Does the behavior of guaranteed copy elision depend on existence of user-defined copy constructor?

by

Quoting from C++17 Working Draft §15.2 Temporary Objects Paragraph 3 (https://timsong-cpp.github.io/cppwp/class.temporary#3):

When an object of class type X is passed to or returned from a function, if each copy constructor, move constructor, and destructor of X is either trivial or deleted, and X has at least one non-deleted copy or move constructor, implementations are permitted to create a temporary object to hold the function parameter or result object. … [ Note: This latitude is granted to allow objects of class type to be passed to or returned from functions in registers. — end note]

In your case, when I made both copy and move constructors defaulted:

S(const S &) = default;
S(S &&) = default;

assertion failed as well with GCC and Clang. Note that implicitly-defined constructors are trivial.

More Related Contents:

  1. How does guaranteed copy elision work?
  2. reinterpret_cast creating a trivially default-constructible object
  3. Is stateful metaprogramming ill-formed (yet)?
  4. What made i = i++ + 1; legal in C++17?
  5. Is a pointer with the right address and type still always a valid pointer since C++17?
  6. How is P0522R0 breaking code?
  7. structured bindings: when something looks like a reference and behaves similarly to a reference, but it’s not a reference
  8. std::ignore with structured bindings?
  9. Deducing first template argument with other template parameters defaulted
  10. Self-initialization of a static constexpr variable, is it well-formed?
  11. Can std::launder be used to convert an object pointer to its enclosing array pointer?
  12. Static member access in constant expressions
  13. Pointer interconvertibility vs having the same address
  14. Why doesn’t an if constexpr make this core constant expression error disappear?
  15. Is it allowed for a compiler to optimize away a local volatile variable?
  16. Will I be able to declare a constexpr lambda inside a template parameter?
  17. What changes to C++ made copy initialization work for class with explicit constructor?
  18. Can I use the result of a C++17 captureless lambda constexpr conversion operator as a function pointer template non-type argument?
  19. Directly write into char* buffer of std::string
  20. trap representation
  21. What is the purpose of std::launder?
  22. How to assert that a constexpr if else clause never happen?
  23. Is a moved-from vector always empty?
  24. Is it UB to access a member by casting an object pointer to `char *`, then doing `*(member_type*)(pointer + offset)`?
  25. Why is modifying a string through a retrieved pointer to its data not allowed?
  26. Is circumventing a class’ constructor legal or does it result in undefined behaviour?
  27. Why is f(i = -1, i = -1) undefined behavior?
  28. Can I Reference Previous Members of an Initializer List?
  29. Use of typename keyword with typedef and new
  30. Appending std::vector to itself, undefined behavior?

Leave a Comment