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

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
Non-member functions
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 conversion (C++11)
Helper classes
Deduction guides(C++17)
 
Defined in header <string>
Compare two basic_string objects
(1)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
template< class CharT, class Traits, class Alloc >

constexpr bool
    operator==( const std::basic_string<CharT,Traits,Alloc>& lhs,

                const std::basic_string<CharT,Traits,Alloc>& rhs ) noexcept;
(since C++20)
(2)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
(3)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
(4)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
(5)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
(6)
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 );
(until 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 ) noexcept;
(since C++11)
(until C++20)
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
(8)
template< class CharT, class Traits, class Alloc >

bool operator==( const std::basic_string<CharT,Traits,Alloc>& lhs,

                 const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

constexpr bool operator==( const std::basic_string<CharT,Traits,Alloc>& lhs,

                           const CharT* rhs );
(since C++20)
template< class CharT, class Traits, class Alloc >

bool operator==( const CharT* lhs,

                 const std::basic_string<CharT,Traits,Alloc>& rhs );
(until C++20)
(9)
template< class CharT, class Traits, class Alloc >

bool operator!=( const std::basic_string<CharT,Traits,Alloc>& lhs,

                 const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

bool operator!=( const CharT* lhs,

                 const std::basic_string<CharT,Traits,Alloc>& rhs );
(until C++20)
(10)
template< class CharT, class Traits, class Alloc >

bool operator<( const std::basic_string<CharT,Traits,Alloc>& lhs,

                const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

bool operator<( const CharT* lhs,

                const std::basic_string<CharT,Traits,Alloc>& rhs );
(until C++20)
(11)
template< class CharT, class Traits, class Alloc >

bool operator<=( const std::basic_string<CharT,Traits,Alloc>& lhs,

                 const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

bool operator<=( const CharT* lhs,

                 const std::basic_string<CharT,Traits,Alloc>& rhs );
(until C++20)
(12)
template< class CharT, class Traits, class Alloc >

bool operator>( const std::basic_string<CharT,Traits,Alloc>& lhs,

                const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

bool operator>( const CharT* lhs,

                const std::basic_string<CharT,Traits,Alloc>& rhs );
(until C++20)
(13)
template< class CharT, class Traits, class Alloc >

bool operator>=( const std::basic_string<CharT,Traits,Alloc>& lhs,

                 const CharT* rhs );
(until C++20)
template< class CharT, class Traits, class Alloc >

bool operator>=( const CharT* lhs,

                 const std::basic_string<CharT,Traits,Alloc>& rhs );
(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 );
(14) (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_compare or std::lexicographical_compare_three_way (since C++20).

The return type of three-way comparison operators (/*comp-cat*/) is Traits::comparison_category if it exists, std::weak_ordering otherwise.

(since C++20)
1-7) Compares two basic_string objects.
8-14) Compares a basic_string object and a null-terminated array of CharT.

Contents

[edit] Parameters

lhs, rhs - strings whose contents to compare

[edit] Return value

1-6,8-13) true if the corresponding comparison holds, false otherwise.
7,14) static_cast</*comp-cat*/>(lhs.compare(rhs) <=> 0).

[edit] Complexity

Linear in the size of the strings.

[edit] 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)

[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
made consistent;
noexcept removed as UB may be raised