operator==,!=,<,<=,>,>=,<=>(std::basic_string)

From cppreference.com
< cpp‎ | string‎ | basic string
C++
Strings library
std::basic_string
Constants
Non-member functions
(C++20)(C++20)
I/O
Comparison
operator==operator!=operator<operator>operator<=operator>=operator<=>
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20) (C++20)
Numeric conversions
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Literals
(C++14)
Helper classes
(C++11)
Deduction guides (C++17)
Defined in header <string>
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
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*/

The <, <=, >, >=, and != operators are synthesized from operator<=> and operator==

(since C++20)

Parameters

lhs, rhs - strings whose contents to compare

Return value

1-6,8-19) true if the corresponding comparison holds, false
7,20) static_cast < /*comp-cat*/ > (lhs.compare (rhs) <=> 0 )

Complexity

Linear in the size of the strings.

Notes

If at least one parameter is of type std::string, std::wstring, std::u8string, std::u16string, or std::u32string, the return type of operator<=> is std::strong_ordering

(since C++20)

Example

This section is incomplete
Reason: no example

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
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/string/basic_string/operator_cmp&oldid=171034