std:: get (std::variant)

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::variant
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 <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

(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)
(C++11)
tuple accesses specified element
(function template)
(C++11)
accesses an element of an array
(function template)
(C++11)
accesses an element of a pair
(function template)
(C++20)
obtains iterator or sentinel from a std::ranges::subrange
(function template)
(C++26)
obtains a reference to real or imaginary part from a std::complex
(function template)
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