Why can we use `std::move` on a `const` object?

by

There’s a trick here you’re overlooking, namely that std::move(cat) doesn’t actually move anything. It merely tells the compiler to try to move. However, since your class has no constructor that accepts a const CAT&&, it will instead use the implicit const CAT& copy constructor, and safely copy. No danger, no trap. If the copy constructor is disabled for any reason, you’ll get a compiler error.

struct CAT
{
   CAT(){}
   CAT(const CAT&) {std::cout << "COPY";}
   CAT(CAT&&) {std::cout << "MOVE";}
};

int main() {
    const CAT cat;
    CAT cat2 = std::move(cat);
}

prints COPY, not MOVE.

http://coliru.stacked-crooked.com/a/0dff72133dbf9d1f

Note that the bug in the code you mention is a performance issue, not a stability issue, so such a bug won’t cause a crash, ever. It will just use a slower copy. Additionally, such a bug also occurs for non-const objects that don’t have move constructors, so merely adding a const overload won’t catch all of them. We could check for the ability to move construct or move assign from the parameter type, but that would interfere with generic template code that is supposed to fall back on the copy constructor.
And heck, maybe someone wants to be able to construct from const CAT&&, who am I to say he can’t?

More Related Contents:

  1. “unpacking” a tuple to call a matching function pointer
  2. Why does reading a record struct fields from std::istream fail, and how can I fix it?
  3. Does C++11 allow vector?
  4. Compiling C++11 with g++
  5. How much is too much with C++11 auto keyword?
  6. reinterpret_cast creating a trivially default-constructible object
  7. How do I pass a unique_ptr argument to a constructor or a function?
  8. Why does C++11’s lambda require “mutable” keyword for capture-by-value, by default?
  9. Where is the lock for a std::atomic?
  10. What are the rules for the “…” token in the context of variadic templates?
  11. Difference between C++03 throw() specifier C++11 noexcept
  12. Is the pass-by-value-and-then-move construct a bad idiom?
  13. Why is this code trying to call the copy constructor?
  14. Why do some people use swap for move assignments?
  15. Default, value and zero initialization mess
  16. Are C++11 thread_local variables automatically static?
  17. Can I typically/always use std::forward instead of std::move?
  18. How to extract the source filename without path and suffix at compile time?
  19. How to atomically update a maximum value?
  20. Why is it not good to use recursive inheritance for std::tuple implementations?
  21. Why is the volatile qualifier used through out std::atomic?
  22. C++11 – declaring non-static data members as ‘auto’
  23. Is the ranged based for loop beneficial to performance?
  24. How to implement timeout for function in c++
  25. With explicitly deleted member functions in C++11, is it still worthwhile to inherit from a noncopyable base class?
  26. generic member function pointer as a template parameter
  27. How can I use std::imbue to set the locale for std::wcout?
  28. Why does destructor disable generation of implicit move methods?
  29. C++11 random numbers
  30. Is there a standard way of moving a range into a vector?

Leave a Comment