std::get_if (std::variant)

Defined in header <variant>.

Declarations

C++17

// 1)
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, std::variant<Types...>>>
    get_if( const std::variant<Types...>* pv ) noexcept;

// 2)
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()
{
    auto check_value = [](const std::variant<int, float>& v)
    {
        if(const int* pval = std::get_if<int>(&v))
          std::cout << "variant value: " << *pval << '\n'; 
        else 
          std::cout << "failed to get value!" << '\n'; 
    };
 
    std::variant<int, float> v{12}, w{3.f};
    check_value(v);
    check_value(w);
}

Result

variant value: 12
failed to get value!