std::get (std::variant)

 

C++

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Technical specifications

Symbols index

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management 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 (C++20) 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_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (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) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Non-member functions

visit

holds_alternative

get

get_if

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

swap

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash

Helper objects

variant_npos

 

Defined in header <variant>
	(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 );
template< std::size_t I, class... Types > constexpr std::variant_alternative_t<I, std::variant<Types...>>&&     get( std::variant<Types...>&& v );
template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>&     get( const std::variant<Types...>& v );
template< std::size_t I, class... Types > constexpr const std::variant_alternative_t<I, std::variant<Types...>>&&     get( const std::variant<Types...>&& v );
			(2)	(since C++17)
template< class T, class... Types > constexpr T& get( std::variant<Types...>& v );
template< class T, class... Types > constexpr T&& get( std::variant<Types...>&& v );
template< class T, class... Types > constexpr const T& get( const std::variant<Types...>& v );
template< class T, class... Types > constexpr const T&& get( const std::variant<Types...>&& v );

1) 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.

2) 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

1,2) Throws std::bad_variant_access on errors.

Example

Run this code

#include <variant>
#include <string>
#include <iostream>
 
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)
std::get(std::tuple) (C++11)	tuple accesses specified element (function template)
std::get(std::array) (C++11)	accesses an element of an array (function template)
std::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)