Advantages of pass-by-value and std::move over pass-by-reference

by 
/* (0) */ 
Creature(const std::string &name) : m_name{name} { }

  • A passed lvalue binds to name, then is copied into m_name.

  • A passed rvalue binds to name, then is copied into m_name.

/* (1) */ 
Creature(std::string name) : m_name{std::move(name)} { }

  • A passed lvalue is copied into name, then is moved into m_name.

  • A passed rvalue is moved into name, then is moved into m_name.

/* (2) */ 
Creature(const std::string &name) : m_name{name} { }
Creature(std::string &&rname) : m_name{std::move(rname)} { }

  • A passed lvalue binds to name, then is copied into m_name.

  • A passed rvalue binds to rname, then is moved into m_name.

As move operations are usually faster than copies, (1) is better than (0) if you pass a lot of temporaries. (2) is optimal in terms of copies/moves, but requires code repetition.

The code repetition can be avoided with perfect forwarding:

/* (3) */
template <typename T,
          std::enable_if_t<
              std::is_convertible_v<std::remove_cvref_t<T>, std::string>, 
          int> = 0
         >
Creature(T&& name) : m_name{std::forward<T>(name)} { }

You might optionally want to constrain T in order to restrict the domain of types that this constructor can be instantiated with (as shown above). C++20 aims to simplify this with Concepts.

In C++17, prvalues are affected by guaranteed copy elision, which – when applicable – will reduce the number of copies/moves when passing arguments to functions.

More Related Contents:

  1. C++: While Looping Amount of Times
  2. C++ press enter to continue
  3. Array of pointers C++ creates of copy
  4. Error c4430 ? visual studio 2012 [closed]
  5. Possibilities to quit a function
  6. Order of evaluation of arguments using std::cout
  7. Visual Studio support for new C / C++ standards?
  8. _DEBUG vs NDEBUG
  9. How I can print the wchar_t values to console?
  10. Using Unicode in C++ source code
  11. stl::multimap – how do i get groups of data?
  12. Unit testing for C++ code – Tools and methodology [closed]
  13. How do you detect/avoid Memory leaks in your (Unmanaged) code? [closed]
  14. How to parse date/time from string?
  15. How to use string.substr() function?
  16. What is the preferred/idiomatic way to insert into a map?
  17. Custom Iterator in C++
  18. meaning of (number) & (-number)
  19. multiset, map and hash map complexity
  20. How does switch compile in Visual C++ and how optimized and fast is it?
  21. copy vs std::move for ints
  22. How performing multiple matrix multiplications in CUDA?
  23. c++ issue with function overloading in an inherited class
  24. Why is auto_ptr being deprecated?
  25. Why was pair range access removed from C++11?
  26. Correctly over-loading a stringbuf to replace cout in a MATLAB mex file
  27. Interfacing C++11 array with Cython
  28. Can I hint the optimizer by giving the range of an integer?
  29. passing object by reference in C++
  30. What happen to pointers when vectors need more memory and realocate memory?

Leave a Comment