std::variant

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Definido no cabeçalho <variant>
template <class... Types> class variant;

The class template std::variant represents a type-safe union. An instance of std::variant a qualquer momento, mantém um valor de um de seus tipos alternativos ou, no caso de erro - nenhum valor (esse estado é difícil de obter, consulte valueless_by_exception).

As with unions, if a variant holds a value of some object type T, the object representation of T is allocated directly within the object representation of the variant itself. Variant is not allowed to allocate additional (dynamic) memory.

A variant is not permitted to hold references, arrays, or the type void. Empty variants are also ill-formed (std::variant<std::monostate> can be used instead).

É permitido a uma variante manter o mesmo tipo mais de uma vez e versões diferentes do mesmo tipo qualificadas para CV.

Consistent with the behavior of unions during aggregate initialization, a default-constructed variant holds a value of its first alternative, unless that alternative is not default-constructible (in which case the variant is not default-constructible either). The helper class std::monostate can be used to make such variants default-constructible.

Template parameters

Types

-

the types that may be stored in this variant. All types must meet the Destructible requirements (in particular, array types and non-object types are not allowed).

Member functions

Predefinição:cpp/utility/variant/dsc constructorPredefinição:cpp/utility/variant/dsc destructorPredefinição:cpp/utility/variant/dsc operator=Predefinição:cpp/utility/variant/dsc indexPredefinição:cpp/utility/variant/dsc valueless by exceptionPredefinição:cpp/utility/variant/dsc emplacePredefinição:cpp/utility/variant/dsc swap

Observers

Modifiers

Non-member functions

Predefinição:cpp/utility/variant/dsc visitPredefinição:cpp/utility/variant/dsc holds alternativePredefinição:cpp/utility/variant/dsc getPredefinição:cpp/utility/variant/dsc get ifPredefinição:cpp/utility/variant/dsc operator cmpPredefinição:cpp/utility/variant/dsc swap2

Helper classes

Predefinição:cpp/utility/variant/dsc monostatePredefinição:cpp/utility/variant/dsc bad variant accessPredefinição:cpp/utility/variant/dsc variant sizePredefinição:cpp/utility/variant/dsc variant alternativePredefinição:cpp/utility/variant/dsc hash

Helper objects

Predefinição:cpp/utility/variant/dsc variant npos

Example

#include <variant>
#include <string>
#include <cassert>

int main()
{
    std::variant<int, float> v, w;
    v = 12; // v contains int
    int i = std::get<int>(v);
    w = std::get<int>(v);
    w = std::get<0>(v); // same effect as the previous line
    w = 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 (const std::bad_variant_access&) {}

    using namespace std::literals;

    std::variant<std::string> x("abc");
    // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous

    std::variant<std::string, void const*> y("abc");
    // casts to void const * when passed a char const *
    assert(std::holds_alternative<void const*>(y)); // succeeds
    y = "xyz"s;
    assert(std::holds_alternative<std::string>(y)); // succeeds
}

See also

Predefinição:cpp/utility/dsc in placePredefinição:cpp/utility/dsc optionalPredefinição:cpp/utility/dsc any