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enumeration declaration

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Uma enumeração é um tipo distinto cujo valor é restrito a uma das várias constantes explicitamente nomeados ("entrevistadores"). Os valores das constantes são valores de um tipo conhecido como parte integrante do tipo subjacente da enumeração.
Original:
An enumeration is a distinct type whose value is restricted to one of several explicitly named constants ("enumerators"). The values of the constants are values of an integral type known as the underlying type of the enumeration.
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enum name : type attr { enumerator = constexpr , enumerator = constexpr , ... None} (1)
enum class name : type attr { enumerator = constexpr , enumerator = constexpr , ... None} (2) (a partir do C++ 11)
enum struct name : type attr { enumerator = constexpr , enumerator = constexpr , ... None} (3) (a partir do C++ 11)
1)
declara um tipo de enumeração sem escopo. Cada enumerator torna-se acessível no escopo limitado, e é implicitamente conversível para o tipo integral, incluindo bool. Se não for especificado explicitamente, o tipo subjacente é um tipo integral, capaz de representar todos os valores de enumerador, que não pode ser maior do que a menos que algum int constexpr avalia a uma constante que não se encaixa em um int
Original:
declares an unscoped enumeration type. Each enumerator becomes accessible in the enclosing scope, and is implicitly-convertible to integral type, including bool. If not explicitly specified, the underlying type is an integral type capable of representing all enumerator values, which cannot be wider than int unless some constexpr evaluates to a constant that does not fit in an int
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2-3)
declara um tipo de enumeração escopo. Cada enumerator só pode ser acessado como name::enumerator. Conversão para tipos integrais é possível com static_cast. Se não for especificado explicitamente, o tipo subjacente é int.
Original:
declares a scoped enumeration type. Each enumerator can only be accessed as name::enumerator. Conversion to integral types is possible with static_cast. If not explicitly specified, the underlying type is int.
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[editar] Explicação

name -
o nome do tipo declarado por esta declaração. Uma enumeração sem escopo pode ser sem nome, caso em que só introduz nomes enumerador como constantes, mas nenhum novo tipo
Original:
the name of the type declared by this declaration. An unscoped enumeration may be nameless, in which case it only introduces enumerator names as constants, but no new type
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type(C++11) -
tipo integral opcional (qualquer cv-qualificação é ignorado), usado como o tipo subjacente da enumeração .
Original:
optional integral type (any cv-qualification is ignored), used as the underlying type of the enumeration.
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attr(C++11) -
zero ou uma aplicação mais atributos específicos do [[attribute]] formulário
Original:
zero or more implementation-specific attributes of the form [[attribute]]
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enumerator -
zero ou mais entrevistadores que são introduzidas por esta declaração. Os nomes dos inquiridores pode ser usado em qualquer lugar as constantes são esperados
Original:
zero or more enumerators which are introduced by this declaration. The names of the enumerators may be used anywhere constants are expected
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constexpr -
expressão constante opcional que avalia o valor a ser atribuído para o enumerador. Se ele for omitido, o valor é o valor do enumerador anterior mais 1. Se omitido para o primeiro enumerador, o valor é 0
Original:
optional constant expression which evaluates to the value to be assigned to the enumerator. If it is omitted, the value is the value of the previous enumerator plus 1. If omitted for the first enumerator, the value is 0
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[editar] Exemplo

#include <iostream>
// color may be red (value 0), yellow (value 1), green (value 20), or blue (value 21)
enum color {
    red,
    yellow,
    green = 20,
    blue
};
// altitude may be altitude::high or altitude::low
enum class altitude : char { 
     high='h',
     low='l', // C++11 allows the extra comma
}; 
// the constant d is 0, the constant e is 1, the constant f is 3
enum { d, e, f=e+2 };
int main()
{
    color col = red;
    altitude a;
    a = altitude::low;
 
    std::cout << "red = " << col << " blue = " << blue << '\n'
              << "a = " << static_cast<char>(a) << '\n'
              << "f = " << f << '\n';
}

Output:

red = 0 blue = 21
a = l
f = 3