C++ : Constructors involved when returning values from functions

  • 0

Trying to figure out how to return huge objects when they are created inside a function or changed inside a function (i.e. avoiding copy constructors).

Code and output should be easy to understand.

Code:

#include <iostream>

using namespace std;

class Dummy {
 int x;
public:
 Dummy() :
   x(0) {
  cout << "\t -> Default Constructor" << endl;
 }
 virtual ~Dummy() {
   cout << "\t -> desctructor" << endl;
  }
 Dummy(int i) :
   x(i) {
  cout << "\t -> Parameter Constructor" << endl;
 }
 Dummy(const Dummy& other) :
   x(other.x) {
  cout << "\t -> Copy Constructor" << endl;
 }
 // Two existing objects!
 Dummy& operator=(const Dummy& other) {
  x = other.x;
  cout << "\t -> Copy Assignment Operator" << endl;
  return *this;
 }
 //C++11
 Dummy(Dummy&& other) {
  x = std::move(other.x);
  cout << "\t -> C++11 Move Constructor" << endl;
 }
 //C++11
 Dummy& operator=(Dummy&& other) {
  x = std::move(other.x);
  cout << "\t -> C++11 Move Operator" << endl;
  return *this;
 }

 void setX(int x) {
  this->x = x;
 }

 friend ostream& operator<<(ostream &out, const Dummy &m);
};

ostream& operator<<(ostream &out, const Dummy &m) {
 out << "\t   -> Dummy.x=" << m.x << endl;
 return out;
}


Dummy fRetClassValue() {
 return Dummy();
}

Dummy fRetClassValue(int x) {
 Dummy d = Dummy(x);
 return d;
}

// Cant't return references to local variables
//Dummy& fRetClassRef() {
// return Dummy();
//}
//
//Dummy& fRetClassRef(int x) {
// Dummy d = Dummy(x);
// return d;
//}

/**
 * This will return a reference for the class passed by param
 * No copy constructors involved
 */
Dummy& fRetClassRef(Dummy &d) {
 return d;
}

Dummy& fRetClassRef(Dummy &d, int x) {
 d.setX(x);
 return d;
}

/**
 * This will return a copy for the class passed by param
 * Copy constructors involved!
 */
Dummy fRetClassValue(Dummy &d) {
 return d;
}

Dummy fRetClassValue(Dummy &d, int x) {
 d.setX(x);
 return d;
}

Dummy& f3(Dummy &m) {
    m.setX(21);
    return m;
}


int main(void) {
 cout << "1........................." <<endl;
 Dummy d1 = fRetClassValue();
 cout << d1;
 Dummy d2 = fRetClassValue(2);
 cout << d2;

 cout << "1_2......................... (Need const to work!)" <<endl;
 const Dummy &d1_2 = fRetClassValue();
 cout << d1_2;
 const Dummy &d2_2 = fRetClassValue(22);
 cout << d2_2;

 cout << "2........................." <<endl;
 Dummy d3 = fRetClassRef(d1);
 cout << d3;
 Dummy d4 = fRetClassRef(d1,4);
 cout << d4;

 cout << "2_2........................." <<endl;
 Dummy &d3_2 = fRetClassRef(d1);
 cout << d3_2;
 Dummy &d4_2 = fRetClassRef(d1,42);
 cout << d4_2;

 cout << "3........................." <<endl;
 Dummy &d5 = fRetClassRef(d1);
 cout << d5;
 Dummy &d6 = fRetClassRef(d1,6);
 cout << "d1" << d1;
 cout << "d6" << d6;
 d6.setX(123);
 cout << "d1" << d1;
 cout << "d6" << d6;

 cout << "4........................." <<endl;
 const Dummy &d7 = fRetClassValue(d1);
 cout << d7;
 const Dummy &d8 = fRetClassValue(d1,7);
 cout << "d1" << d1;
 cout << "d8" << d8;
 d6.setX(1234);
 cout << "d1" << d1;
 cout << "d8" << d8;

 cout << "END ........................." <<endl;

 return 0;
}

Output:

1.........................
  -> Default Constructor
    -> Dummy.x=0
  -> Parameter Constructor
    -> Dummy.x=2
1_2......................... (Need const to work!)
  -> Default Constructor
    -> Dummy.x=0
  -> Parameter Constructor
    -> Dummy.x=22
2.........................
  -> Copy Constructor
    -> Dummy.x=0
  -> Copy Constructor
    -> Dummy.x=4
2_2.........................
    -> Dummy.x=4
    -> Dummy.x=42
3.........................
    -> Dummy.x=42
d1    -> Dummy.x=6
d6    -> Dummy.x=6
d1    -> Dummy.x=123
d6    -> Dummy.x=123
4.........................
  -> Copy Constructor
    -> Dummy.x=123
  -> Copy Constructor
d1    -> Dummy.x=7
d8    -> Dummy.x=7
d1    -> Dummy.x=1234
d8    -> Dummy.x=7
END .........................
  -> desctructor
  -> desctructor
  -> desctructor
  -> desctructor
  -> desctructor
  -> desctructor
  -> desctructor
  -> desctructor

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