operator==,!=,<,<=,>,>=,<=>(std::basic_string) - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| Compare two basic_string objects | ||
| template< class CharT, class Traits, class Alloc > bool operator==( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (1) | (noexcept since C++11)(constexpr since C++20) |
| template< class CharT, class Traits, class Alloc > bool operator!=( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (2) | (until C++20) (noexcept since C++11) |
| template< class CharT, class Traits, class Alloc > bool operator<( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (3) | (until C++20) (noexcept since C++11) |
| template< class CharT, class Traits, class Alloc > bool operator<=( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (4) | (until C++20) (noexcept since C++11) |
| template< class CharT, class Traits, class Alloc > bool operator>( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (5) | (until C++20) (noexcept since C++11) |
| template< class CharT, class Traits, class Alloc > bool operator>=( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (6) | (until C++20) (noexcept since C++11) |
| template< class CharT, class Traits, class Alloc > constexpr /*comp-cat*/ operator<=>( const std::basic_string<CharT,Traits,Alloc>& lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ) noexcept; | (7) | (since C++20) |
| Compare a basic_string object and null-terminated array of T | ||
| template< class CharT, class Traits, class Alloc > bool operator==( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (8) | (constexpr since C++20) |
| template< class CharT, class Traits, class Alloc > bool operator==( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (9) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator!=( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (10) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator!=( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (11) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator<( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (12) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator<( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (13) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator<=( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (14) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator<=( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (15) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator>( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (16) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator>( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (17) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator>=( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (18) | (until C++20) |
| template< class CharT, class Traits, class Alloc > bool operator>=( const CharT* lhs, const std::basic_string<CharT,Traits,Alloc>& rhs ); | (19) | (until C++20) |
| template< class CharT, class Traits, class Alloc > constexpr /*comp-cat*/ operator<=>( const std::basic_string<CharT,Traits,Alloc>& lhs, const CharT* rhs ); | (20) | (since C++20) |
Compares the contents of a string with another string or a null-terminated array of CharT.
All comparisons are done via the compare() member function (which itself is defined in terms of Traits::compare()):
- Two strings are equal if both the size of lhs and rhs are equal and each character in lhs has equivalent character in rhs at the same position.
- The ordering comparisons are done lexicographically – the comparison is performed by a function equivalent to std::lexicographical_compareor std::lexicographical_compare_three_way(since C++20).
1-7) Compares two basic_string objects.
8-20) Compares a basic_string object and a null-terminated array of CharT.
| The return type of three-way comparison operators (/*comp-cat*/) is Traits::comparison_category if that qualified-id exists and denotes a type, std::weak_ordering otherwise. If /*comp-cat*/ is not a comparison category type, the program is ill-formed.The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively. | (since C++20) |
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[edit] Parameters
| lhs, rhs | - | strings whose contents to compare |
|---|
[edit] Return value
1-6,8-19) true if the corresponding comparison holds, false otherwise.
7,20) static_cast</*comp-cat*/>(lhs.compare(rhs) <=> 0).
[edit] Complexity
Linear in the size of the strings.
[edit] Notes
[edit] Example
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2064 | C++11 | whether overloads taking two basic_strings are noexcept was inconsistent;overloads taking a CharT* were noexcept but might raise UB | made consistent;noexcept removed |
| LWG 3432 | C++20 | the return type of operator<=> was not required to be a comparison category type | required |