c++ - 在没有虚方法或父类(super class)的类中，假设(第一个成员变量的地址)== this 是否安全？

Question

我创建了一个私有(private) API，它假定类中第一个成员对象的地址与类的 this 指针相同......这样成员对象就可以简单地派生出指向该对象的指针它是的成员，而不必显式存储指针。

鉴于我愿意确保容器类不会从任何父类(super class)继承，不会有任何虚拟方法，并且执行此技巧的成员对象将是声明的第一个成员对象，将该假设对任何 C++ 编译器都有效，还是我需要使用 offsetof() 运算符(或类似运算符)来保证正确性？

换句话说，下面的代码在 g++ 下实现了我所期望的，但它能在任何地方运行吗？

class MyContainer
{
public:
   MyContainer() {}
   ~MyContainer() {}  // non-virtual dtor

private:
   class MyContained
   {
   public:
      MyContained() {}
      ~MyContained() {}

      // Given that the only place Contained objects are declared is m_contained
      // (below), will this work as expected on any C++ compiler?
      MyContainer * GetPointerToMyContainer()
      {
         return reinterpret_cast<MyContainer *>(this);
      }
   };

   MyContained m_contained;  // MUST BE FIRST MEMBER ITEM DECLARED IN MyContainer
   int m_foo;                // other member items may be declared after m_contained
   float m_bar;
};

Answer 1

目前的标准似乎只对 POD 类型保证这一点。

9.2.17

A pointer to a POD-struct object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [Note: There might therefore be unnamed padding within a POD-struct object, but not at its beginning, as necessary to achieve appropriate alignment. ]

然而，C++0x 标准似乎将这种保证扩展到“标准布局结构对象”

A standard-layout class is a class that:

— has no non-static data members of type non-standard-layout class (or array of such types) or reference,

— has no virtual functions (10.3) and no virtual base classes (10.1),

— has the same access control (Clause 11) for all non-static data members,

— has no non-standard-layout base classes,

— either has no non-static data members in the most-derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and

— has no base classes of the same type as the first non-static data member.

A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class.

这个假设很可能在实践中成立(前者不仅有这些区别，尽管这可能是故意的)？