offsetof

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C++
 
实用工具库
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tuple（C++11）
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Swap, forward and move
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类型的支持
基本类型
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Basic types
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基本类型
固定宽度的整数类型 （C++11）
nullptr_t（C++11）
offsetof
NULL
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is_constructible
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is_nothrow_copy_constructible		（C++11）
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is_move_constructible
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is_nothrow_move_constructible		（C++11）
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is_assignable
is_trivially_assignable
is_nothrow_assignable		（C++11）
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is_copy_assignable
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is_nothrow_copy_assignable		（C++11）
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is_move_assignable
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is_nothrow_move_assignable		（C++11）
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键入特征常数
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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*/
宏offsetof扩展类型std::size_t，，它的值的偏移量，以字节为单位，从一个指定类型的对象的开始到其指定的部件，包括填充物，如果没有..恒定
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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目录

[编辑] 注释

type是不是一个标准的布局类型，其行为是未定义.
Original:
If type is not a standard-layout type, the behavior is undefined.
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member是一个静态成员或函数成员，该行为是未定义.
Original:
If member is a static member or a function member, the behavior is undefined.
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一个标准的布局类型的第一个成员的偏移量始终为零（空基的优化是强制性的）
Original:
The offset of the first member of a standard-layout type is always zero (空基的优化 is mandatory)
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[编辑] 可能的实现

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

[编辑] 为例

#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

[编辑] 另请参阅
返回无符号整数类型的sizeof运营商
Original:
unsigned integer type returned by the sizeof operator
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（的typedef） [edit]
（C++11）
检查如果一个类型是标准的布局类型
Original:
checks if a type is standard-layout type
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（类模板） [edit]
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/types/offsetof&oldid=26840”