None; no copies are done.
The address of the caller’s Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller’s stack frame.
This is known as the named return value optimisation. Also see the c++ faq on this topic.
commercial-grade C++ compilers implement return-by-value in a way that lets them eliminate the overhead, at least in simple cases
…
When yourCode() calls rbv(), the compiler secretly passes a pointer to the location where rbv() is supposed to construct the “returned” object.
You can demonstrate that this has been done by adding a destructor with a printf to your struct. The destructor should only be called once if this return-by-value optimisation is in operation, otherwise twice.
Also you can check the assembly to see that this happens:
Data createData()
{
Data data;
// initialize data values...
data.values[5] = 6;
return data;
}
here’s the assembly:
__Z10createDatav:
LFB2:
pushl %ebp
LCFI0:
movl %esp, %ebp
LCFI1:
subl $1032, %esp
LCFI2:
movl 8(%ebp), %eax
movl $6, 20(%eax)
leave
ret $4
LFE2:
Curiously, it allocated enough space on the stack for the data item subl $1032, %esp
, but note that it takes the first argument on the stack 8(%ebp)
as the base address of the object, and then initialises element 6 of that item. Since we didn’t specify any arguments to createData, this is curious until you realise this is the secret hidden pointer to the parent’s version of Data.