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

From cppreference.com
C++
Strings library
std::basic_string_view
Defined in header <string_view>
(1)
template < class CharT, class Traits >

constexpr bool operator== ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr bool operator== (
std::basic_string_view <CharT,Traits> lhs,

std::type_identity_t < std::basic_string_view <CharT,Traits>> rhs ) noexcept ;
(since C++20)
template < class CharT, class Traits >

constexpr bool operator! = ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(2) (since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr bool operator< ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(3) (since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr bool operator<= ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(4) (since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr bool operator> ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(5) (since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr bool operator>= ( std::basic_string_view <CharT,Traits> lhs,

std::basic_string_view <CharT,Traits> rhs ) noexcept ;
(6) (since C++17)
(until C++20)
template < class CharT, class Traits >

constexpr /*comp-cat*/ operator<=> (
std::basic_string_view <CharT,Traits> lhs,

std::type_identity_t < std::basic_string_view <CharT,Traits>> rhs ) noexcept ;
(7) (since C++20)

Compares two views.

All comparisons are done via the compare() member function (which itself is defined in terms of Traits::compare()):

  • Two views are equal if both the size of lhs and rhs are equal and each character in lhs has an equivalent character in rhs
  • The ordering comparisons are done lexicographically – the comparison is performed by a function equivalent to std::lexicographical_compare

The implementation provides sufficient additional constexpr and noexcept overloads of these functions so that a basic_string_view<CharT,Traits> object sv may be compared to another object t with an implicit conversion to basic_string_view<CharT,Traits>, with semantics identical to comparing sv and basic_string_view<CharT,Traits>(t)

(until C++20)

The return type of three-way comparison operators (/*comp-cat*/) is Traits::comparison_category if that qualified-id 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 - views to compare

Return value

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

Complexity

Linear in the size of the views.

Notes

Sufficient additional overloads can be implemented through non-deduced context in one parameter type.

(until C++20)

Three-way comparison result type of std::string_view, std::wstring_view, std::u8string_view, std::u16string_view and std::u32string_view is std::strong_ordering

std::type_identity_t is used for non-deduced context, which makes arguments that implicitly convertible to the string view type comparable with the string view.

(since C++20)

Example

Run this code
#include <string_view>
 
int main()
{
    using namespace std::literals;
 
    static_assert(""sv == ""sv);
 
    static_assert(""sv == "", "Selects an additional overload until C++20.");
 
    static_assert("" == ""sv, "Selects an additional overload until C++20."
                              "Uses a rewritten candidate since C++20.");
 
    static_assert(!(""sv != ""sv), "Uses the rewritten candidate since C++20.");
 
    static_assert(!(""sv != ""), "Selects an additional overload until C++20;"
                                 "Uses a rewritten candidate since C++20.");
 
    static_assert(!("" != ""sv), "Selects an additional overload until C++20."
                                 "Uses a rewritten candidate since C++20.");
}

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 3432 C++20 the return type of operator<=> was not required to be a comparison category type required
LWG 3950 C++20 redundant additional overloads were still required overload sets reduced
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