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

 

C++

Compiler support

Freestanding and hosted

Language

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)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

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

Member functions basic_string::basic_string basic_string::operator= basic_string::assign basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (C++11) basic_string::back (C++11) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (C++11)   Operations basic_string::clear basic_string::insert basic_string::erase basic_string::push_back basic_string::pop_back (C++11) basic_string::append basic_string::operator+= basic_string::compare basic_string::starts_with (C++20)   basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::replace basic_string::substr basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23) basic_string::swap

Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Constants basic_string::npos Deduction guides (C++17) Non-member functions operator+ swap(std::basic_string) operator""s (C++14) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline 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 stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11) stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Helper classes hash<std::string>hash<std::wstring>hash<std::u32string>hash<std::u16string>hash<std::u8string> (C++11)  (C++11)(C++11)(C++11)(C++20) hash<std::pmr::string>hash<std::pmr::wstring>hash<std::pmr::u32string>hash<std::pmr::u16string>hash<std::pmr::u8string> (C++17)(C++17)(C++17)(C++17)(C++20)

 

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).

1-7) Compares two basic_string objects.

8-14) 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 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)

Parameters

lhs, rhs

-

strings whose contents to compare

Return value

1-6,8-13) true if the corresponding comparison holds, false otherwise.

7,14) 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)

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