offsetof — cppreference.com

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Déclaré dans l'en-tête `<cstddef>`
`#define offsetof(type, member) /implementation-defined/`

offsetof la macro s'étend à une constante de std::size_t type, la valeur de ce qui est le décalage en octets du début d'un objet du type spécifié à son élément de donnée, y compris le cas échéant rembourrage .

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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Notes

Si type n'est pas un type standard mise en page, le comportement est indéfini .

Original:

If type is not a standard-layout type, the behavior is undefined.

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Si member est un membre statique ou un membre de la fonction, le comportement est indéfini .

Original:

If member is a static member or a function member, the behavior is undefined.

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Le décalage du premier membre d'un type standard-layout est toujours zéro (vide de base de l'optimisation est obligatoire)

Original:

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Mise en œuvre possible

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

Exemple

#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';
}

Résultat :

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

Voir aussi


	unsigned integer retournée par l'opérateur `sizeof` Original: unsigned integer type returned by the `sizeof` operator The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (typedef) [edit]

	vérifie si un type est standard disposition type Original: checks if a type is standard-layout type The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe générique) [edit]