Class initialisation and constructors

There is often lots of confusion with regards to class initialisation. People may forget to take care while using default constructors and copy constructors. Here is a simple example that tries to explain lots of concepts.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//This program gives an example of constructors and initialisations

#include<iostream>

using namespace std;

//Example of a class
class someClass1
{
public:
  //implicit default constructor
  int x;
  int *y;
};

//Example of a class with constructor
class someClass2
{
public:
  someClass2(const someClass2& xyz)
  {
    cout<<"** copy constructor called"<<endl;
    x = xyz.x;
    y = new int(*xyz.y);
  }
  //default constructor will have to be explicitly defined
  someClass2() {};
  //overloading operator '='
  const someClass2& operator = (const someClass2& xyz)
  {
    cout<<"** operator '=' called"<<endl;
    x = xyz.x;
    y = new int(*xyz.y);
    return *this;
  }
  int x;
  int *y;
};

int main()
{
  someClass1 a; //Default initialisation
  a.x = 1234;
  a.y = new int(6789);

  cout<<"someClass1: a.x = "<<a.x<<"       a.y( " <<a.y<< " ) = "<<*(a.y)<<endl;

  someClass1 b = a; //Copy Initialisation

  cout<<"someClass1: b.x = "<<b.x<<"       b.y( " <<b.y<< " ) = "<<*(b.y)<<endl;

  someClass1 c(a); //Direct Initialisation

  cout<<"someClass1: c.x = "<<c.x<<"       c.y( " <<c.y<< " ) = "<<*(c.y)<<endl;

  //Calling default constructor
  someClass2 aa;
  aa.x = 2468;
  aa.y = new int(3579);

  cout<<"someClass2: aa.x = "<<aa.x<<"      aa.y( " <<aa.y<< " ) = "<<*(aa.y)<<endl;

  //calling copy constructor
  someClass2 bb = aa; //Copy Initialisation - note copy constructor will be called

  cout<<"someClass2: bb.x = "<<bb.x<<"      bb.y( " <<bb.y<< " ) = "<<*(bb.y)<<endl;

  //calling copy constructor
  someClass2 cc(aa); //Direct Initialisation - note copy constructor called in this case as well

  cout<<"someClass2: cc.x = "<<cc.x<<"      cc.y( " <<cc.y<< " ) = "<<*(cc.y)<<endl;

  someClass2 dd;
  //calling operator =
  dd = aa;

  cout<<"someClass2: dd.x = "<<dd.x<<"      dd.y( " <<dd.y<< " ) = "<<*(dd.y)<<endl;

  return 0;
}

The output is as follows:

There are certain things worth noting in the above example:

• In case of someClass1, since only the default constructor is used, the same pointer is used for y in all the cases. This can cause serious problems in the code if one of the classes delete the memory pointed by y for class someClass1. This problem is not present in someClass2
• If any constructor is defined, it becomes necessary to define the default constructor. You can make sure that nobody uses default constructor in case of someClass2 by making it private (I havent done that because I wanted to show operator =)
• As you can see in case of variable cc, copy constructor would always be called instead of operator =.
• operator = would only be called in case of assignment. This is a common mistake.
• In the code above, cc(aa) is better than using bb = aa even though the results are the same. The main advantage being that it will avoid confusion with operator '=' if its overloaded for a novice and the constructor can be overloaded to take more than one input in future.

Instantiating a Multimap inside a class

The following is a very simple example of Instantiating a Multimap. This example was posted as a result of a comment on the actual Multimap example here.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//This program shows use of multi-maps in a class
#include<iostream>
#include<map>
#include <string>

using namespace std;

class mapInstantiator
{
public:
  ~mapInstantiator();
  void createMultiMap(void);
  void insertElements(pair<string, int> element);
  void printer(void);
private:
  multimap<string, int> *phoneNums;
};

void mapInstantiator::createMultiMap(void)
{
  //Instantiate
  phoneNums = new multimap<string, int>;
}

void mapInstantiator::insertElements(pair<string, int> element)
{
  phoneNums->insert(element);
}

void mapInstantiator::printer(void)
{
  cout<<"\n\nMultimap printer method"<<endl;
  cout<<"Map size = "<<phoneNums->size()<<endl;
  multimap<string, int>::iterator it = phoneNums->begin();
  while(it != phoneNums->end())
  {
    cout<<"Key = "<<it->first<<"    Value = "<<it->second<<endl;
    it++;
  }
}

mapInstantiator::~mapInstantiator()
{
  //Dont forget to delete the pointer
  delete phoneNums;
}

int main()
{
  mapInstantiator aClass;
  aClass.createMultiMap();

  //Insert key, value as pairs
  aClass.insertElements(pair<string, int>("Joe",123));
  aClass.insertElements(pair<string, int>("Will",444));
  aClass.insertElements(pair<string, int>("Joe",369));
  aClass.insertElements(pair<string, int>("Joe",812));
  aClass.insertElements(pair<string, int>("Will",4556));
  aClass.insertElements(pair<string, int>("Smith",71));

  aClass.printer();

  return 0;
}

The Output is as follows:

Difference between MyClass p; and MyClass p();

Sometimes there are interesting questions asken in an interview. I am going to add some of them when I come across under the 'Interview Questions' tag.

So, what is the difference between MyClass p; and MyClass p();?

MyClass p; creates an instance of class MyClass by calling a constructor for MyClass.

MyClass p(); declares function p which takes no parameters and returns an object of class MyClass by value.

Example to show the difference between struct and class in C++

This is a very simple example showing the difference between struct and class in C++. In reality there is hardly any difference between struct and class in C++. They are exactly the same except that the default is public in struct and private in class. You can do everything you can do in a class in a struct but its not a good practice. The reason being struct's are inherited from C programming and as the name suggests it was meant to represent a structure. You can also read the C++ FAQ that explains it better.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//This very simple example demonstrates the basic difference 
//between struct and class in C++

#include<iostream>

using namespace std;

class A {
  int a; //default in class is private

public:
  int b;
  void initVariables(int c1, int c2, int c3)
  {
    a=c1, b=c2, c=c3;
  }
  void printVariables()
  {
    cout<<"a = "<<a<<"    b = "<<b<<"    c = "<<c<<endl;
  }

private:
  int c;
};

struct B {
  int a; //default in struct is public

public:
  int b;
  void initVariables(int s1, int s2, int s3)
  {
    a=s1, b=s2, c=s3;
  }
  void printVariables()
  {
    cout<<"a = "<<a<<"    b = "<<b<<"    c = "<<c<<endl;
  }

private:
  int c;
};

int main()
{
  A var1;
  var1.initVariables(10, 11, 12);
  //var1.a = 15; //Cant be changed as its private
  var1.b = 16;
  //var1.c = 17; //Cant be changed as its private
  cout<<"Printing the variables from the class"<<endl;
  var1.printVariables();

  B var2;
  var2.initVariables(20, 21, 22);
  var2.a = 25; //possible
  var2.b = 26;
  //var2.c = 27; //Cant be changed as its private
  cout<<"\n\nPrinting the variables from the struct"<<endl;
  var2.printVariables();
  cout<<"\n\n";

  return 0;
}

The output is as follows: