C++ copy on member access

Question

I was doing some experiments to see when copy is performed apart from copy elision, RVO, NRVO cases.

So I've written some code like this:

class X {
 public:
  X() { std::cout << "Default constructor" << std::endl; }

  X(const X&) { std::cout << "Copy constructor" << std::endl; }

  X(X&&) { std::cout << "Move constructor" << std::endl; }

  X& operator=(const X) {
    std::cout << "Assignment operator" << std::endl;
    return *this;
  }

  X& operator=(X&&) {
    std::cout << "Move assignment operator" << std::endl;
    return *this;
  }

  ~X() { std::cout << "Destructor" << std::endl; }
};

class Y {
 private:
  X x;

 public:
  const X& getX() const {
    std::cout << "getX" << std::endl;
    return x;
  }
};

int main() {
  Y y;
  std::cout << "assign to ref" << std::endl;
  const X& x1 = y.getX();
  (void)x1;
  std::cout << "assign to const" << std::endl;
  const X x2 = y.getX();
  return 0;
}

and I receive the following as output:

Default constructor
assign to ref
getX
assign to const
getX
Copy constructor
Destructor
Destructor

Both when compiled with gcc or clang with -O3 and tried -std=c++{11,14,17} all produced the same output.

Which surprised me was, I wasn't expecting any copy to be performed when using y.getX(); to a const variable. It is something I used frequently just to ease my access to that variable and its members in the following code, but I wasn't doing it over a const reference instead I was just using const hoping the compiler would regard it just as a renaming.

Does anyone knows why exactly is that copy performed? Only reason that comes to my mind is that it is to make code thread-safe. If there are multiple threads working with object y, then my assignment to const would not be that const after all. Since it would just reference the member x in object y. Which might be changed by other threads. But I am not sure whether that's the real intention or not.

Answer 1

To see the effect of RVO verses compiler forced use of NRVO, play with -fno-elide-constructors compiler switch on the following modified program below. With the usual options you get:

Default constructor 1
assign to ref
getX (with id: 1)
x1 (id:1)
assign to const
getX (with id: 1)
Copy constructor 2
x2 (id:2)
make_X copy
Default constructor 3
make_X (with id: 3)
x3 (id:3)
make_X ref
Default constructor 4
make_X (with id: 4)
x4 (id:4)
Destructor 4
Destructor 3
Destructor 2
Destructor 1

But with NRVO you get:

Default constructor 1
assign to ref
getX (with id: 1)
x1 (id:1)
assign to const
getX (with id: 1)
Copy constructor 2
x2 (id:2)
additional 1
Default constructor 3
make_X (with id: 3)
Move constructor 4
Destructor 3
Move constructor 5
Destructor 4
x3 (id:5)
additional 2
Default constructor 6
make_X (with id: 6)
Move constructor 7
Destructor 6
x4 (id:7)
Destructor 7
Destructor 5
Destructor 2
Destructor 1

Code example:

#include <iostream>
int global_id;
class X {
public:
    X() : id(++global_id) {
        std::cout << "Default constructor " << id << std::endl;
    }
    X(const X&) : id(++global_id) {
        std::cout << "Copy constructor " << id << std::endl;
    }
    X(X&&) : id(++global_id) {
        std::cout << "Move constructor " << id << std::endl;
    }
    X& operator=(const X&) {
        std::cout << "Assignment operator " << id << std::endl;
        return *this;
    }
    X& operator=(X&&) {
        std::cout << "Move assignment operator " << id << std::endl;
        return *this;
    }
    ~X() {
        std::cout << "Destructor " << id << std::endl;
    }
    int id;
};

class Y {
    X x;
public:
    const X& getX() const {
        std::cout << "getX (with id: " << x.id << ')' << std::endl;
        return x;
    }
    X make_X() const {
        X extra;
        std::cout << "make_X (with id: " << extra.id << ')' << std::endl;
        return extra;
    }
};

int main()
{
    Y y;
    std::cout << "assign to ref" << std::endl;
    const X& x1 = y.getX();
    std::cout << "x1 (id:" << x1.id << ")\n";
    (void) x1;
    std::cout << "assign to const" << std::endl;
    const X x2 = y.getX();
    std::cout << "x2 (id:" << x2.id << ")\n";
    std::cout << "make_X copy" << std::endl;
    const X x3 = y.make_X();
    std::cout << "x3 (id:" << x3.id << ")\n";
    std::cout << "make_X ref" << std::endl;
    const X& x4 = y.make_X();
    std::cout << "x4 (id:" << x4.id << ")\n";
    return 0;
}

As you see, the RVO really only comes to play with local variables.