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

| Defined in header | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------- | | | (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....

[edit] Template parameters

[edit] Parameters

[edit] Return value

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

[edit] Example

#include #include   int main() { auto check_value = [](const std::variant<int, float>& v) { if (const int* pval = std::get_if(&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!

[edit] 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] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |