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std::get_if (std::variant) - cppreference.com

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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, std::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....

Template parameters

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

Parameters

pv - pointer to a variant

Return value

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

Example

#include <iostream>
#include <variant>

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);
}

Output: 

variant value: 12
failed to get value!

See also

reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template) [edit]