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offsetof

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< cpp‎ | types

 
 
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Defined in header <cstddef>
#define offsetof(type, member) /*implementation-defined*/
O offsetof macro expande para uma constante de std::size_t tipo, cujo valor é o deslocamento, em bytes, a partir do início de um objecto do tipo especificado para o seu membro especificado, incluindo estofamento, se qualquer.
Original:
The macro offsetof expands to a constant of type std::size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified member, including padding if any.
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Índice

[editar] Notas

Se type não é um tipo padrão de layout, o comportamento é indefinido.
Original:
If type is not a standard-layout type, the behavior is undefined.
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Se member é um membro estático ou um membro da função, o comportamento é indefinido.
Original:
If member is a static member or a function member, the behavior is undefined.
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O deslocamento do primeiro membro de um tipo padrão de layout é sempre zero (empty-base de otimização é obrigatório)
Original:
The offset of the first member of a standard-layout type is always zero (empty-base de otimização is mandatory)
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[editar] Possível implementação

#define offsetof(type,member) ((std::size_t) &(((type*)0)->member))

[editar] Exemplo

#include <iostream>
#include <cstddef>
struct S {
    char c;
    double d;
};
int main()
{
    std::cout << "the first element is at offset " << offsetof(S, c) << '\n'
              << "the double is at offset " << offsetof(S, d) << '\n';
}

Output:

the first element is at offset 0
the double is at offset 8

[editar] Veja também

tipo inteiro sem sinal retornado pelo operador sizeof
Original:
unsigned integer type returned by the sizeof operator
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(typedef) [edit]
verifica se um tipo é o layout padrão tipo
Original:
checks if a type is standard-layout type
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(modelo de classe) [edit]