[libc++] Mark string functions as [[nodiscard]] #166524

philnik777 · 2025-11-05T09:42:52Z

This applies [[nodiscard]] according to our coding guidelines to basic_string.

github-actions · 2025-11-05T09:44:34Z

✅ With the latest revision this PR passed the C/C++ code formatter.

H-G-Hristov · 2025-11-06T05:20:33Z

@philnik777 I have a related off-topic question. The coding guidelines are quite clear about applying [[nodiscard]].
What about applying those at a scale to an already existing code? Was this already discussed and is it desirable? Is there a policy about that?
For example math functions should be marked [[nodiscard]]. I have prepared a PR for the saturation math functions, etc. Should I go ahead with revisiting mine and other older code and adding the attribute?

philnik777 · 2025-11-06T07:42:21Z

@philnik777 I have a related off-topic question. The coding guidelines are quite clear about applying [[nodiscard]]. What about applying those at a scale to an already existing code? Was this already discussed and is it desirable?

The entire reason for having the guideline was that we want to apply [[nodiscard]] throughout the code base. Unlike most coding guidelines this has a direct impact on our QoI, so yeah, it's desirable.

Is there a policy about that? For example math functions should be marked [[nodiscard]].

Some math functions should be marked [[nodiscard]].

I have prepared a PR for the saturation math functions, etc. Should I go ahead with revisiting mine and other older code and adding the attribute?

That would be great.

…card]]` ...according to Coding Guidelines: - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant - llvm#166524 (comment)

…card]]` (#166898) ...according to Coding Guidelines: *`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.* # References - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant - #166524 (comment) Co-authored-by: Hristo Hristov <[email protected]>

…as `[[nodiscard]]` (#166898) ...according to Coding Guidelines: *`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.* # References - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant - llvm/llvm-project#166524 (comment) Co-authored-by: Hristo Hristov <[email protected]>

llvmbot · 2025-11-12T09:19:25Z

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

This applies [[nodiscard]] according to our coding guidelines to basic_string.

Patch is 39.37 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/166524.diff

3 Files Affected:

(modified) libcxx/include/string (+103-80)
(modified) libcxx/test/libcxx/diagnostics/string.nodiscard.verify.cpp (+112-3)
(modified) libcxx/test/libcxx/strings/basic.string/nonnull.verify.cpp (+2)

diff --git a/libcxx/include/string b/libcxx/include/string
index ede42467b99fe..79da203cfdf6f 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1253,45 +1253,55 @@ public:
 #  endif
   _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& operator=(value_type __c);
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 iterator begin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 iterator begin() _NOEXCEPT {
     return __make_iterator(__get_pointer());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator begin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator begin() const _NOEXCEPT {
     return __make_const_iterator(__get_pointer());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 iterator end() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 iterator end() _NOEXCEPT {
     return __make_iterator(__get_pointer() + size());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator end() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator end() const _NOEXCEPT {
     return __make_const_iterator(__get_pointer() + size());
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reverse_iterator rbegin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reverse_iterator rbegin() _NOEXCEPT {
     return reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator rbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator
+  rbegin() const _NOEXCEPT {
     return const_reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reverse_iterator rend() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reverse_iterator rend() _NOEXCEPT {
     return reverse_iterator(begin());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator rend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator rend() const _NOEXCEPT {
     return const_reverse_iterator(begin());
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator cbegin() const _NOEXCEPT { return begin(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator cend() const _NOEXCEPT { return end(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator crbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator cbegin() const _NOEXCEPT {
+    return begin();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_iterator cend() const _NOEXCEPT {
+    return end();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator
+  crbegin() const _NOEXCEPT {
     return rbegin();
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator crend() const _NOEXCEPT { return rend(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator crend() const _NOEXCEPT {
+    return rend();
+  }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type size() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type size() const _NOEXCEPT {
     return __is_long() ? __get_long_size() : __get_short_size();
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type length() const _NOEXCEPT { return size(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type length() const _NOEXCEPT {
+    return size();
+  }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type max_size() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type max_size() const _NOEXCEPT {
     if (size_type __m = __alloc_traits::max_size(__alloc_); __m <= std::numeric_limits<size_type>::max() / 2) {
       size_type __res = __m - __alignment;
 
