Returning unique_ptr from functions

by

is there some other clause in the language specification that this exploits?

Yes, see 12.8 §34 and §35:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object […]
This elision of copy/move operations, called copy elision, is permitted […]
in a return statement in a function with a class return type, when the expression is the name of
a non-volatile automatic object with the same cv-unqualified type as the function return type […]

When the criteria for elision of a copy operation are met and the object to be copied is designated by an lvalue,
overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.

Just wanted to add one more point that returning by value should be the default choice here because a named value in the return statement in the worst case, i.e. without elisions in C++11, C++14 and C++17 is treated as an rvalue. So for example the following function compiles with the -fno-elide-constructors flag

std::unique_ptr<int> get_unique() {
  auto ptr = std::unique_ptr<int>{new int{2}}; // <- 1
  return ptr; // <- 2, moved into the to be returned unique_ptr
}

...

auto int_uptr = get_unique(); // <- 3

With the flag set on compilation there are two moves (1 and 2) happening in this function and then one move later on (3).

More Related Contents:

  1. Is std::unique_ptr required to know the full definition of T?
  2. Why can I not push_back a unique_ptr into a vector?
  3. make_unique and perfect forwarding
  4. How do I pass a unique_ptr argument to a constructor or a function?
  5. Differences between std::make_unique and std::unique_ptr with new
  6. Differences between unique_ptr and shared_ptr [duplicate]
  7. Is there any use for unique_ptr with array?
  8. How to capture a unique_ptr into a lambda expression?
  9. “Downcasting” unique_ptr to unique_ptr
  10. Copy constructor for a class with unique_ptr
  11. How to implement make_unique function in C++11? [duplicate]
  12. How do I use unique_ptr for pimpl?
  13. Proper way to create unique_ptr that holds an allocated array
  14. So can unique_ptr be used safely in stl collections?
  15. Exception safety and make_unique
  16. std::unique_ptr, deleters and the Win32 API
  17. std::auto_ptr to std::unique_ptr
  18. How does the custom deleter of std::unique_ptr work?
  19. How can I pass std::unique_ptr into a function
  20. Should I use shared_ptr or unique_ptr
  21. error::make_unique is not a member of ‘std’
  22. shared_ptr is to weak_ptr as unique_ptr is to… what?
  23. Raw pointer lookup for sets of unique_ptrs
  24. unique_ptr and OpenSSL’s STACK_OF(X509)*
  25. How can I compile C++11 code with Orwell Dev-C++?
  26. Why does unique_ptr take two template parameters when shared_ptr only takes one?
  27. Should std::unique_ptr be permitted
  28. stl container with std::unique_ptr’s vs boost::ptr_container
  29. Does std::bind work with move-only types in general, and std::unique_ptr in particular?
  30. Does C++11 unique_ptr and shared_ptr able to convert to each other’s type?

Leave a Comment