Why is the construction of std::optional more expensive than a std::pair?

Question

Consider these two approaches that can represent an \"optional int\":

using std_optional_int = std::optional;
using my_optional_int =         


        

Answer 1

Why does get_std_optional_int() require three mov instructions, while get_my_optional() only needs a single movabs?

The direct cause is that optional is returned through a hidden pointer while pair is returned in a register. Why is that, though? The SysV ABI specification, section 3.2.3 Parameter Passing says:

If a C++ object has either a non-trivial copy constructor or a non-trivial destructor, it is passed by invisible reference.

Sorting out the C++ mess that is optional is not easy, but there seem to be a non-trivial copy constructor at least in the optional_base class of the implementation I checked.