std::get(std::variant) (3) - Linux Manuals

std::get(std::variant): std::get(std::variant)

NAME

std::get(std::variant) - std::get(std::variant)

Synopsis

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

Parameters

I - index to look up
Type - unique type to look up
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>

  int main()
  {
      std::variant<int, float> v{12}, w;
      int i = std::get<int>(v);
      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 {
        std::get<float>(w); // w contains int, not float: will throw
      }
      catch (std::bad_variant_access&) {}
  }