Solved

C++, copy constructor

Posted on 2013-01-17
3
433 Views
Last Modified: 2013-01-18
Hello experts, please have a look at the following code:


#include <iostream>

using namespace std;

class Line
{
   public:
      int getLength( void );
      Line( int len );             // simple constructor, headers only
      Line( const Line &obj);      // copy constructor, headers only
      ~Line();                      // destructor

   private:
      int *ptr;
};

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;
   *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( )
{
   Line line(10);

   display(line);//****** why does it call copy constructor here? ******

   return 0;
}

I do not understand why calling 'display' with 'line' as argument makes a call to constructor here...

Any help?

Thank you

panJames
0
Comment
Question by:panJames
  • 3
3 Comments
 
LVL 22

Accepted Solution

by:
ambience earned 500 total points
ID: 38788153
because

void display(Line obj)

does not take Line by reference. It takes it by value so a copy happens
0
 
LVL 22

Expert Comment

by:ambience
ID: 38788160
void display(Line& obj)

should not invoke the constructor.

Looking at the purpose of display its better to have it like

void display(const Line& obj)

but that wont compile unless you also change

int getLength( void ) const;

int Line::getLength( void ) const
{
    return *ptr;
}
0
 
LVL 22

Expert Comment

by:ambience
ID: 38788176
As a general rule, always consider passing objects as const references unless there is a compelling reason not to do so. Passing it by reference ensures there is no overhead of copying and const ensure there are no side-affects because you can only access const methods and data on the object.
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Often, when implementing a feature, you won't know how certain events should be handled at the point where they occur and you'd rather defer to the user of your function or class. For example, a XML parser will extract a tag from the source code, wh…
Basic understanding on "OO- Object Orientation" is needed for designing a logical solution to solve a problem. Basic OOAD is a prerequisite for a coder to ensure that they follow the basic design of OO. This would help developers to understand the b…
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.
The viewer will learn how to clear a vector as well as how to detect empty vectors in C++.

919 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

18 Experts available now in Live!

Get 1:1 Help Now