std::get_if (3) - Linux Manuals

std::get_if: std::get_if

NAME

std::get_if - std::get_if

Synopsis

Defined in header <variant>
template <std::size_t I, class... Types>
constexpr std::add_pointer_t<
std::variant_alternative_t<I, std::variant<Types...>>
> get_if(std::variant<Types...>* pv) noexcept;
template <std::size_t I, class... Types>
constexpr std::add_pointer_t< (1) (since C++17)
const std::variant_alternative_t<I, variant<Types...>>
> get_if(const std::variant<Types...>* pv) noexcept;
template <class T, class... Types>
constexpr std::add_pointer_t<T> get_if(std::variant<Types...>* pv) noexcept; (2) (since C++17)
template <class T, class... Types>
constexpr std::add_pointer_t<const T> get_if(const std::variant<Types...>* pv) noexcept;

1) Index-based non-throwing accessor: If pv is not a null pointer and pv->index() == I, returns a pointer to the value stored in the variant pointed to by pv. Otherwise, returns a null pointer value. The call is ill-formed if I is not a valid index in the variant.
2) Type-based non-throwing accessor: Equivalent to (1) with I being the zero-based index of T in Types.... 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
pv - pointer to a variant

Return value

Pointer to the value stored in the pointed-to variant or null pointer on error.

Example

// Run this code

  #include <variant>
  #include <iostream>

  int main()
  {
      std::variant<int, float> v{12};

      if(auto pval = std::get_if<int>(&v))
        std::cout << "variant value: " << *pval << '\n';
      else
        std::cout << "failed to get value!" << '\n';
  }

Output:

  variant value: 12