Why is copy constructor called only twice here (should be thrice?)

[TechCrazy]

I am compiling like

g++ -O0 MyString.cpp

Should the copy cnstrs be called thrice? Any optimization going on
here?

--------------------------
#include <iostream>
#include <string>
using namespace std;
class MyString: public string {
public:
MyString(const char * str): string(str) {
cout << "copy ctr 1 called" << endl;
}
MyString(MyString const & str): string(str) {
cout << "copy ctr 2 called" << endl;
}
};

MyString getString() {
MyString x("rrr"); //copy ctr 1 called

//This should call copy ctr once to create the temporary
//that will be returned
return x;
}

int main() {
//This should call copy ctr again with the temporary
//created at the end of getString() to create y
MyString y (getString());
}

------------
This program is printing:

copy ctr 1 called
copy ctr 2 called

[benben]

"TechCrazy" <Te*******@hotpop.com> wrote in message
news:11*********************@g43g2000cwa.googlegro ups.com...

I am compiling like

g++ -O0 MyString.cpp

Should the copy cnstrs be called thrice? Any optimization going on
here?

The getString() function is eligible for Named Return Value (NRV)
optimization, which eliminates the temporary.

Regards,
Ben

[TechCrazy]

But, I turned off all optimization with -O0

[Ian]

TechCrazy wrote:

I am compiling like

g++ -O0 MyString.cpp

Should the copy cnstrs be called thrice? Any optimization going on
here?
Yes, the object returned by getString is y.
--------------------------
#include <iostream>
#include <string>
using namespace std;
class MyString: public string {
public:
MyString(const char * str): string(str) {
cout << "copy ctr 1 called" << endl;
}

Not a copy constructor.

Ian

[benben]

Ian, I think the OP wants to ask why the constructor was called twice than
three times.

ben

[Karl Heinz Buchegger]

TechCrazy wrote:

But, I turned off all optimization with -O0

Then ask the guys writing your compiler, why NRVO is still
done in that case.

I guess the answer will be something like: NRVO is always on, because
it is an optimization which affects the callee as well as the caller.
If we would allow to disable it, then there could be a situation, where
the caller is compiled for NRVO, while the callee is not. And that will
give you a big crash.

--
Karl Heinz Buchegger
kb******@gascad.at