc:
Test new overloads.
* testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc:
Ditto.
* testsuite/21_strings/basic_string/modifiers/assign/char/3.cc:
Ditto.
* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc:
Ditto.
---
libstdc++-v3/include/bits/basic_string.h | 28 +++++++++++++++++++
libstdc++-v3/include/bits/cow_string.h | 26 +++++++++++++++++
.../basic_string/modifiers/append/char/2.cc | 16 +++++++++++
.../modifiers/append/wchar_t/2.cc | 16 +++++++++++
.../basic_string/modifiers/assign/char/3.cc | 16 +++++++++++
.../modifiers/assign/wchar_t/3.cc | 16 +++++++++++
6 files changed, 118 insertions(+)
diff --git a/libstdc++-v3/include/bits/basic_string.h
b/libstdc++-v3/include/bits/basic_string.h
index 65d92ebfbcf..706def2c967 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -1639,6 +1639,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
return _M_append(__s, __n);
}
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 3662. basic_string::append/assign(NTBS, pos, n) suboptimal
+ /**
+ * @brief Append a C substring.
+ * @param __s The C string to append.
+ * @param __pos The position in the C string to append from.
+ * @param __n The number of characters to append.
+ * @return Reference to this string.
+ */
+ _GLIBCXX20_CONSTEXPR
+ basic_string&
+ append(const _CharT* __s, size_type __pos, size_type __n)
+ { return append(__sv_type(__s).substr(__pos, __n)); }
+
/**
* @brief Append multiple characters.
* @param __n The number of characters to append.
@@ -1902,6 +1916,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
traits_type::length(__s));
}
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 3662. basic_string::append/assign(NTBS, pos, n) suboptimal
+ /**
+ * @brief Set value to a C substring.
+ * @param __s The C string to use.
+ * @param __pos The position in the C string to assign from.
+ * @param __n Number of characters to use.
+ * @return Reference to this string.
+ */
+ _GLIBCXX20_CONSTEXPR
+ basic_string&
+ assign(const _CharT* __s, size_type __pos, size_type __n)
+ { return assign(__sv_type(__s).substr(__pos, __n)); }
+
/**
* @brief Set value to multiple characters.
* @param __n Length of the resulting string.
diff --git a/libstdc++-v3/include/bits/cow_string.h
b/libstdc++-v3/include/bits/cow_string.h
index 7c945f6b998..5148bc78ac7 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -1345,6 +1345,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return this->append(__s, traits_type::length(__s));
}
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 3662. basic_string::append/assign(NTBS, pos, n) suboptimal
+ /**
+ * @brief Append a C substring.
+ * @param __s The C string to append.
+ * @param __pos The position in the C string to append from.
+ * @param __n The number of characters to append.
+ * @return Reference to this string.
+ */
+ basic_string&
+ append(const _CharT* __s, size_type __pos, size_type __n)
+ { return append(__sv_type(__s).substr(__pos, __n)); }
+
/**
* @brief Append multiple characters.
* @param __n The number of characters to append.
@@ -1517,6 +1530,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return this->assign(__s, traits_type::length(__s));
}
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 3662. basic_string::append/assign(NTBS, pos, n) suboptimal
+ /**
+ * @brief Set value to a C substring.
+ * @param __s The C string to use.
+ * @param __pos The position in the C string to assign from.
+ * @param __n Number of characters to use.
+ * @return Reference to this string.
+ */
+ basic_string&
+ assign(const _CharT* __s, size_type __pos, size_type __n)
+ { return assign(__sv_type(__s).substr(__pos, __n)); }
+
/**
* @brief Set value to multiple characters.
* @param __n Length of the resulting string.
diff --git
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
index d70fc190b45..d9ccbd76896 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/char/2.cc
@@ -19,11 +19,13 @@
// 21.3.5 string modifiers
+#include <stdexcept>
#include <string>
#include <testsuite_hooks.h>
// append(const _CharT* __s, size_type __n)
// append(const _CharT* __s)
+// append(const _CharT* __s, size_type __pos, size_type __n)
void
test02()
{
@@ -55,6 +57,20 @@ test02()
two.append(two.c_str(), 3);
VERIFY( two == "Written in your eyeseyesWri" );
+
+ two.append(two.c_str(), 8, 2);
+ VERIFY( two == "Written in your eyeseyesWriin" );
+
+ try {
+ two.append("a\0b", 2, 1);
+ VERIFY( false );
+ }
+ catch(std::out_of_range& fail) {
+ VERIFY( true );
+ }
+ catch(...) {
+ VERIFY( false );
+ }
}
int main()
diff --git
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
index 1922874de7f..d0d95fb3f27 100644
---
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
+++
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/append/wchar_t/2.cc
@@ -19,11 +19,13 @@
// 21.3.5 string modifiers
+#include <stdexcept>
#include <string>
#include <testsuite_hooks.h>
// append(const _CharT* __s, size_type __n)
// append(const _CharT* __s)
+// append(const _CharT* __s, size_type __pos, size_type __n)
void
test02()
{
@@ -55,6 +57,20 @@ test02()
two.append(two.c_str(), 3);
VERIFY( two == L"Written in your eyeseyesWri" );
+
+ two.append(two.c_str(), 8, 2);
+ VERIFY( two == L"Written in your eyeseyesWriin" );
+
+ try {
+ two.append(L"a\0b", 2, 1);
+ VERIFY( false );
+ }
+ catch(std::out_of_range& fail) {
+ VERIFY( true );
+ }
+ catch(...) {
+ VERIFY( false );
+ }
}
int main()
diff --git
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
index 1bd3deb30f4..75f2dcf0130 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/char/3.cc
@@ -19,11 +19,13 @@
// 21.3.5 string modifiers
+#include <stdexcept>
#include <string>
#include <testsuite_hooks.h>
// assign(const _CharT* __s, size_type __n)
// assign(const _CharT* __s)
+// assign(const _CharT* __s, size_type __pos, size_type __n)
void
test03()
{
@@ -47,6 +49,20 @@ test03()
one.assign(one.c_str() + 8, 6);
VERIFY( one == "by the" );
+
+ one.assign(one.c_str(), 3, 3);
+ VERIFY( one == "the" );
+
+ try {
+ one.assign("a\0b", 2, 1);
+ VERIFY( false );
+ }
+ catch(std::out_of_range& fail) {
+ VERIFY( true );
+ }
+ catch(...) {
+ VERIFY( false );
+ }
}
int main()
diff --git
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
index e7fcb231dff..821cde246bd 100644
---
a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
+++
b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/assign/wchar_t/3.cc
@@ -19,11 +19,13 @@
// 21.3.5 string modifiers
+#include <stdexcept>
#include <string>
#include <testsuite_hooks.h>
// assign(const _CharT* __s, size_type __n)
// assign(const _CharT* __s)
+// assign(const _CharT* __s, size_type __pos, size_type __n)
void
test03()
{
@@ -47,6 +49,20 @@ test03()
one.assign(one.c_str() + 8, 6);
VERIFY( one == L"by the" );
+
+ one.assign(one.c_str(), 3, 3);
+ VERIFY( one == L"the" );
+
+ try {
+ one.assign(L"a\0b", 2, 1);
+ VERIFY( false );
+ }
+ catch(std::out_of_range& fail) {
+ VERIFY( true );
+ }
+ catch(...) {
+ VERIFY( false );
+ }
}
int main()
--
2.54.0
