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

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::tuple
Defined in header <tuple>
template < class ... TTypes , class ... UTypes >

bool operator == ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(1) (since C++11)
(constexpr since C++14)
template < class ... TTypes , class ... UTypes >

bool operator ! = ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(2) (since C++11)
(constexpr since C++14)
(until C++20)
template < class ... TTypes , class ... UTypes >

bool operator < ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(3) (since C++11)
(constexpr since C++14)
(until C++20)
template < class ... TTypes , class ... UTypes >

bool operator <= ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(4) (since C++11)
(constexpr since C++14)
(until C++20)
template < class ... TTypes , class ... UTypes >

bool operator > ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(5) (since C++11)
(constexpr since C++14)
(until C++20)
template < class ... TTypes , class ... UTypes >

bool operator >= ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(6) (since C++11)
(constexpr since C++14)
(until C++20)
template < class ... TTypes , class ... UTypes >

constexpr std:: common_comparison_category_t <
synth - three - way - result < TTypes, Elems > ... >
operator <=> ( const std:: tuple < TTypes... > & lhs,

const std:: tuple < UTypes... > & rhs ) ;
(7) (since C++20)
template < class ... TTypes , tuple - like UTuple >
constexpr bool operator == ( const tuple < TTypes... > & lhs, const UTuple & rhs ) ;
(8) (since C++23)
template < class ... TTypes , tuple - like UTuple >

constexpr std:: common_comparison_category_t <
synth - three - way - result < TTypes, /* Elems */ > ... >

operator <=> ( const tuple < TTypes... > & lhs, const UTuple & rhs ) ;
(9) (since C++23)
1,2) Compares every element of the tuple lhs with the corresponding element of the tuple rhs by operator == .
1) Returns true if all pairs of corresponding elements are equal.
2) Returns ! ( lhs == rhs ) .
If sizeof... ( TTypes ) does not equal sizeof... ( UTypes ) , or std :: get < i > ( lhs ) == std :: get < i > ( rhs ) is not a valid expression for any i in [ ​ 0 ​ , sizeof... ( Types ) ) , the program is ill-formed.
If the type and value category of std :: get < i > ( lhs ) == std :: get < i > ( rhs ) do not meet the BooleanTestable requirements for any i in [ ​ 0 ​ , sizeof... ( Types ) ) , the behavior is undefined.
(until C++26)
This overload participates in overload resolution only if sizeof... ( TTypes ) equals sizeof... ( UTypes ) , std :: get < i > ( lhs ) == std :: get < i > ( rhs ) is a valid expression and decltype ( std :: get < i > ( lhs ) == std :: get < i > ( rhs ) ) model boolean-testable for every i in [ ​ 0 ​ , sizeof... ( Types ) ) .
(since C++26)
3-6) Compares lhs and rhs lexicographically by operator < , that is, compares the first elements, if they are equivalent, compares the second elements, if those are equivalent, compares the third elements, and so on.
3) For empty tuples, returns false . For non-empty tuples, the effect is equivalent to
if ( std :: get < 0 > ( lhs ) < std :: get < 0 > ( rhs ) ) return true ;

if ( std :: get < 0 > ( rhs ) < std :: get < 0 > ( lhs ) ) return false ;
if ( std :: get < 1 > ( lhs ) < std :: get < 1 > ( rhs ) ) return true ;
if ( std :: get < 1 > ( rhs ) < std :: get < 1 > ( lhs ) ) return false ;
...

return std :: get < N - 1 > ( lhs ) < std :: get < N - 1 > ( rhs ) ;
4) Returns ! ( rhs < lhs ) .
5) Returns rhs < lhs .
6) Returns ! ( lhs < rhs ) .
If sizeof... ( TTypes ) does not equal sizeof... ( UTypes ) , or any of the comparison expression shown in the equivalent-to statements is not a valid expression, the program is ill-formed.
If the type and value category of any of the comparison expression shown in the equivalent-to statements do not meet the BooleanTestable requirements, the behavior is undefined.
7) Compares lhs and rhs lexicographically by synth-three-way , that is, compares the first elements, if they are equivalent, compares the second elements, if those are equivalent, compares the third elements, and so on.

if ( auto c = synth-three-way ( std :: get < 0 > ( lhs ) , std :: get < 0 > ( rhs ) ) ; c ! = 0 ) return c ;
if ( auto c = synth-three-way ( std :: get < 1 > ( lhs ) , std :: get < 1 > ( rhs ) ) ; c ! = 0 ) return c ;
...
return synth-three-way ( std :: get < N - 1 > ( lhs ) , std :: get < N - 1 > ( rhs ) ) ;

8) Same as (1) , except that rhs is a tuple-like object, and the number of elements of rhs is determined by std:: tuple_size_v < UTuple > instead. This overload can only be found via argument-dependent lookup .
9) Same as (7) , except that rhs is a tuple-like object. /* Elems */ denotes the pack of types std:: tuple_element_t < i, UTuple > for each i in [ ​ 0 ​ , std:: tuple_size_v < UTuple > ) in increasing order. This overload can only be found via argument-dependent lookup .

All comparison operators are short-circuited; they do not access tuple elements beyond what is necessary to determine the result of the comparison.

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

(since C++20)

Parameters

lhs, rhs - tuples to compare

Return value

1,8) true if std :: get < i > ( lhs ) == std :: get < i > ( rhs ) for all i in [ ​ 0 ​ , sizeof... ( Types ) ) , otherwise false . For two empty tuples returns true .
2) ! ( lhs == rhs )
3) true if the first non-equivalent element in lhs is less than the one in rhs , false if the first non-equivalent element in rhs is less than the one in lhs or there is no non-equivalent element. For two empty tuples, returns false .
4) ! ( rhs < lhs )
5) rhs < lhs
6) ! ( lhs < rhs )
7,9) The relation between the first pair of non-equivalent elements if there is any, std::strong_ordering::equal otherwise. For two empty tuples, returns std::strong_ordering::equal .

Notes

The relational operators are defined in terms of each element's operator < .

(until C++20)

The relational operators are defined in terms of synth-three-way , which uses operator <=> if possible, or operator < otherwise.

Notably, if an element type does not itself provide operator <=> , but is implicitly convertible to a three-way comparable type, that conversion will be used instead of operator < .

(since C++20)
Feature-test macro Value Std Feature
__cpp_lib_constrained_equality 202403L (C++26) Constrained operator == for std::tuple

Example

Because operator < is defined for tuples, containers of tuples can be sorted.

Run this code 
#include <algorithm>
#include <iostream>
#include <tuple>
#include <vector>
 
int main()
{
    std::vector<std::tuple<int, std::string, float>> v
    {
        {2, "baz", -0.1},
        {2, "bar", 3.14},
        {1, "foo", 10.1},
        {2, "baz", -1.1},
    };
    std::sort(v.begin(), v.end());
 
    for (const auto& p: v)
        std::cout << "{ " << get<0>(p)
                  << ", " << get<1>(p)
                  << ", " << get<2>(p)
                  << " }\n";
}

Output: 

{ 1, foo, 10.1 }
{ 2, bar, 3.14 }
{ 2, baz, -1.1 }
{ 2, baz, -0.1 }

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 2114
( P2167R3 ) 		C++11 type preconditions for boolean operations were missing added

See also

(removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (C++20)
lexicographically compares the values in the pair
(function template)
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