std::get_if (std::variant)

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std::variant
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
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

Run this code
#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

(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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