__builtin_offsetof运算符的用途和返回类型是什么？ [英] what is the purpose and return type of the __builtin_offsetof operator?

问题描述

C ++中的__builtin_offsetof运算符（或Symbian中的_FOFF运算符）的用途是什么？

What is the purpose of the __builtin_offsetof operator (or _FOFF operator in Symbian) in C++?

此外，它返回什么？指针？字节数？

In addition what does it return? Pointer? Number of bytes?

推荐答案

这是一个由GCC编译器提供的内置函数，用于实现 offsetof 由C和C ++标准指定的宏：

It's a builtin provided by the GCC compiler to implement the offsetof macro that is specified by the C and C++ Standard:

GCC - offsetof

它返回POD结构/联合成员的偏移量。

It returns the offset in bytes that a member of a POD struct/union is at.

示例：

struct abc1 { int a, b, c; };
union abc2 { int a, b, c; };
struct abc3 { abc3() { } int a, b, c; }; // non-POD
union abc4 { abc4() { } int a, b, c; };  // non-POD

assert(offsetof(abc1, a) == 0); // always, because there's no padding before a.
assert(offsetof(abc1, b) == 4); // here, on my system
assert(offsetof(abc2, a) == offsetof(abc2, b)); // (members overlap)
assert(offsetof(abc3, c) == 8); // undefined behavior. GCC outputs warnings
assert(offsetof(abc4, a) == 0); // undefined behavior. GCC outputs warnings

@Jonathan提供了一个很好的例子，我记得看过它用来实现入侵的列表（数据项包括next和prev指针本身的列表），但我不记得它在实现它是有帮助的，可悲的是。

@Jonathan provides a nice example of where you can use it. I remember having seen it used to implement intrusive lists (lists whose data items include next and prev pointers itself), but i can't remember where it was helpful in implementing it, sadly.

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