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[libc++][char_traits] Applied [[nodiscard]] #172244

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Merged
frederick-vs-ja merged 1 commit into from
Dec 15, 2025
+206 −31
Merged

[libc++][char_traits] Applied [[nodiscard]] #172244

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75 changes: 44 additions & 31 deletions libcxx/include/__string/char_traits.h
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Original file line number Diff line number Diff line change
Expand Up @@ -94,16 +94,17 @@ struct char_traits<char> {
}

// TODO: Make this _LIBCPP_HIDE_FROM_ABI
static inline _LIBCPP_HIDDEN _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDDEN _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT {
return __c1 == __c2;
}
static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool
lt(char_type __c1, char_type __c2) _NOEXCEPT {
return (unsigned char)__c1 < (unsigned char)__c2;
}

// __constexpr_memcmp requires a trivially lexicographically comparable type, but char is not when char is a signed
// type
static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 int
compare(const char_type* __lhs, const char_type* __rhs, size_t __count) _NOEXCEPT {
if (__libcpp_is_constant_evaluated()) {
#ifdef _LIBCPP_COMPILER_CLANG_BASED
Expand All @@ -126,11 +127,12 @@ struct char_traits<char> {
}
}

static inline _LIBCPP_HIDE_FROM_ABI size_t _LIBCPP_CONSTEXPR_SINCE_CXX17 length(const char_type* __s) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI size_t _LIBCPP_CONSTEXPR_SINCE_CXX17
length(const char_type* __s) _NOEXCEPT {
return std::__constexpr_strlen(__s);
}

static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
[[__nodiscard__]] static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
return std::__constexpr_memchr(__s, __a, __n);
}
Expand All @@ -154,19 +156,24 @@ struct char_traits<char> {
return __s;
}

static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type not_eof(int_type __c) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type not_eof(int_type __c) _NOEXCEPT {
return eq_int_type(__c, eof()) ? ~eof() : __c;
}
static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR char_type
to_char_type(int_type __c) _NOEXCEPT {
return char_type(__c);
}
static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type
to_int_type(char_type __c) _NOEXCEPT {
return int_type((unsigned char)__c);
}
static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT {
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool
eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT {
return __c1 == __c2;
}
static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }
[[__nodiscard__]] static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT {
return int_type(EOF);
}
};

template <class _CharT, class _IntT, _IntT _EOFVal>
Expand All @@ -187,11 +194,11 @@ struct __char_traits_base {
__lhs = __rhs;
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool eq(char_type __lhs, char_type __rhs) _NOEXCEPT {
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool eq(char_type __lhs, char_type __rhs) _NOEXCEPT {
return __lhs == __rhs;
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool lt(char_type __lhs, char_type __rhs) _NOEXCEPT {
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool lt(char_type __lhs, char_type __rhs) _NOEXCEPT {
return __lhs < __rhs;
}

Expand All @@ -213,19 +220,22 @@ struct __char_traits_base {
return __str;
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT {
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT {
return char_type(__c);
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT { return int_type(__c); }
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT {
return int_type(__c);
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool eq_int_type(int_type __lhs, int_type __rhs) _NOEXCEPT {
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR bool
eq_int_type(int_type __lhs, int_type __rhs) _NOEXCEPT {
return __lhs == __rhs;
}

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return _EOFVal; }
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return _EOFVal; }

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type not_eof(int_type __c) _NOEXCEPT {
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR int_type not_eof(int_type __c) _NOEXCEPT {
return eq_int_type(__c, eof()) ? static_cast<int_type>(~eof()) : __c;
}
};
Expand All @@ -235,18 +245,19 @@ struct __char_traits_base {
#if _LIBCPP_HAS_WIDE_CHARACTERS
template <>
struct char_traits<wchar_t> : __char_traits_base<wchar_t, wint_t, static_cast<wint_t>(WEOF)> {
static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 int
compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
if (__n == 0)
return 0;
return std::__constexpr_wmemcmp(__s1, __s2, __n);
}

static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t length(const char_type* __s) _NOEXCEPT {
[[__nodiscard__]] static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t
length(const char_type* __s) _NOEXCEPT {
return std::__constexpr_wcslen(__s);
}

static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
[[__nodiscard__]] static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
return std::__constexpr_wmemchr(__s, __a, __n);
}
Expand All @@ -257,16 +268,16 @@ struct char_traits<wchar_t> : __char_traits_base<wchar_t, wint_t, static_cast<wi

template <>
struct char_traits<char8_t> : __char_traits_base<char8_t, unsigned int, static_cast<unsigned int>(EOF)> {
static _LIBCPP_HIDE_FROM_ABI constexpr int
[[nodiscard]] static _LIBCPP_HIDE_FROM_ABI constexpr int
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@frederick-vs-ja frederick-vs-ja Dec 15, 2025

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Note for review: Non-uglified nodiscard occurrences can only appear since C++20 currently, while other uglified __nodiscard__ occurrences can appear in C++03.

compare(const char_type* __s1, const char_type* __s2, size_t __n) noexcept {
return std::__constexpr_memcmp(__s1, __s2, __element_count(__n));
}

static _LIBCPP_HIDE_FROM_ABI constexpr size_t length(const char_type* __str) noexcept {
[[nodiscard]] static _LIBCPP_HIDE_FROM_ABI constexpr size_t length(const char_type* __str) noexcept {
return std::__constexpr_strlen(__str);
}

