Difference between revisions of "cpp/utility/variant/get if"
From cppreference.com
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{{dcl rev begin | num=1 | since=c++17 }} | {{dcl rev begin | num=1 | since=c++17 }} | ||
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| − | template <std::size_t I, class... Types> | + | template< std::size_t I, class... Types > |
constexpr std::add_pointer_t<std::variant_alternative_t<I, std::variant<Types...>>> | constexpr std::add_pointer_t<std::variant_alternative_t<I, std::variant<Types...>>> | ||
get_if( std::variant<Types...>* pv ) noexcept; | get_if( std::variant<Types...>* pv ) noexcept; | ||
}} | }} | ||
{{dcl | | {{dcl | | ||
| − | template <std::size_t I, class... Types> | + | template< std::size_t I, class... Types > |
constexpr std::add_pointer_t<const std::variant_alternative_t<I, variant<Types...>>> | constexpr std::add_pointer_t<const std::variant_alternative_t<I, variant<Types...>>> | ||
get_if( const std::variant<Types...>* pv ) noexcept; | get_if( const std::variant<Types...>* pv ) noexcept; | ||
Revision as of 15:09, 29 October 2019
| Defined in header <variant>
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| (1) | (since C++17) | |
| template< std::size_t I, class... Types > constexpr std::add_pointer_t<std::variant_alternative_t<I, std::variant<Types...>>> |
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| template< std::size_t I, class... Types > constexpr std::add_pointer_t<const std::variant_alternative_t<I, variant<Types...>>> |
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| (2) | (since C++17) | |
| template< class T, class... Types > constexpr std::add_pointer_t<T> get_if( std::variant<Types...>* pv ) noexcept; |
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| template< class T, class... Types > constexpr std::add_pointer_t<const T> get_if( const std::variant<Types...>* pv ) noexcept; |
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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....Contents |
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
Run this code
#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) |
