offsetof
De cppreference.com
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<tbody> </tbody>| Definido no cabeçalho <cstddef>
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#define offsetof(type, member) /*implementation-defined*/ |
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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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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.The text has been machine-translated via Google Translate.
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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.The text has been machine-translated via Google Translate.
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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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Possível implementação
#define offsetof(type,member) ((std::size_t) &(((type*)0)->member))
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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';
}
Saída:
the first element is at offset 0
the double is at offset 8