std::get (std::variant)

Defined in header <variant>.

Declarations

C++17

// 1)
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)
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

#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 Result

12
42
Unexpected index: w contained int, not float