与成员指针的偏移量，没有临时实例 [英] Offset from member pointer without temporary instance

问题描述

当我为标准版式成员变量提供变量时，我想获取它的偏移量.我不能使用 offsetof ，因为我有一个指针而不是一个名称.我目前使用的代码看起来像这样，我想知道是否存在一种符合标准的方法来消除 dummy 变量.

I would like to get the offset of a standard layout member variable when provided with a poiner to that variable. I cannot use offsetof since I have a pointer and not a name. The current code I have looks something like this, and I am wondering if there is a standards compliant way to get rid of the dummy variable.

template<class T>
struct {
  ptrdiff_t get_offset( int (T::*mem) )
  {
    T dummy;
    return reinterpret_cast<char*>(&(dummy.*mem)) 
      - reinterpret_cast<char*>(&dummy);
  }
};

此函数只能与 int 成员变量点一起调用(这是有意的).

This function should only be callable with int member variable points (this is intentional).

我敢肯定，编译器实际上并没有创建 dummy 变量，但是如果我能摆脱它的话，它仍然很不错.我不能使用null指针，因为未定义对null的取消引用(尽管它可能适用于所有常见的编译器).C ++ 03解决方案会很好，或者C ++ 11解决方案也很有趣(但我现在无法使用).

I am quite certain that the compiler doesn't actually create the dummy variable but it'd still be nice if I could get rid of it. I can't use a null pointer since dereferencing null is not defined (though it probably works on all common compilers). A C++03 solution would be good, or a C++11 solution is also of interest (but not usable by me now).

注意:我已经知道这只是符合标准的标准，而T是标准的布局类型.

推荐答案

怎么样:

template<class T>
struct {
  ptrdiff_t get_offset( int (T::*mem) )
  {
    union {
      int used;
      T unused;
    } dummy;
    return reinterpret_cast<char*>(&(dummy.unused.*mem)) 
      - reinterpret_cast<char*>(&dummy.unused);
  }
};

工会成员的地址不取决于所构建的工会成员.在C ++ 03中已经可以使用，但仅适用于POD.

The address of a union member doesn't depend on the union member being constructed. Works already in C++03, but then only for PODs.

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