@@ -1309,7 +1319,7 @@ public:
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type capacity() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type capacity() const _NOEXCEPT {
     return (__is_long() ? __get_long_cap() : static_cast<size_type>(__min_cap)) - 1;
   }
 
@@ -1344,7 +1354,8 @@ public:
     return size() == 0;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __pos) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference
+  operator[](size_type __pos) const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
     if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
       return *(__get_long_pointer() + __pos);
@@ -1352,7 +1363,8 @@ public:
     return *(data() + __pos);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __pos) _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference
+  operator[](size_type __pos) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
     if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
       return *(__get_long_pointer() + __pos);
@@ -1360,8 +1372,8 @@ public:
     return *(__get_pointer() + __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference at(size_type __n) const;
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 reference at(size_type __n);
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference at(size_type __n) const;
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 reference at(size_type __n);
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& operator+=(const basic_string& __str) {
     return append(__str);
@@ -1473,22 +1485,22 @@ public:
   _LIBCPP_CONSTEXPR_SINCE_CXX20 void push_back(value_type __c);
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void pop_back();
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference front() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference front() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string::front(): string is empty");
     return *__get_pointer();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference front() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference front() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string::front(): string is empty");
     return *data();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference back() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference back() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string::back(): string is empty");
     return *(__get_pointer() + size() - 1);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference back() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference back() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string::back(): string is empty");
     return *(data() + size() - 1);
   }
@@ -1761,16 +1773,16 @@ public:
   _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type copy(value_type* __s, size_type __n, size_type __pos = 0) const;
 
 #  if _LIBCPP_STD_VER <= 20
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string
-  substr(size_type __pos = 0, size_type __n = npos) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string substr(size_type __pos = 0, size_type __n = npos) const {
     return basic_string(*this, __pos, __n);
   }
 #  else
-  _LIBCPP_HIDE_FROM_ABI constexpr basic_string substr(size_type __pos = 0, size_type __n = npos) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr basic_string substr(size_type __pos = 0, size_type __n = npos) const& {
     return basic_string(*this, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr basic_string substr(size_type __pos = 0, size_type __n = npos) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr basic_string substr(size_type __pos = 0, size_type __n = npos) && {
     return basic_string(std::move(*this), __pos, __n);
   }
 #  endif
@@ -1790,231 +1802,238 @@ public:
   // [string.ops]
   // ------------
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const value_type* c_str() const _NOEXCEPT { return data(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const value_type* data() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const value_type* c_str() const _NOEXCEPT {
+    return data();
+  }
+
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const value_type* data() const _NOEXCEPT {
     return std::__to_address(__get_pointer());
   }
 #  if _LIBCPP_STD_VER >= 17
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 value_type* data() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 value_type* data() _NOEXCEPT {
     return std::__to_address(__get_pointer());
   }
 #  endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 allocator_type get_allocator() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 allocator_type get_allocator() const _NOEXCEPT {
     return __alloc_;
   }
 
   // find
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __str.data(), __pos, __str.size());
   }
 
   template <class _Tp, __enable_if_t<__can_be_converted_to_string_view_v<_CharT, _Traits, _Tp>, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT {
     __self_view __sv = __t;
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __sv.data(), __pos, __sv.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type find(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  find(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::find(): received nullptr");
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string::find(): received nullptr");
     return std::__str_find<value_type, size_type, traits_type, npos>(
         data(), size(), __s, __pos, traits_type::length(__s));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type find(value_type __c, size_type __pos = 0) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type find(value_type __c, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
   // rfind
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   rfind(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_rfind<value_type, size_type, traits_type, npos>(
         data(), size(), __str.data(), __pos, __str.size());
   }
 
