c++ design: cast from base to derived class with no extra data members

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Here is why I would not use this technique:

  1. It is a violation of the Standard and causes the behavior to be undefined. It is probably true that this works nearly all the time, but you can’t rule out problems in the future. Compilers have been seen to make use of undefined behavior in optimizations, much to the disadvantage of the unsuspecting programmer. And you can’t predict when and under what circumstances this will happen.

  2. You can’t guarantee that neither you nor a team mate will ever add some data members to the derived type. Your class hierarchy will grow and more code will be added over time; at some point it may not be obvious to you or another programmer that adding an innocent data member to the derived type (even temporarily, perhaps for some debugging purpose) can spell disaster.

  3. There are clean and legal alternatives, for example using wrappers based on references:

    #include <iostream>

struct Elem
{ };

struct ElemWrapper
{
  Elem &elem_;

  ElemWrapper(Elem &elem) : elem_(elem)
  { }
};

struct ElemWrapper1 : ElemWrapper
{
  using ElemWrapper::ElemWrapper;

  void foo()
  { std::cout << "foo1" << std::endl; }
};

struct ElemWrapper2 : ElemWrapper
{
  using ElemWrapper::ElemWrapper;

  void foo()
  { std::cout << "foo2" << std::endl; }
};

int main()
{
  Elem e;

  ElemWrapper1(e).foo();

  return 0;
}

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  3. What are the rules for calling the base class constructor?
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  14. Private inheritance VS composition : when to use which?
  15. accessing a protected member of a base class in another subclass
  16. C++ inheritance and function overriding
  17. Function with same name but different signature in derived class not found
  18. What’s the point of a final virtual function?
  19. Do ALL virtual functions need to be implemented in derived classes?
  20. C++ inheritance and member function pointers
  21. C++ Access derived class member from base class pointer
  22. C++11 virtual destructors and auto generation of move special functions
  23. Why must virtual base classes be constructed by the most derived class?
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  25. Class inherited from class without default constructor
  26. Struct inheritance in C++
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  28. Inheriting private members in C++
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  30. C++ inheritance and name hiding [duplicate]

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