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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string_view

Member functions basic_string_view::basic_string_view    basic_string_view::operator= Iterators basic_string_view::beginbasic_string_view::cbegin basic_string_view::endbasic_string_view::cend basic_string_view::rbeginbasic_string_view::crbegin basic_string_view::rendbasic_string_view::crend Element access basic_string_view::at basic_string_view::operator[] basic_string_view::front basic_string_view::back basic_string_view::data Capacity basic_string_view::sizebasic_string_view::length basic_string_view::max_size basic_string_view::empty Modifiers basic_string_view::remove_prefix basic_string_view::remove_suffix basic_string_view::swap

Operations basic_string_view::copy basic_string_view::substr basic_string_view::compare basic_string_view::starts_with (C++20) basic_string_view::ends_with (C++20) basic_string_view::contains (C++23) basic_string_view::find basic_string_view::rfind basic_string_view::find_first_of basic_string_view::find_last_of basic_string_view::find_first_not_of basic_string_view::find_last_not_of Constants basic_string_view::npos Non-member functions operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) operator<< operator""sv Helper classes hash<std::string_view>hash<std::wstring_view>hash<std::u8string_view>hash<std::u16string_view>hash<std::u32string_view> (C++20) Deduction guides(C++20)

 

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 at the same position.

• 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*/ is not a comparison category type, the program is ill-formed. The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively.	(since C++20)

Parameters

lhs, rhs

-

views to compare

Return value

1-6) true if the corresponding comparison holds, false otherwise.

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