_LIBCPP_HIDE_FROM_ABI static constexpr const char_type*
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr const char_type*
find(const char_type* __s, size_t __n, const char_type& __a) noexcept {
return std::__constexpr_memchr(__s, __a, __n);
}
Expand All @@ -276,11 +287,11 @@ struct char_traits<char8_t> : __char_traits_base<char8_t, unsigned int, static_c

template <>
struct char_traits<char16_t> : __char_traits_base<char16_t, uint_least16_t, static_cast<uint_least16_t>(0xFFFF)> {
_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 int
compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t length(const char_type* __s) _NOEXCEPT;

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
__identity __proj;
const char_type* __match = std::__find(__s, __s + __n, __a, __proj);
Expand All @@ -290,7 +301,7 @@ struct char_traits<char16_t> : __char_traits_base<char16_t, uint_least16_t, stat
}
};

inline _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] inline _LIBCPP_CONSTEXPR_SINCE_CXX17 int
char_traits<char16_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
for (; __n; --__n, ++__s1, ++__s2) {
if (lt(*__s1, *__s2))
Expand All @@ -301,7 +312,8 @@ char_traits<char16_t>::compare(const char_type* __s1, const char_type* __s2, siz
return 0;
}

inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t char_traits<char16_t>::length(const char_type* __s) _NOEXCEPT {
[[__nodiscard__]] inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t
char_traits<char16_t>::length(const char_type* __s) _NOEXCEPT {
size_t __len = 0;
for (; !eq(*__s, char_type(0)); ++__s)
++__len;
Expand All @@ -310,11 +322,11 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t char_traits<char16_t>::length(const

template <>
struct char_traits<char32_t> : __char_traits_base<char32_t, uint_least32_t, static_cast<uint_least32_t>(0xFFFFFFFF)> {
_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 int
compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t length(const char_type* __s) _NOEXCEPT;

_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
__identity __proj;
const char_type* __match = std::__find(__s, __s + __n, __a, __proj);
Expand All @@ -324,7 +336,7 @@ struct char_traits<char32_t> : __char_traits_base<char32_t, uint_least32_t, stat
}
};

inline _LIBCPP_CONSTEXPR_SINCE_CXX17 int
[[__nodiscard__]] inline _LIBCPP_CONSTEXPR_SINCE_CXX17 int
char_traits<char32_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
for (; __n; --__n, ++__s1, ++__s2) {
if (lt(*__s1, *__s2))
Expand All @@ -335,7 +347,8 @@ char_traits<char32_t>::compare(const char_type* __s1, const char_type* __s2, siz
return 0;
}

inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t char_traits<char32_t>::length(const char_type* __s) _NOEXCEPT {
[[__nodiscard__]] inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t
char_traits<char32_t>::length(const char_type* __s) _NOEXCEPT {
size_t __len = 0;
for (; !eq(*__s, char_type(0)); ++__s)
++__len;
Expand Down
162 changes: 162 additions & 0 deletions libcxx/test/libcxx/strings/char.traits/nodiscard.verify.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,162 @@
//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

// Check that functions are marked [[nodiscard]]

#include <string>

#include "test_macros.h"

void test_char() {
typedef char char_t;
typedef std::char_traits<char_t> traits;
typedef typename traits::int_type int_t;

const char_t buf[1] = {};

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq(char_t(), char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::lt(char_t(), char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::compare(buf, buf, 0);
traits::length(buf); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::find(buf, 0, char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::not_eof(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_char_type(int_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_int_type(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq_int_type(int_t(), int_t());
traits::eof(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}

#ifndef TEST_HAS_NO_CHAR8_T
void test_char8_t() {
Comment on lines +44 to +45
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@frederick-vs-ja frederick-vs-ja Dec 15, 2025

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Note for review: I attempted to use a function template to handle five character types. However, the count of expected warnings varies in different modes, and it's infeasible to write the different counts in comments. As a result, I chose to repeat the function body 5 times.

typedef char8_t char_t;
typedef std::char_traits<char_t> traits;
typedef typename traits::int_type int_t;

const char_t buf[1] = {};

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq(char_t(), char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::lt(char_t(), char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::compare(buf, buf, 0);
traits::length(buf); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::find(buf, 0, char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::not_eof(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_char_type(int_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_int_type(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq_int_type(int_t(), int_t());
traits::eof(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}
#endif

void test_char16_t() {
typedef char16_t char_t;
typedef std::char_traits<char_t> traits;
typedef typename traits::int_type int_t;

const char_t buf[1] = {};

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq(char_t(), char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::lt(char_t(), char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::compare(buf, buf, 0);
traits::length(buf); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::find(buf, 0, char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::not_eof(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_char_type(int_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_int_type(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq_int_type(int_t(), int_t());
traits::eof(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}

void test_char32_t() {
typedef char32_t char_t;
typedef std::char_traits<char_t> traits;
typedef typename traits::int_type int_t;

const char_t buf[1] = {};

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq(char_t(), char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::lt(char_t(), char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::compare(buf, buf, 0);
traits::length(buf); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::find(buf, 0, char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::not_eof(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_char_type(int_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_int_type(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq_int_type(int_t(), int_t());
traits::eof(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}

#ifndef TEST_HAS_NO_WIDE_CHARACTERS
void test_wchar_t() {
typedef wchar_t char_t;
typedef std::char_traits<char_t> traits;
typedef typename traits::int_type int_t;

const char_t buf[1] = {};

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq(char_t(), char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::lt(char_t(), char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::compare(buf, buf, 0);
traits::length(buf); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::find(buf, 0, char_t());

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::not_eof(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_char_type(int_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::to_int_type(char_t());
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
traits::eq_int_type(int_t(), int_t());
traits::eof(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}
#endif
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