   template <class _Tp, __enable_if_t<__can_be_converted_to_string_view_v<_CharT, _Traits, _Tp>, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   rfind(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT {
     __self_view __sv = __t;
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __sv.data(), __pos, __sv.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type rfind(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  rfind(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::rfind(): received nullptr");
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   rfind(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = npos) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string::rfind(): received nullptr");
     return std::__str_rfind<value_type, size_type, traits_type, npos>(
         data(), size(), __s, __pos, traits_type::length(__s));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type rfind(value_type __c, size_type __pos = npos) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  rfind(value_type __c, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
   // find_first_of
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(
         data(), size(), __str.data(), __pos, __str.size());
   }
 
   template <class _Tp, __enable_if_t<__can_be_converted_to_string_view_v<_CharT, _Traits, _Tp>, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_of(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT {
     __self_view __sv = __t;
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(
         data(), size(), __sv.data(), __pos, __sv.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::find_first_of(): received nullptr");
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_of(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string::find_first_of(): received nullptr");
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s, __pos, traits_type::length(__s));
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_of(value_type __c, size_type __pos = 0) const _NOEXCEPT {
     return find(__c, __pos);
   }
 
   // find_last_of
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_last_of(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(
         data(), size(), __str.data(), __pos, __str.size());
   }
 
   template <class _Tp, __enable_if_t<__can_be_converted_to_string_view_v<_CharT, _Traits, _Tp>, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_last_of(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT {
     __self_view __sv = __t;
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(
         data(), size(), __sv.data(), __pos, __sv.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_last_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::find_last_of(): received nullptr");
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_last_of(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = npos) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string::find_last_of(): received nullptr");
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s, __pos, traits_type::length(__s));
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_last_of(value_type __c, size_type __pos = npos) const _NOEXCEPT {
     return rfind(__c, __pos);
   }
 
   // find_first_not_of
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_not_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(
         data(), size(), __str.data(), __pos, __str.size());
   }
 
   template <class _Tp, __enable_if_t<__can_be_converted_to_string_view_v<_CharT, _Traits, _Tp>, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_not_of(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT {
     __self_view __sv = __t;
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(
         data(), size(), __sv.data(), __pos, __sv.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_not_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string::find_first_not_of(): received nullptr");
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
   find_first_not_of(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string::find_first_not_of(): received nullptr");
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s, __pos, traits_type::lengt...
[truncated]

This applies `[[nodiscard]]` according to our coding guidelines to `basic_string`.

philnik777 force-pushed the string_nodiscard branch from 55eebcd to 93f64f1 Compare November 6, 2025 07:39

H-G-Hristov mentioned this pull request Nov 7, 2025

[libc++][numeric] Marked saturation artithmetic functions as [[nodiscard]] #166898

Merged

[libc++] Mark string functions as [[nodiscard]]

761b111

philnik777 force-pushed the string_nodiscard branch from 93f64f1 to 761b111 Compare November 10, 2025 10:18

philnik777 marked this pull request as ready for review November 12, 2025 09:18

philnik777 requested a review from a team as a code owner November 12, 2025 09:18

philnik777 merged commit 36c1273 into llvm:main Nov 12, 2025
81 checks passed

philnik777 deleted the string_nodiscard branch November 12, 2025 09:19

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Nov 12, 2025

git-crd pushed a commit to git-crd/crd-llvm-project that referenced this pull request Nov 13, 2025

[libc++] Mark string functions as [[nodiscard]] (llvm#166524)

309b078

This applies `[[nodiscard]]` according to our coding guidelines to `basic_string`.

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[libc++] Mark string functions as [[nodiscard]] #166524

[libc++] Mark string functions as [[nodiscard]] #166524

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[libc++] Mark string functions as [[nodiscard]] #166524

[libc++] Mark string functions as [[nodiscard]] #166524

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