Hi all,
I have doubt regarding how objects are passed in C++. The
primary problem of passing by value in C++, is that the destructor of
the object passed will be called twice, thus creating possible damage
to the object passed. If that is the case, why isnt the following
program producing the unexpected o/p. I expect the program to print
In B::printValues : 22 33
instead of
In B::printValues : 1 0
Since i have overloaded the copy constructor, i have introduced the
problem mentioned above ( 2 calls to destructor ). But why is the copy
of A in printValues() not printing the actual values in the first call
? Where is the program going wrong?
# include <iostream>
using std::cout;
using std::endl;
class A
{
int x, y;
public:
~A();
A();
A(int , int );
int getX();
int getY();
A(A&);
};
/* Default constructor of A.*/
A::A()
{
cout << "A:: Def constructor" << endl;
}
/* Overloaded constructor of A. */
A::A(int a, int b)
{
x=a;
y=b;
cout << "A:: Prm constructor" << endl;
}
/* Overloaded copy constructor of A. */
A::A(A &a)
{
cout << "In Copy Constructor : " << a.getX() << " " << a.getY() <<
endl;
}
/* Destructor of A. */
A::~A()
{
x=-1;
y=-1;
cout << "A:: Def destructor" << endl;
}
/* Getters. */
int A::getX() { return x; }
int A::getY() { return y; }
class B
{
public:
void printValues(A);
};
void B::printValues( A a)
{
cout << "In B::printValues : " << a.getX() << " " << a.getY() << endl;
cout.flush();
}
int main()
{
A a(22,33);
B b;
b.printValues(a );
b.printValues(a );
return 0;
}
OUTPUT
-------------
A:: Prm constructor
In Copy Constructor : 22 33
In B::printValues : 1 0
A:: Def destructor
In Copy Constructor : 22 33
In B::printValues : -1 -1
A:: Def destructor
A:: Def destructor 3 1770
sarathy schrieb:
Hi all,
I have doubt regarding how objects are passed in C++. The
primary problem of passing by value in C++, is that the destructor of
the object passed will be called twice, thus creating possible damage
to the object passed.
It is not the same object that is destructed twice. The original is
desctructed once, and every copy is destructed once.
If that is the case, why isnt the following
program producing the unexpected o/p. I expect the program to print
In B::printValues : 22 33
instead of
In B::printValues : 1 0
[...]
See below.
# include <iostream>
using std::cout;
using std::endl;
class A
{
int x, y;
public:
~A();
A();
A(int , int );
int getX();
int getY();
int getX() const;
int getY() const;
A(A&);
};
[...]
/* Overloaded copy constructor of A. */
A::A(A &a)
Should better be:
A::A(const A& a)
{
cout << "In Copy Constructor : " << a.getX() << " " << a.getY() <<
endl;
}
Your copy constructor doesn't copy construct your object. You have to
initialize x and y of the constructed object with some values.
--
Thomas
"sarathy" <sp*********@gm ail.comskrev i meddelandet
news:11******** **************@ h48g2000cwc.goo glegroups.com.. .
Hi all,
I have doubt regarding how objects are passed in C++.
The
primary problem of passing by value in C++, is that the destructor
of
the object passed will be called twice, thus creating possible
damage
to the object passed.
Why? If you make a copy, you will later have to destruct both the
original and the copy.
If that is the case, why isnt the following
program producing the unexpected o/p. I expect the program to print
In B::printValues : 22 33
instead of
In B::printValues : 1 0
Since i have overloaded the copy constructor, i have introduced the
problem mentioned above ( 2 calls to destructor ). But why is the
copy
of A in printValues() not printing the actual values in the first
call
? Where is the program going wrong?
You have a copy constructor that prints values, but it doesn't copy
them.
>
/* Overloaded copy constructor of A. */
A::A(A &a)
{
cout << "In Copy Constructor : " << a.getX() << " " << a.getY() <<
endl;
}
>
OUTPUT
-------------
A:: Prm constructor
In Copy Constructor : 22 33
In B::printValues : 1 0
A:: Def destructor
In Copy Constructor : 22 33
In B::printValues : -1 -1
A:: Def destructor
A:: Def destructor
I see one construction of an A, and two copies. That matches the three
destructors.
Bo Persson
sarathy wrote:
Hi all,
I have doubt regarding how objects are passed in C++. The
primary problem of passing by value in C++, is that the destructor of
the object passed will be called twice, thus creating possible damage
to the object passed.
How did you come to that conclusion? It's not true.
If that is the case, why isnt the following program producing the
unexpected o/p. I expect the program to print
In B::printValues : 22 33
instead of
In B::printValues : 1 0
If you wanted the values copied over to the new object, you'd actually have
to copy them in your copy constructor.
>
Since i have overloaded the copy constructor, i have introduced the
problem mentioned above ( 2 calls to destructor ).
No, you haven't. When you pass an object by value, it gets copied. Storage
is acquired for that copy, then your constructor is called to initialize
it. Your constructor just happens to keep it uninitialized.
But why is the copy of A in printValues() not printing the actual values
in the first call ?
The actual values are undefined.
Where is the program going wrong?
Where your copy constructor happens to keep the "copy" uninitialized. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: Edward Diener |
last post by:
Coming from the C++ world I can not understand the reason why copy
constructors are not used in the .NET framework. A copy constructor creates
an object from a copy of another object of the same kind. It sounds simple
but evidently .NET has difficulty with this concept for some reason. I do
understand that .NET objects are created on the GC heap but that doesn't
mean that they couldn't be copied from another object of the same kind when...
|
by: A |
last post by:
Hi,
A default copy constructor is created for you when you don't specify one
yourself. In such case, the default copy constructor will simply do a
bitwise copy for primitives (including pointers) and for objects types call
their default constructor.
Any others points i should know?
|
by: Jesper |
last post by:
Hi,
Does the concept "copy constructor" from c++ excist in
c#. What is the syntax.
best regards Jesper.
|
by: utab |
last post by:
Dear all,
So passing and returning a class object is the time when to include the
definition of the copy constructor into the class definition. But if we
don't call by value or return by value, we do not need to use the
copy-constructor. So depending on the above reasoning I can avoid call
by value and return by value for class objects, this bypasses the
problem or it seems to me like that.
Could any one give me some simple examples...
|
by: shuisheng |
last post by:
Dear All,
I am wondering how the default copy constructor of a derived class
looks like. Does it look like
class B : public A
{
B(const B& right)
: A(right)
{}
| |
by: clicwar |
last post by:
A simple program with operator overloading and copy constructor:
#include <iostream>
#include <string>
using namespace std;
class Vector {
private:
float x,y;
public:
Vector(float u, float v);
|
by: puzzlecracker |
last post by:
From my understanding, if you declare any sort of constructors,
(excluding copy ctor), the default will not be included by default. Is
this correct?
class Foo{
public:
Foo(int); // no Foo() is included, i believe.
};
|
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in...
|
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth.
The Art of Business Website Design
Your website is...
|
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
|
by: tracyyun |
last post by:
Dear forum friends,
With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
| |
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms.
Adolph will...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
|
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
|
by: muto222 |
last post by:
How can i add a mobile payment intergratation into php mysql website.
|
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
| |