C++ copy constructor calling other constructors

Question

I have this code. I'm not that proficient in C++, so I'm going to ask several questions:

#include <iostream>

using namespace std;

class Line {

   public:
      int getLength( void );
      Line( int len );             // simple constructor
      Line( const Line &obj);  // copy constructor
      ~Line();                     // destructor

   private:
      int *ptr;
};

// Member functions definitions including constructor
Line::Line(int len) {
   cout << "Normal constructor allocating ptr" << endl;

   // allocate memory for the pointer;
   ptr = new int;
   *ptr = len;
}

Line::Line(const Line &obj) {
   cout << "Copy constructor allocating ptr." << endl;
   ptr = new int;
   cout<<&obj<<endl;
   *ptr = *obj.ptr; // copy the value
}

Line::~Line(void) {
   cout << "Freeing memory!" << endl;
   delete ptr;
}

int Line::getLength( void ) {
   return *ptr;
}

void display(Line obj) {
   cout << "Length of line : " << obj.getLength() <<endl;
}

// Main function for the program
int main() {

   int a = 10;
   cout<<&a<<endl;
   Line line(a);

   display(line);

   return 0;
}

Here I don't see where we call copy contructor.
Destructor is being called twice. Where is the second object is being created?
Line::Line(const Line &obj) receives address of a as a parameter? I guess not. But why? a by itself is not an instance of Line, so why function accepts that?
What would be the difference between Line::Line(const Line &obj) and Line::Line(const Line obj)
And could you please explain this *ptr = *obj.ptr; . From here i know only lhs, which dereferences ptr (aka sets value of an object). But I didn't get what's in rhs?

I'd appreciate if you explain with less technical terms and more examples, for clarity

Above code outputs this:

0x7fff692196a4
Normal constructor allocating ptr
Copy constructor allocating ptr.
0x7fff69219698
Length of line : 10
Freeing memory!
Freeing memory!

So what is the output of this program? How did you figure out that default constructor is called if it is not implemented here? Destructor called for `line` `main` function local variable and for `obj` `display` function argument (which is copy-constructed). — user7860670, May 07 '18 at 18:09
You don't have a default constructor. Are you talking about `Line(int)`? — R Sahu, May 07 '18 at 18:11
`Line line(a)` is calling the `int` constructor, that should be obvious. — Mark Ransom, May 07 '18 at 18:13
You still haven't explained where you see a call to a default constructor. In any case, what you need is a [book for beginners](https://stackoverflow.com/q/388242/241631). — Praetorian, May 07 '18 at 18:13
There is also a thing called debugger that allows you to execute code line-by line and figure out what is called where. — user7860670, May 07 '18 at 18:15
@NathanOliver , yes, I know. But I still don't feel confident about it — Yura, May 07 '18 at 18:16
@Yura `void display(Line obj)` accepts it's argument by copy. `obj` is a *copy* of the object given as the argument (`line` in this case). — François Andrieux, May 07 '18 at 18:18
@VTT , no all people smart enough to figure out things from debugger. If everyone was, number of questions here would reduce by 80% — Yura, May 07 '18 at 18:21
@NathanOliver I would have closed this as "too broad". There are 5 separate questions here. Only one of them is a duplicate. — melpomene, May 07 '18 at 18:22
@melpomene I felt all the questions stemmed from them not knowing that pasing by value causes a copy. — NathanOliver, May 07 '18 at 18:26

score 1 · Answer 1 · edited May 07 '18 at 18:25

Here for some reason default constructor is called. I don't see any place where we create object without parameter (aka that goes to default constructor)

I don't see any place either! How can you be so sure the default constructor is called? In fact, your class is not constructible using a default constructor because there isn't one. If you try this:

Line l;

It won't compile since there is no default constructor.

Destructor is being called twice. Where is the second object is being created?

It this line:

display(line);

You function receives a value. To have a value both in your main and your display function, you have to copy it somehow. Passing line will copy it from your main to your function.

Line::Line(const Line &obj) receives address of a as a parameter? I guess not. But why? a by itself is not an instance of Line, so why function accepts that?

No. This is a reference. The copy constructor receive a reference from other context to copy it.

What would be the difference between Line::Line(const Line &obj) and Line::Line(const Line obj)

One is a reference, the other is passing by value (a copy, move or other construction). If the copy constructor had to receive a value, how would you copy the value into the copy constructor? The copy constructor must receive a reference.

And could you please explain this *ptr = *obj.ptr; . From here i know only lhs, which dereferences ptr (aka sets value of an object). But I didn't get what's in rhs?

I don't know what you mean by rhs and lhs, is not in the code you posted. But indeed, *ptr = *obj.ptr will set the value from the integer pointed by obj.ptr into the variable pointed by this->ptr.

I recommend getting a good C++ book and follow a tutorial that will teach you the basics.

C++ copy constructor calling other constructors

1 Answers1