Hi All,
I know this is very silly question about singleton pattern but still it is first choice of interviewer.
Could you let me know in below code snippet.
(1) After deleting singleton object why still i am able to call show() method and it works fine.
delete obj;
obj=NULL;
obj->show();
(2) After creating obj1 object why i am not able to print the content of acquire_lock and release_lock function even print statment "one Time" will be printed once and if we increment counter i then instead of 2 it is printing 1 only, why?
Foo *obj1=MySingleton<Foo>::GetInstance();
code snippet:
#include <iostream>
#include <fstream>
#include <memory>
#include <string>
using namespace std;
static int i;
class Lock
{
public:
Lock(){};
~Lock(){};
void acquire_lock()
{
cout<<"aquired lock for class";
}
void release_lock()
{
cout<<"released lock for class";
}
};
class Foo
{
public:
void show()
{
cout<<"\ndone\n";
}
};
template <class T>
class MySingleton
{
protected:
MySingleton()
{
}
private:
//holds one and only object of MySingleton
MySingleton(const MySingleton <T> &) {};
MySingleton <T> & operator=(const MySingleton <T> &) {};
static T* m_pOnlyOneInstance;
~MySingleton() {};
public:
// MySingleton functionalities
static T * GetInstance();
void foo()
{
cout<<"Mohan";
}
};
//with thread approach
template <class T>
T* MySingleton<T>::GetInstance()
{ Lock lock;
if (m_pOnlyOneInstance == NULL)
{
lock.acquire_lock();
cout<<"one Time"<<endl;
i++;
if(m_pOnlyOneInstance == NULL)
{
m_pOnlyOneInstance = new T();
}
lock.release_lock();
}
return m_pOnlyOneInstance;
}
template <class T> T* MySingleton<T> :: m_pOnlyOneInstance=NULL;
int main()
{
//std::unique_ptr <Foo> obj (MySingleton<Foo>::GetInstance());
Foo *obj=MySingleton<Foo>::GetInstance();
//obj->show();
delete obj;
obj=NULL;
obj->show();
cout<<"\ncalling again\n";
Foo *obj1=MySingleton<Foo>::GetInstance();
obj1->show();
cout<<"i="<<i;
return 1;
}
Note: lock related function are dummy implementationonly
