[百度空间] [原] Empty base class optimization

最近遇到了一个诡异的问题, 数组的数据不对, 最后发现是两个类型的大小不一样导致的.

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class alloc { public: void* operator new(size_t n){...} void operator delete(void* p) {...} };    class alloc2 { public: void* operator new(size_t n){...} void operator delete(void* p) {...} };    class DualQuaterion : public class alloc {...};    //override base with different allocation strategy class BoneDQ : public dualquaterion, public class alloc2 { public: using alloc2::operator new; using alloc2::operator delete; };

两个地方使用的数组类型不一样, 导致数据错误.也就是sizeof(BoneDQ) != sizeof(DualQuaternion).

这是VS2012下的结果, 换用GCC, 两个类型大小一样.

这个真的没有想到.

最后查到问题在这里:

http://en.cppreference.com/w/cpp/language/ebo

上面说的基本限制: 如果同时有连续两个一样的子对象, 那么这两个对象地址不一样(C++标准规定), 导致empty base被阻止.

但跟我遇到的问题仍然不一样.

但是下面又提到,

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Empty base optimization is required for StandardLayoutTypes in order to maintain the requirement that the pointer to a standard-layout object, converted using reinterpret_cast, points to its initial member, which is why one of the requirements for a standard layout type is "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". (since C++11)

也就是说, 只有C++11以后才要求empty base optimization是必须的. 所以VS2012的处理也是正确的.