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Please help me understand whats happening in following 3 cases.

In the provided code

#include <iostream>

using namespace std;

class Dummy {
public:
  string content;
  Dummy(string c) : content(c) { cout << "Constructor" << endl; }
  Dummy(const Dummy &o) : content(o.content) {
    cout << "Copy Constructor" << endl;
  }
  // Dummy(const Dummy &o) = delete;
  Dummy(Dummy &&o) noexcept {
    content = std::move(o.content);
    cout << "Move Constructor" << endl;
  }
  Dummy &operator=(const Dummy &o) {
    cout << "Assignment" << endl;
    content = o.content;
    return *this;
  }
  Dummy &operator=(Dummy &&o) {
    cout << "Move Assignment" << endl;
    content = std::move(o.content);
    return *this;
  }
  ~Dummy() { cout << "Destructor" << endl; }
};

Dummy returnDummyObject(string c) {
  Dummy d{c};
  return d;
}

void takeDummyParam(Dummy d) { cout << d.content << endl; }

int main() {

  cout << "Case 1" << endl;
  Dummy d1 = returnDummyObject("some string 1");

  cout << "Case 2" << endl;
  Dummy d2 = std::move(returnDummyObject("some string 2"));

  cout << "Case 3" << endl;
  Dummy d3 = Dummy{"some string 3"};

  cout << "Case 4" << endl;
  Dummy d4 = d3;
}

Output is:

Case 1
Constructor
Case 2
Constructor
Move Constructor
Destructor
Case 3
Constructor
Case 4
Copy Constructor
Destructor
Destructor
Destructor
Destructor

What exactly is happening in Case 1? I created a dummy object inside that function and then created another dummy object using that. It should have used a constructor and then a copy constructor (or move constructor based on whether it returns lvalue or rvalue).

Why does Case 2 work? Shouldn't object returned by a function be an rvalue? I am confused because std::move turns lvalue to rvalue.

In Case 3 it just uses a constructor. My assumption was that it will use constructor and then a copy constructor. Can I use construct like this everywhere without it doing any unnecessary copy?

Type t = Type{args}

Thanks in advance. I just started learning c++ using c++ primer 5th edition. Confused about above cases.

Rahul Gupta
  • 1
  • For case 1 you might want to read about [copy elision and return value optimization](http://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization). – Some programmer dude Jan 08 '17 at 11:47
  • @Someprogrammerdude Makes sense. It's using nrvo. Case 2 also works because it returned lvalue. Case 3 is rvo. Thanks for your help. – Rahul Gupta Jan 08 '17 at 11:55

0 Answers0