[libcxx-commits] [libcxx] a5c06c0 - [libc++] Implement LWG3662 basic_string::append/assign(NTBS, pos, n) suboptimal (#206320)
via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Jul 1 04:20:30 PDT 2026
Author: Connector Switch
Date: 2026-07-01T19:20:26+08:00
New Revision: a5c06c0b4c7d713ef7ea63cd43554cc674dd167d
URL: https://github.com/llvm/llvm-project/commit/a5c06c0b4c7d713ef7ea63cd43554cc674dd167d
DIFF: https://github.com/llvm/llvm-project/commit/a5c06c0b4c7d713ef7ea63cd43554cc674dd167d.diff
LOG: [libc++] Implement LWG3662 basic_string::append/assign(NTBS, pos, n) suboptimal (#206320)
Closes https://github.com/llvm/llvm-project/issues/189809
This patch implements LWG 3662 and adds the related functional and
nonnull tests.
---------
Co-authored-by: A. Jiang <de34 at live.cn>
Added:
libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer_size_size.pass.cpp
libcxx/test/std/strings/basic.string/string.modifiers/string_assign/pointer_size_size.pass.cpp
Modified:
libcxx/docs/Status/Cxx26Issues.csv
libcxx/include/string
libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp
Removed:
################################################################################
diff --git a/libcxx/docs/Status/Cxx26Issues.csv b/libcxx/docs/Status/Cxx26Issues.csv
index 10cad5d52e2f1..3f2a446f7e1f3 100644
--- a/libcxx/docs/Status/Cxx26Issues.csv
+++ b/libcxx/docs/Status/Cxx26Issues.csv
@@ -264,7 +264,7 @@
"`LWG2746 <https://wg21.link/LWG2746>`__","Inconsistency between requirements for ``emplace`` between ``optional`` and ``variant``","2026-03 (Croydon)","","","`#189806 <https://github.com/llvm/llvm-project/issues/189806>`__",""
"`LWG3504 <https://wg21.link/LWG3504>`__","``condition_variable::wait_for`` is overspecified","2026-03 (Croydon)","","","`#189807 <https://github.com/llvm/llvm-project/issues/189807>`__",""
"`LWG3599 <https://wg21.link/LWG3599>`__","The ``const`` overload of ``lazy_split_view::begin`` should be constrained by ``const Pattern``","2026-03 (Croydon)","","","`#189808 <https://github.com/llvm/llvm-project/issues/189808>`__",""
-"`LWG3662 <https://wg21.link/LWG3662>`__","``basic_string::append/assign(NTBS, pos, n)`` suboptimal","2026-03 (Croydon)","","","`#189809 <https://github.com/llvm/llvm-project/issues/189809>`__",""
+"`LWG3662 <https://wg21.link/LWG3662>`__","``basic_string::append/assign(NTBS, pos, n)`` suboptimal","2026-03 (Croydon)","|Complete|","23","`#189809 <https://github.com/llvm/llvm-project/issues/189809>`__",""
"`LWG3777 <https://wg21.link/LWG3777>`__","Common ``cartesian_product_view`` produces an invalid range if the first range is input and one of the ranges is empty","2026-03 (Croydon)","","","`#189810 <https://github.com/llvm/llvm-project/issues/189810>`__",""
"`LWG3797 <https://wg21.link/LWG3797>`__","``elements_view`` insufficiently constrained","2026-03 (Croydon)","","","`#189811 <https://github.com/llvm/llvm-project/issues/189811>`__",""
"`LWG3831 <https://wg21.link/LWG3831>`__","Two-digit formatting of negative ``year`` is ambiguous","2026-03 (Croydon)","","","`#189812 <https://github.com/llvm/llvm-project/issues/189812>`__",""
diff --git a/libcxx/include/string b/libcxx/include/string
index e5bc854cd34f3..6516d26e288a9 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -201,6 +201,7 @@ public:
basic_string& append(const T& t, size_type pos, size_type n=npos); // C++17, constexpr since C++20
basic_string& append(const value_type* s, size_type n); // constexpr since C++20
basic_string& append(const value_type* s); // constexpr since C++20
+ basic_string& append(const value_type* s, size_type pos, size_type n); // constexpr since C++20
basic_string& append(size_type n, value_type c); // constexpr since C++20
template<class InputIterator>
basic_string& append(InputIterator first, InputIterator last); // constexpr since C++20
@@ -224,6 +225,7 @@ public:
basic_string& assign(const T& t, size_type pos, size_type n=npos); // C++17, constexpr since C++20
basic_string& assign(const value_type* s, size_type n); // constexpr since C++20
basic_string& assign(const value_type* s); // constexpr since C++20
+ basic_string& assign(const value_type* s, size_type pos, size_type n); // constexpr since C++20
basic_string& assign(size_type n, value_type c); // constexpr since C++20
template<class InputIterator>
basic_string& assign(InputIterator first, InputIterator last); // constexpr since C++20
@@ -1342,6 +1344,8 @@ public:
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(const value_type* __s, size_type __n)
_LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero");
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s);
+ _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(const value_type* __s, size_type __pos, size_type __n)
+ _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero");
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(size_type __n, value_type __c);
template <class _InputIterator, __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value, int> = 0>
@@ -1468,6 +1472,8 @@ public:
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& assign(const value_type* __s, size_type __n)
_LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero");
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& assign(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s);
+ _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& assign(const value_type* __s, size_type __pos, size_type __n)
+ _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero");
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& assign(size_type __n, value_type __c);
template <class _InputIterator, __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value, int> = 0>
@@ -2757,6 +2763,13 @@ basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_ty
return (__builtin_constant_p(__n) && __fits_in_sso(__n)) ? __assign_short(__s, __n) : __assign_external(__s, __n);
}
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_type __pos, size_type __n) {
+ _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::assign received nullptr");
+ return assign(__self_view(__s).substr(__pos, __n));
+}
+
template <class _CharT, class _Traits, class _Allocator>
_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::assign(size_type __n, value_type __c) {
@@ -3024,6 +3037,13 @@ basic_string<_CharT, _Traits, _Allocator>::append(const value_type* __s) {
return append(__s, traits_type::length(__s));
}
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::append(const value_type* __s, size_type __pos, size_type __n) {
+ _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::append received nullptr");
+ return append(__self_view(__s).substr(__pos, __n));
+}
+
// insert
template <class _CharT, class _Traits, class _Allocator>
diff --git a/libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp b/libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp
index d51ce1fcf2ba6..bdb3189b7fd35 100644
--- a/libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp
@@ -25,8 +25,12 @@ void func() {
std::string str2(np, std::allocator<char>{}); // expected-warning {{null passed}}
str2 = np; // expected-warning {{null passed}}
str2 += np; // expected-warning {{null passed}}
+ str2.assign(np, 1); // expected-warning {{null passed}}
str2.assign(np); // expected-warning {{null passed}}
+ str2.assign(np, 0, 1); // expected-warning {{null passed}}
+ str2.append(np, 1); // expected-warning {{null passed}}
str2.append(np); // expected-warning {{null passed}}
+ str2.append(np, 0, 1); // expected-warning {{null passed}}
str2.insert(0, np); // expected-warning {{null passed}}
str2.find(np); // expected-warning {{null passed}}
str2.rfind(np); // expected-warning {{null passed}}
diff --git a/libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer_size_size.pass.cpp b/libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer_size_size.pass.cpp
new file mode 100644
index 0000000000000..e70a11fced589
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer_size_size.pass.cpp
@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// constexpr basic_string& append(const charT* s, size_type pos, size_type n);
+
+#include <cassert>
+#include <stdexcept>
+#include <string>
+
+#include "asan_testing.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+template <class S>
+TEST_CONSTEXPR_CXX20 void
+test(S s, const typename S::value_type* str, typename S::size_type pos, typename S::size_type n, S expected) {
+ typename S::size_type str_len = S::traits_type::length(str);
+ if (pos <= str_len) {
+ s.append(str, pos, n);
+ LIBCPP_ASSERT(s.__invariants());
+ assert(s == expected);
+ LIBCPP_ASSERT(is_string_asan_correct(s));
+ }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+ else if (!TEST_IS_CONSTANT_EVALUATED) {
+ try {
+ s.append(str, pos, n);
+ assert(false);
+ } catch (std::out_of_range&) {
+ assert(pos > str_len);
+ }
+ }
+#endif
+}
+
+template <class S>
+TEST_CONSTEXPR_CXX20 void test_string() {
+ test(S(), "", 0, 0, S());
+ test(S(), "", 1, 0, S("not happening"));
