Confusion in C++ constructor

Question

#include <iostream>
using namespace std;

class Obj {
public:
    Obj(){cout <<"create obj" << endl;}
    Obj(const Obj& other){cout<<"copy create obj"<<endl;}
    ~Obj(){cout<<"destructed obj"<<endl;}
};

int main() {
    Obj(Obj((Obj())));
    cout<<"---- exit main ----"<<endl;
}

I have no idea why this program only prints out 1 create obj and 1 destructed obj. Help.

Just for fun try this instead: `Obj{Obj{{Obj{}}}}; ` – juanchopanza Oct 20 '17 at 06:06 — juanchopanza, Oct 20 '17 at 06:06
Oh! c++ compiler does not optimise this format? why??? – chenyuandong Oct 20 '17 at 06:10 — chenyuandong, Oct 20 '17 at 06:10

score 9 · Accepted Answer · answered Oct 20 '17 at 06:06

9

Because of Copy Elision. Read more about it here. Your compiler understands, that it can avoid copying the object around, and just creates one object.

answered Oct 20 '17 at 06:06

gsamaras

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Oh! Thank you very much! Just hard to imagine what c++ compiler does under the hood. – chenyuandong Oct 20 '17 at 06:09
Another mini question: c++ compiler does not optimise this format, like Obj{Obj{{Obj{}}}}, why??? – chenyuandong Oct 20 '17 at 06:12
@chenyuandong it does for me, under GCC 8. – gsamaras Oct 20 '17 at 06:14

Confusion in C++ constructor

1 Answers1