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

 

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)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Visitation

variant::visit (C++26)

Non-member functions

visit(std::variant)

holds_alternative

get(std::variant)

get_if

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

swap(std::variant)

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash<std::variant>

Helper objects

variant_npos

 

Defined in header <variant>
template< class... Types > constexpr bool operator==( const std::variant<Types...>& lhs,                            const std::variant<Types...>& rhs );	(1)	(since C++17)
template< class... Types > constexpr bool operator!=( const std::variant<Types...>& lhs,                            const std::variant<Types...>& rhs );	(2)	(since C++17)
template< class... Types > constexpr bool operator<( const std::variant<Types...>& lhs,                           const std::variant<Types...>& rhs );	(3)	(since C++17)
template< class... Types > constexpr bool operator>( const std::variant<Types...>& lhs,                           const std::variant<Types...>& rhs );	(4)	(since C++17)
template< class... Types > constexpr bool operator<=( const std::variant<Types...>& lhs,                            const std::variant<Types...>& rhs );	(5)	(since C++17)
template< class... Types > constexpr bool operator>=( const std::variant<Types...>& lhs,                            const std::variant<Types...>& rhs );	(6)	(since C++17)
template< class... Types > constexpr std::common_comparison_category_t               <std::compare_three_way_result_t<Types>...>     operator<=>( const std::variant<Types...>& lhs,                  const std::variant<Types...>& rhs );	(7)	(since C++20)
Helper function template
template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>&     GET( const variant<Types...>& v );	(8)	(exposition only*)

Performs comparison operations on std::variant objects.

1-7) Compares two std::variant objects lhs and rhs. The contained values are compared (using the corresponding operator of T) only if both lhs and rhs contain values corresponding to the same index. Otherwise,

• lhs is considered equal to rhs if, and only if, both lhs and rhs do not contain a value.
• lhs is considered less than rhs if, and only if, either rhs contains a value and lhs does not, or lhs.index() is less than rhs.index().

1-6) Let @ denote the corresponding comparison operator, for each of these functions:

If, for some values of I, the corresponding expression GET <I>(lhs) @ GET <I>(rhs) is ill-formed or its result is not convertible to bool, the program is ill-formed.	(until C++26)
This overload participates in overload resolution only if for all values of I, the corresponding expression GET <I>(lhs) @ GET <I>(rhs) is well-formed and its result is convertible to bool.	(since C++26)

8) The exposition-only function template GET behaves like std::get(std::variant), except that std::bad_variant_access is never thrown.

If I < sizeof...(Types) is false, the program is ill-formed.

If I == v.index() is false, the behavior is undefined.

Parameters

lhs,rhs

-

variants to compare

Return value

Operator	Both operands contains a value (let I be lhs.index() and J be rhs.index())		lhs or rhs is valueless (let lhs_empty be lhs.valueless_by_exception() and rhs_empty be rhs.valueless_by_exception())
	I and J are equal	I and J are unequal
==	GET <I>(lhs) == GET <I>(rhs)	false	lhs_empty && rhs_empty
!=	GET <I>(lhs) != GET <I>(rhs)	true	lhs_empty != rhs_empty
<	GET <I>(lhs) < GET <I>(rhs)	lhs.index() < rhs.index()	lhs_empty && !rhs_empty
>	GET <I>(lhs) > GET <I>(rhs)	lhs.index() > rhs.index()	!lhs_empty && rhs_empty
<=	GET <I>(lhs) <= GET <I>(rhs)	lhs.index() < rhs.index()	lhs_empty
>=	GET <I>(lhs) >= GET <I>(rhs)	lhs.index() > rhs.index()	rhs_empty
<=>	GET <I>(lhs) <=> GET <I>(rhs)	lhs.index() <=> rhs.index()	see below

For operator<=>:

• If only lhs is valueless, returns std::strong_ordering::less.
• If only rhs is valueless, returns std::strong_ordering::greater.
• If both lhs and rhs are valueless, returns std::strong_ordering::equal.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_constrained_equality	202403L	(C++26)	constrained comparison operators for std::variant

Example

Run this code

#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::cout << std::boolalpha;
    std::string cmp;
    bool result;
 
    auto print2 = [&cmp, &result](const auto& lhs, const auto& rhs)
    {
        std::cout << lhs << ' ' << cmp << ' ' << rhs << " : " << result << '\n';
    };
 
    std::variant<int, std::string> v1, v2;
 
    std::cout << "operator==\n";
    {
        cmp = "==";
 
        // by default v1 = 0, v2 = 0;
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
 
        v1 = v2 = 1;
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
 
        v2 = 2;
        result = v1 == v2; // false
        std::visit(print2, v1, v2);
 
        v1 = "A";
        result = v1 == v2; // false: v1.index == 1, v2.index == 0
        std::visit(print2, v1, v2);
 
        v2 = "B";
        result = v1 == v2; // false
        std::visit(print2, v1, v2);
 
        v2 = "A";
        result = v1 == v2; // true
        std::visit(print2, v1, v2);
    }
 
    std::cout << "operator<\n";
    {
        cmp = "<";
 
        v1 = v2 = 1;
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
 
        v2 = 2;
        result = v1 < v2; // true
        std::visit(print2, v1, v2);
 
        v1 = 3;
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
 
        v1 = "A"; v2 = 1;
        result = v1 < v2; // false: v1.index == 1, v2.index == 0
        std::visit(print2, v1, v2);
 
        v1 = 1; v2 = "A";
        result = v1 < v2; // true: v1.index == 0, v2.index == 1
        std::visit(print2, v1, v2);
 
        v1 = v2 = "A";
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
 
        v2 = "B";
        result = v1 < v2; // true
        std::visit(print2, v1, v2);
 
        v1 = "C";
        result = v1 < v2; // false
        std::visit(print2, v1, v2);
    }
 
    {
        std::variant<int, std::string> v1;
        std::variant<std::string, int> v2;
    //  v1 == v2; // Compilation error: no known conversion
    }
 
    // TODO: C++20 three-way comparison operator <=> for variants
}

Output:

operator==
0 == 0 : true
1 == 1 : true
1 == 2 : false
A == 2 : false
A == B : false
A == A : true
operator<
1 < 1 : false
1 < 2 : true
3 < 2 : false
A < 1 : false
1 < A : true
A < A : false
A < B : true
C < B : false

See also

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)

compares optional objects (function template)