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offsetof

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< cpp‎ | types

 
 
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offsetof
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Type trait constants
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integral_constant(C++11)
 
Defined in header <cstddef>
#define offsetof(type, member) /*implementation-defined*/
El offsetof macro se expande a una constante de std::size_t tipo, el valor de los cuales es el desplazamiento, en bytes, desde el principio de un objeto del tipo especificado para su miembro especificado, incluido el relleno, si cualquier .
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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Contenido

[editar] Notas

Si type no es un tipo de normas de diseño, el comportamiento no está definido .
Original:
If type is not a standard-layout type, the behavior is undefined.
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Si member es un miembro estático o un miembro de la función, el comportamiento es indefinido .
Original:
If member is a static member or a function member, the behavior is undefined.
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El desplazamiento del primer elemento de un tipo de diseño de estándar es siempre cero (vacío-base optimización es obligatorio)
Original:
The offset of the first member of a standard-layout type is always zero (vacío-base optimización is mandatory)
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[editar] Posible implementación

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

[editar] Ejemplo

#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] Ver también

tipo entero sin signo devuelto por el operador sizeof
Original:
unsigned integer type returned by the sizeof operator
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(typedef) [edit]
comprueba si un tipo es estándar de diseño de tipo
Original:
checks if a type is standard-layout type
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(clase de plantilla) [edit]