Answer / srikanth

We can make a constructor Private or protected.
1. Private constructor is classic example of implementing a
singleton class( A class with a single instance)
2. Protected constructors can be used when the class cannot
be instantiated but can only be inherited.

Answer / prabha govind

yes
and if a constructor is made private/protected...then that
class cannot be inherited

Answer / baikunta

yes , we can creat private constructor trrough static
method we can access the class (constructor), for example
singleton, there are a lot of use in design pattern
here is example of single ton
class Singleton {
static Singleton s;
int i;
Singleton(int x) : i(x) { }
void operator=(Singleton&);
Singleton(const Singleton&);
public:
static Singleton& getHandle() {
return s;
}
int getValue() { return i; }
void setValue(int x) { i = x; }
};
Singleton Singleton::s(47);
int main() {
Singleton& s = Singleton::getHandle();
cout << s.getValue() << endl;
Singleton& s2 = Singleton::getHandle();
s2.setValue(9);
cout << s.getValue() << endl;
} ///:~

Answer / arun

1. Yes we can make a constructor private. By implementing
this concept we can create a singleTon class.

2. Suppose we have a static method is a class that is used
to create the object of the class by using private
constructor then that member function is named as "Named
Constructor".

3. Using this named constructor concept we can create
SingleTon class as well as normal class.

Example:
class Singleton
{
public:
static Singleton* Instance();
protected:
Singleton();
Singleton(const Singleton&);
Singleton& operator= (const Singleton&);
private:
static Singleton* pinstance;
};
Singleton* Singleton::pinstance = 0;// initialize pointer
Singleton* Singleton::Instance ()
{
if (pinstance == 0) // is it the first call?
{
pinstance = new Singleton; // create sole instance
}
return pinstance; // address of sole instance
}
Singleton::Singleton()
{
//... perform necessary instance initializations
}
Singleton *p1 = Singleton::Instance();
Singleton *p2 = p1->Instance();
Singleton & ref = * Singleton::Instance();

Answer / arun

#include<iostream>
using namespace std;

class Singleton
{
public:
static Singleton* Instance();
private:
static Singleton* pinstance;
Singleton();
};

Singleton* Singleton::pinstance = 0;// initialize pointer
Singleton* Singleton::Instance ()
{
if (pinstance == 0) // is it the first call?
{
pinstance = new Singleton; // create sole instance
}
return pinstance; // address of sole instance
}
Singleton::Singleton()
{
//... perform necessary instance initializations
}

void main()
{
Singleton *p1 = Singleton::Instance();
cout<<p1<<endl;
Singleton *p2 = p1->Instance();
cout<<p2<<endl;
Singleton & ref = * Singleton::Instance();
}

Answer / manjunath

Used mainly to control the object creation...
Ex: the Number of user Login's we can restrict to 15 by
creating 15 objects...

Answer / aj

Yes, only if we want to prevent instantiation from other
class, but then the class itself cannot be sub classed.

this the heart of singleton design pattern

Answer / manjunath

#include<iostream>
using namespace std;
class A
{
int value;
A* ptr;
A()
{
cout<<"\n\t\tConctructor\n";
}
public:
static A* CreateObject()
{
A* ptr=NULL;
ptr=new A;
return ptr;
}

void getdata()
{
cout<<"\n\tEnter the Value of A class\t:\t";
cin>>value;
}
void putdata()
{
cout<<"\n\t\tThe Value of A Class\t:\t";
cout<<value<<endl;
}
~A()
{
cout<<"\n\t\tDestructor\n";
}
};
int main()
{
A *ptr,*ptr1,*ptr3;
ptr=A::CreateObject();
ptr1=A::CreateObject();
ptr3=A::CreateObject();
ptr->getdata();
ptr1->getdata();
ptr3->getdata();
ptr->putdata();
ptr1->putdata();
ptr3->putdata();
delete ptr;
delete ptr1;
delete ptr3;
return 0;
}


Ref:
Singleton and Factory
Classes...

Answer / ashwini

Yes, We can.
By doing so, we can create only single instance of the
class.(singleton)

Answer / arpitha

no, its of no use having a private constructor,,, bcoz
declaring constructor as private makes it blocked.

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