std::get(std::variant)

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) 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)     type_order (C++26) is_eqis_ltis_lteq (C++20)(C++20)(C++20) is_neqis_gtis_gteq (C++20)(C++20)(C++20)

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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< std::size_t I, class... Types > constexpr std::variant_alternative_t<I, std::variant<Types...>>& get( std::variant<Types...>& v );	(1)	(since C++17)
template< std::size_t I, class... Types > constexpr std::variant_alternative_t<I, std::variant<Types...>>&& get( std::variant<Types...>&& v );	(2)	(since C++17)
template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>& get( const std::variant<Types...>& v );	(3)	(since C++17)
template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>&& get( const std::variant<Types...>&& v );	(4)	(since C++17)
template< class T, class... Types > constexpr T& get( std::variant<Types...>& v );	(5)	(since C++17)
template< class T, class... Types > constexpr T&& get( std::variant<Types...>&& v );	(6)	(since C++17)
template< class T, class... Types > constexpr const T& get( const std::variant<Types...>& v );	(7)	(since C++17)
template< class T, class... Types > constexpr const T&& get( const std::variant<Types...>&& v );	(8)	(since C++17)

1-4) Index-based value accessor: If v.index() == I, returns a reference to the value stored in v. Otherwise, throws std::bad_variant_access. The call is ill-formed if I is not a valid index in the variant.

5-8) Type-based value accessor: If v holds the alternative T, returns a reference to the value stored in v. Otherwise, throws std::bad_variant_access. The call is ill-formed if T is not a unique element of Types....

Template parameters

I	-	index to look up
T	-	unique type to look up
Types...	-	types forming the variant

Parameters

v

-

a variant

Return value

Reference to the value stored in the variant.

Exceptions

Throws std::bad_variant_access on errors.

Example

Run this code

#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::variant<int, float> v{12}, w;
    std::cout << std::get<int>(v) << '\n';
    w = std::get<int>(v);
    w = std::get<0>(v); // same effect as the previous line
 
//  std::get<double>(v); // error: no double in [int, float]
//  std::get<3>(v);      // error: valid index values are 0 and 1
 
    try
    {
        w = 42.0f;
        std::cout << std::get<float>(w) << '\n'; // ok, prints 42
        w = 42;
        std::cout << std::get<float>(w) << '\n'; // throws
    }
    catch (std::bad_variant_access const& ex)
    {
        std::cout << ex.what() << ": w contained int, not float\n";
    }
}

Possible output:

12
42
Unexpected index: w contained int, not float

See also

get_if (C++17)	obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error (function template)
get(std::tuple) (C++11)	tuple accesses specified element (function template)
get(std::array) (C++11)	accesses an element of an array (function template)
get(std::pair) (C++11)	accesses an element of a pair (function template)
get(std::ranges::subrange) (C++20)	obtains iterator or sentinel from a std::ranges::subrange (function template)
get(std::complex) (C++26)	obtains a reference to real or imaginary part from a std::complex (function template)