do i need copy constructor ? and how copy const object to non const object using copy constructor

Question

TARGET: I have to copy one object into another object .

I thought of copying it without using assignment or copy contructor but it doesnt compile. SO I have use copy contructor /assignment operator.

ISSUES
. Now I have 2 questions?

1) Do I really need copy constructor & assignment operator ? As far as I know I am not using any pointers in member variables so there is no need to use copy constructor?

Butt then my object is an pointer so may be I need it?

2) IF my below code is right why I am getting error shown below
can not convert from 'const Home_instance *' to 'Home_instance *'

home_data.h

struct home_data
{
    QString                           area_address_;
    QVector<QString>                   home_numbers_;
    QMap< QString, QList<QByteArray> > area_to_home_mapping;
};


Home_instance.h

class Home_instance : public QObject
{
    Q_OBJECT

public:
    Home_instance( const Home_data& data, QObject* parent_p = 0 );
    Home_instance();
    Home_instance( const Home_instance& rhs );
    Home_instance& operator=( const Home_instance& rhs );
    ~Home_instance();
// some otehr functions

private:
    Home_data data_;


}

Home_instance.cpp

//constructors

Firmware_instance::Home_instance( )
{
}

Home_instance::Home_instance( const Home_data& data, QObject* parent_p )
: QObject( parent_p ),
  data_  ( data )
{}

Home_instance::Home_instance( const Home_instance& rhs )
{
    data_.ip_address_            = rhs.data_.ip_address_;
    data_.serial_numbers_        = rhs.data_.serial_numbers_;
    data_.serial_to_slot_mapping = rhs.data_.serial_to_slot_mapping;

}

 Home_instance& Home_instance::operator=( const Home_instance& rhs )
 {
    data_.ip_address_            = rhs.data_.ip_address_;
    data_.serial_numbers_        = rhs.data_.serial_numbers_;
    data_.serial_to_slot_mapping = rhs.data_.serial_to_slot_mapping;
    return *this;
 }

Home_instance::~Home_instance()
{
   // do we actually need destructor ?
}

//  in some other Dialog window say Council.h &cpp


class Council
{

// some other variable decl
private:
Home_instance*            Home_instance_p_;
Home_data                 Home_data_p_;

}


in council.cpp

// in constructor 

{


connect( udp_p_,    SIGNAL( discovered_Home( const Home_instance* ) ), this, SLOT( new_Home( const Home_instance* ) ) );
}


// in function new_home

void Council::new_home( const Home_instance* Home_old_instance )

{

// bla bla 

Home_instance_p_  =     Home_old_instance; // error can not convert from 'const Home_instance *' to 'Home_instance *'
}

Thanks

Henrik · Accepted Answer · 2014-01-22T14:09:51.827

2

This

Home_instance_p_  =     Home_old_instance;

doesn't copy the objects, it tries to copy the pointer.

You want

Home_instance_p_ = new Home_instance(*Home_old_instance);

and then delete Home_instance_p_ in ~Council.

edited Jan 22 '14 at 14:09

answered Jan 22 '14 at 14:01

Henrik

23,186
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Probably worth mentioning he'd need a Council destructor at that point to delete the Home_instance_p_ that was created. – Jmc Jan 22 '14 at 14:03
@Henrik, Thanks , but one wondering do i need copy constructor ? – samprat Jan 22 '14 at 14:17
@samprat as already stated in Jmc's answer, you probably don't need one. – Henrik Jan 22 '14 at 14:25

score 2 · Answer 2 · edited May 23 '17 at 11:49

Do I really need copy constructor & assignment operator ?

Normally, I would tell you YES, but when having a class inheriting from QObject, then the answer is NO. QObject classes are not meant to be copied.

See this : How do I copy object in Qt?

can not convert from 'const Home_instance *' to 'Home_instance *'

Yes, because the right hand operator is a const pointer, and left is non-const pointer. There is something called const correctness, that prevent this conversion.

This answers why : newbie question: c++ const to non-const conversion

score 1 · Answer 3 · answered Jan 22 '14 at 14:00

Assuming that QList, QMap, and QByteArray all have proper copy constructors (or can use the synthesized one) you do not need one, because, as you said, you have no data that requires a deep copy. However, if the map, list, and bytearray do not copy correctly, you would need to implement one (and the best place would be within the Q-classes)
You are trying to copy a pointer to a const object to a pointer to a non-const object. This doesn't have anything to do with your copy constructor. You should just remove the const from the method definition (if Council::home_instance_p_ needs to be used to call non-const methods OR make your member variable const).

score -1 · Answer 4 · answered Jan 22 '14 at 14:02

-1

1) Do I really need copy constructor & assignment operator ? As far as I know I am not using any pointers in member variables so there is no need to use copy constructor?

If no pointers, no need of copy constructor & assignment operator.

answered Jan 22 '14 at 14:02

Pranit Kothari

9,721
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do i need copy constructor ? and how copy const object to non const object using copy constructor

4 Answers4