Singleton Class in C++

Singleton pattern is a design pattern that restricts the instantiation of a class to one object.

Application:

• This pattern can be used in "Database class" to that insert and retrieve record's in threads.
• Can be used in Logger class.

How to create?

• Define the constructor as private.
• Declare copy constructor and Assignment operator as private to avoid creating copy of object

Sample 1( Allocating memory dynamically with new operator):

 

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;


class Singleton
{
private:
    static Singleton *m_ptrInstance;
    Singleton(){};
    Singleton(Singleton const&);             
    void operator=(Singleton const&);

public:
    static Singleton* GetInstance()
    {
        if(m_ptrInstance==NULL)
        {
            m_ptrInstance=new Singleton();
        }
        return m_ptrInstance;
    }
    void Display()
    {
        cout<<"This is singleton class"; 
    }
};
Singleton* Singleton::m_ptrInstance=NULL;


int _tmain(int argc, _TCHAR* argv[])
{
    //How to call    
    Singleton::GetInstance()->Display();
    getch();
    return 0;
}

Sample 2( Without allocating memory dynamically with new operator):

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;


class Singleton
{
private:
    Singleton(){}
    Singleton(Singleton const&);             
    void operator=(Singleton const&);

public:
    static Singleton& GetInstance()
    {
        static Singleton _instance;
        return _instance;
    }
    void Display()
    {
        cout<<"This is singleton class"; 
    }
};


int _tmain(int argc, _TCHAR* argv[])
{
    //How to call    
    Singleton::GetInstance().Display();
    getch();
    return 0;
}

How to Intialize the Array elements value to NULL?

Usually we initialize the dynamically allocated array values to NULL using the memset.

We can do it while we allocate memory itself, by adding parenthesis ()  after the size.

Sample Code snippet:
 
int* nIntializedArray=new int[3]();

Example:

#include "stdafx.h"
#include <iostream>
#include <string>
#include <conio.h>
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    int i=0;
    cout<<"Uninitialazied Arrary of int* \r\n\r\n";

    int* nUnIntializedArrayptr=new int[3];
    for( i=0; i<3; i++)
    {
        cout<<nUnIntializedArrayptr[i]<<"\r\n";
        
    }
    delete[] nUnIntializedArrayptr;
    cout<<"\r\n\r\n";


    cout<<"Initialazied Arrary of int* \r\n\r\n";
    int* nIntializedArray=new int[3]();
    for( i=0; i<3; i++)
    {
        cout<<nIntializedArray[i]<<"\r\n";
        
    }
    delete[] nIntializedArray;


    cout<<"UnInitialazied char* \r\n\r\n";
    char* sztxt=new char[3];
    cout<<sztxt<<"\r\n";    
    delete[] sztxt;

    cout<<"Initialazied char* \r\n\r\n";
    char* szIntialTxt=new char[3]();
    cout<<szIntialTxt<<"\r\n";    
    delete[] szIntialTxt;


    getch();
    return 0;
}

How to find the size of a dynamically allocated array?

Usually when we pass a pointer to an array to another function, we pass its size also as the parameter.
Which is not required, if the array is dynamically allocated.

We can find the size of a dynamically allocated array using _msize() function.

    TCHAR *szTxt=new TCHAR[256];
    int nArraySize=0;
    nArraySize=    _msize(szTxt)/sizeof(TCHAR);

Example:

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;

void GetIntPtrArray(int* ptrInt)
{
    //To find size of Array
    int nSize=_msize(ptrInt)/sizeof(int);
    cout<<"\r\nThe size of the Integer Array is "<<nSize;
}

void GetCharPtrArray(char* ptrchr)
{
    //To find size of Array
    int nSize=_msize(ptrchr)/sizeof(char);
    cout<<"\r\nThe size of the Char Array is "<<nSize;
}
int _tmain(int argc, _TCHAR* argv[])
{
    int *nArr=new int[10];
    GetIntPtrArray(nArr);

    char *szTxt=new char[256];
    GetCharPtrArray(szTxt);

    getch();
    return 0;
}

How to update the icon changes when installing?

BMP files can be opened using MS Paint or Windows photo viewer, or with some other programs.
But Icon of that file, is the default program that associated with that extension. you can also change the default program to open files.

Even you can create your own file extension documents for your application. To update the icon of the files and to refresh the windows after default program changed or your application installation can be done using the following code.

SHChangeNotify(SHCNE_ASSOCCHANGED,0,NULL,NULL);

How to check whether internet connection is available?

Some times we need to check the internet connection before doing updates or online license check.

How to find  it? Use the following code.

#include "intshcut.h"
#pragma comment(lib,"url.lib")

if( !InetIsOffline( 0 ))
{
    // Internet Connection is available  
}

Virtual destructor

One of the common C++ interview question is what is virtual destructor? Or what is the use of virtual destructor?

You are assigning a derived class object to a base class pointer and deleting the base class pointer.

If the base class destructor is not declared as virtual, then derived class destructor won’t get called.

If you are deleting some variables allocated heap memory in derived class destructor, then it will cause memory leak.

// CPPSamples.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;
class Base
{
    public:
    Base()
    {
        cout<<"\n Base class Constructor\r\n";
    }
    ~Base()
    {
        cout<<"\n Base class Destructor\r\n";
    }

};
class Derived:Base
{
public:
    Derived()
    {
        cout<<"\n Derived class Constructor\r\n";
    }
    ~Derived()
    {
        cout<<"\n Derived class Destructor\r\n";
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    Base *ptr=(Base*)new Derived();
    delete ptr;
    getch();
    return 0;
}

If the base class Destructor is virtual then,

// CPPSamples.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;
class Base
{
    public:
    Base()
    {
        cout<<"\n Base class Constructor\r\n";
    }
    virtual ~Base()
    {
        cout<<"\n Base class Destructor\r\n";
    }

};
class Derived:Base
{
public:
    Derived()
    {
        cout<<"\n Derived class Constructor\r\n";
    }
    ~Derived()
    {
        cout<<"\n Derived class Destructor\r\n";
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    Base *ptr=(Base*)new Derived();
    delete ptr;
    getch();
    return 0;
}

Rollback is not working in PhpMyAdmin MySQL

Once I had problem, in rollback my changes in  MySQL.
I tried disabling autocomit. But problem was not resolved.

/* disable autocommit */
$mysqli->autocommit(FALSE);

$mysqli->rollback();
mysql_query("ROLLBACK");


Then finally, I found the actual problem.
By default in PhpMyadmin table type is MyISAM. Rollback will not work in MyISAM.

Then I changed the table in to InnoDB, It worked!!!