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std::get_if

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

#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

See also

(C++17)
 reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template)

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http://en.cppreference.com/w/cpp/utility/variant/get_if