Why isn’t memcpy guaranteed to be safe for non-POD types?

by

Try bit-wise copying std::shared_ptr<>. You might find that your program blows up in your face more often than not.

You’ll encounter this problem with any class whose copy constructor does something other than a bit-wise copy. In the case of std::shared_ptr<>, it’ll copy the pointer but won’t increment the reference count, so you’ll end up freeing the shared object and its reference count early, and then blowing up when the copied shared_ptr tries to decrement the freed reference count.

UPDATE: It was pointed out that this doesn’t quite answer the question, which is fair, since I mainly addressed the idea of copying shared_ptr to shared_ptr, not shared_ptr to char[] and back again. However, the principle still holds.

If you bit-wise copy a shared_ptr to a char[], assign a different value to the shared_ptr, then copy the char[] back over, the end result may be to leak one object and double-delete another, i.e., UB.

The same might happen with a POD, but that would be a bug in the program logic. Bit-wise copying back into the POD equivalent of a modified shared_ptr would be perfectly valid as long as the program understands and accommodates such an event. Doing so for a std::shared_ptr generally won’t work.

More Related Contents:

  1. expain following code snippet [closed]
  2. Accessing members in same namespace in C++ [closed]
  3. Why when i add number to string it shows random text in c++?
  4. C++ lambda with captures as a function pointer
  5. static initialization order fiasco
  6. Capturing function exit time with __gnu_mcount_nc
  7. Output Unicode to console Using C++, in Windows
  8. Are C++ Reads and Writes of an int Atomic?
  9. Can I assume (bool)true == (int)1 for any C++ compiler?
  10. Why should exceptions be used conservatively?
  11. plotting real time Data on (qwt )Oscillocope
  12. When to use the brace-enclosed initializer?
  13. What is a non-trivial constructor in C++?
  14. Allowing signed integer overflows in C/C++
  15. OpenCv 2.3 C – How to isolate object inside image
  16. Get index of QPushButton on 2D array QPushButton
  17. Why aren’t static const floats allowed? [duplicate]
  18. How do I tell CMake to use Clang on Windows?
  19. C/C++ NaN constant (literal)?
  20. Does a standard implementation of a Circular List exist for C++?
  21. Should I use wchar_t when using UTF-8?
  22. `std::list::sort()` – why the sudden switch to top-down strategy?
  23. lvalue to rvalue implicit conversion
  24. How to test if preprocessor symbol is #define’d but has no value?
  25. Why must const members be initialized in the constructor initializer rather than in its body?
  26. vector memory allocation strategy
  27. c++ convert from LPCTSTR to const char *
  28. New (std::nothrow) vs. New within a try/catch block
  29. What’s the motivation behind having copy and direct initialization behave differently?
  30. Is a Linked-List implementation without using pointers possible or not?

Leave a Comment