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Difference between revisions of "cpp/utility/variant/get"

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Revision as of 08:54, 12 July 2016

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::variant
 
Defined in header <variant>
template <std::size_t I, class... Types>

constexpr std::variant_alternative_t<
  I, std::variant<Types...>
>& get(std::variant<Types...>& v);
template <std::size_t I, class... Types>
constexpr std::variant_alternative_t<
  I, std::variant<Types...>
>&& get(std::variant<Types...>&& v);
template <std::size_t I, class... Types>
constexpr std::variant_alternative_t<
  I, std::variant<Types...>
> const& get(const std::variant<Types...>& v);
template <std::size_t I, class... Types>
constexpr std::variant_alternative_t<
  I, std::variant<Types...>

> const&& get(const std::variant<Types...>&& v);
 (1) (since C++17)
template <class T, class... Types>

constexpr T& get(std::variant<Types...>& v);
template <class T, class... Types>
constexpr T&& get(std::variant<Types...>&& v);
template <class T, class... Types>
constexpr const T& get(const std::variant<Types...>& v);
template <class T, class... Types>

constexpr const T&& get(const std::variant<Types...>&& v);
 (2) (since C++17)
1) Index-based value accessor: If v.index() == I, returns a reference to the value stored in v. Otherwise, throws std::bad_variant_access. The call is ill-formed if I is not a valid index in the variant or if T_I is a (possibly cv-qualified) type void.
2) Type-based value accessor: If v holds the alternative T, returns a reference to the value stored in v. Otherwise, throws std::bad_variant_access. The call is ill-formed if T is not a unique element of Types... or if T_I is a (possibly cv-qualified) type void.

Contents

Parameters

I - index to look up
Type - unique type to look up
v - a variant

Return value

Reference to the value stored in the variant.

Exceptions

1,2) Throws std::bad_variant_access on errors.

Example

Run this code
#include <variant>
#include <string>
 
int main()
{
    std::variant<int, float> v{12}, w;
    int i = std::get<int>(v);
    w = std::get<int>(v);
    w = std::get<0>(v); // same effect as the previous line
 
//  std::get<double>(v); // error: no double in [int, float]
//  std::get<3>(v);      // error: valid index values are 0 and 1
 
    try {
      std::get<float>(w); // w contains int, not float: will throw
    }
    catch (std::bad_variant_access&) {}
}

See also

(C++17)
 obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error
(function template) [edit]
(C++11)
 tuple accesses specified element
(function template) [edit]
(C++11)
 accesses an element of an array
(function template) [edit]
(C++11)
 accesses an element of a pair
(function template) [edit]
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