std::thread Why object is copied twice?

by

For anything you want to move or avoid copies, prefer move constructors and std::move.

But Why doesn’t this happen automatically for me?

Move in C++ is conservative. It generally will only move if you explicitly write std::move(). This was done because move semantics, if extended beyond very explicit circumstances, might break older code. Automatic-moves are often restricted to very careful set of circumstances for this reason.

In order to avoid copies in this situation, you need to shift a around by using std::move(a) (even when passing it into std::thread). The reason it makes a copy the first time around is because std::thread can’t guarantee that the value will exist after you have finished constructing the std::thread (and you haven’t explicitly moved it in). Thusly, it will do the safe thing and make a copy (not take a reference/pointer to what you passed in and store it: the code has no idea whether or not you’ll keep it alive or not).

Having both a move constructor and using std::move will allow the compiler to maximally and efficiently move your structure. If you’re using VC++ (with the CTP or not), you must explicitly write the move constructor, otherwise MSVC will (even sometimes erroneously) declare and use a Copy constructor.

More Related Contents:

  1. How do I take values from text file and input into a 2D array? c++
  2. How can I sort two vectors in the same way, with criteria that uses only one of the vectors?
  3. const vs constexpr on variables
  4. How is “=default” different from “{}” for default constructor and destructor?
  5. Does const mean thread-safe in C++11?
  6. How to implement make_unique function in C++11? [duplicate]
  7. Pointers in c++ after delete
  8. Why does std::move prevent RVO (return value optimization)?
  9. Narrowing conversions in C++0x. Is it just me, or does this sound like a breaking change?
  10. Can the use of C++11’s ‘auto’ improve performance?
  11. Why are by-value parameters excluded from NRVO?
  12. How to use C++11 enum class for flags
  13. Is it possible to use std::string in a constexpr?
  14. Are int8_t and uint8_t intended to be char types?
  15. Double delete in initializer_list vs 2013
  16. Cannot create constexpr std::vector
  17. How to immediately invoke a C++ lambda?
  18. What’s the point of a final virtual function?
  19. Should I still return const objects in C++11? [duplicate]
  20. Lambdas and capture by reference local variables : Accessing after the scope
  21. Forcing or preventing use of a particular minor version of libstdc++
  22. Are lambdas inlined like functions in C++?
  23. Why Aren’t `std::uniform_int_distribution` and `std::uniform_int_distribution` Allowed?
  24. how to specify vc11 lambda calling convention
  25. Partial specialization of function templates
  26. Dynamic dispatching of template functions?
  27. Reference invalidation after applying reverse_iterator on a custom made iterator
  28. C++11 Implementation of Spinlock using header “
  29. Move semantics == custom swap function obsolete?
  30. How to assert if a std::mutex is locked?

Leave a Comment