+ test(S(), "12345", 0, 3, S("123"));
+ test(S(), "12345", 1, 4, S("2345"));
+ test(S(), "12345", 3, 15, S("45"));
+ test(S(), "12345", 5, 15, S(""));
+ test(S(), "12345", 6, 15, S("not happening"));
+ test(S(), "12345678901234567890", 0, 0, S());
+ test(S(), "12345678901234567890", 1, 1, S("2"));
+ test(S(), "12345678901234567890", 2, 3, S("345"));
+ test(S(), "12345678901234567890", 12, 13, S("34567890"));
+ test(S(), "12345678901234567890", 21, 13, S("not happening"));
+
+ test(S("12345"), "", 0, 0, S("12345"));
+ test(S("12345"), "12345", 2, 2, S("1234534"));
+ test(S("12345"), "1234567890", 0, 100, S("123451234567890"));
+
+ test(S("12345678901234567890"), "", 0, 0, S("12345678901234567890"));
+ test(S("12345678901234567890"), "12345", 1, 3, S("12345678901234567890234"));
+ test(S("12345678901234567890"), "12345678901234567890", 5, 10, S("123456789012345678906789012345"));
+
+ // Starting from long string (no SSO)
+ test(S("123456789012345678901234567890"), "", 0, 0, S("123456789012345678901234567890"));
+ test(S("123456789012345678901234567890"), "12345", 1, 3, S("123456789012345678901234567890234"));
+ test(S("123456789012345678901234567890"),
+ "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890",
+ 5,
+ 10,
+ S("1234567890123456789012345678906789012345"));
+}
+
+TEST_CONSTEXPR_CXX20 bool test() {
+ test_string<std::string>();
+#if TEST_STD_VER >= 11
+ test_string<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
+ test_string<std::basic_string<char, std::char_traits<char>, safe_allocator<char>>>();
+ test_string<std::basic_string<char, std::char_traits<char>, limited_allocator<char, 96>>>();
+#endif
+
+ return true;
+}
+
+int main(int, char**) {
+ test();
+#if TEST_STD_VER >= 20
+ static_assert(test());
+#endif
+
+ return 0;
+}
diff --git a/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/pointer_size_size.pass.cpp b/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/pointer_size_size.pass.cpp
new file mode 100644
index 0000000000000..a6ba75653d1a4
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/pointer_size_size.pass.cpp
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// constexpr basic_string& assign(const charT* s, size_type pos, size_type n);
+
+#include <cassert>
+#include <stdexcept>
+#include <string>
+
+#include "asan_testing.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+template <class S>
+TEST_CONSTEXPR_CXX20 void
+test(S s, const typename S::value_type* str, typename S::size_type pos, typename S::size_type n, S expected) {
+ typename S::size_type str_len = S::traits_type::length(str);
+ if (pos <= str_len) {
+ s.assign(str, pos, n);
+ LIBCPP_ASSERT(s.__invariants());
+ assert(s == expected);
+ LIBCPP_ASSERT(is_string_asan_correct(s));
+ }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+ else if (!TEST_IS_CONSTANT_EVALUATED) {
+ try {
+ s.assign(str, pos, n);
+ assert(false);
+ } catch (std::out_of_range&) {
+ assert(pos > str_len);
+ }
+ }
+#endif
+}
+
+template <class S>
+TEST_CONSTEXPR_CXX20 void test_string() {
+ test(S(), "", 0, 0, S());
+ test(S(), "", 1, 0, S("not happening"));
+ test(S(), "12345", 0, 3, S("123"));
+ test(S(), "12345", 1, 4, S("2345"));
+ test(S(), "12345", 3, 15, S("45"));
+ test(S(), "12345", 5, 15, S(""));
+ test(S(), "12345", 6, 15, S("not happening"));
+ test(S(), "12345678901234567890", 0, 0, S());
+ test(S(), "12345678901234567890", 1, 1, S("2"));
+ test(S(), "12345678901234567890", 2, 3, S("345"));
+ test(S(), "12345678901234567890", 12, 13, S("34567890"));
+ test(S(), "12345678901234567890", 21, 13, S("not happening"));
+
+ test(S("12345"), "", 0, 0, S());
+ test(S("12345"), "12345", 2, 2, S("34"));
+ test(S("12345"), "1234567890", 0, 100, S("1234567890"));
+
+ test(S("12345678901234567890"), "", 0, 0, S());
+ test(S("12345678901234567890"), "12345", 1, 3, S("234"));
+ test(S("12345678901234567890"),
+ "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890",
+ 5,
+ 10,
+ S("6789012345"));
+}
+
+TEST_CONSTEXPR_CXX20 bool test() {
+ test_string<std::string>();
+#if TEST_STD_VER >= 11
+ test_string<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
+ test_string<std::basic_string<char, std::char_traits<char>, safe_allocator<char>>>();
+ test_string<std::basic_string<char, std::char_traits<char>, limited_allocator<char, 33>>>();
+#endif
+
+ return true;
+}
+
+int main(int, char**) {
+ test();
+#if TEST_STD_VER >= 20
+ static_assert(test());
+#endif
+
+ return 0;
+}
More information about the libcxx-commits
mailing list