UNIT-2

Pointers, Arrays, Dynamic allocation Operator

2.1 Arrays of Object

• Like array of other user-defined data types, an array of type class can also be created.
• The array of type class contains the objects of the class as its individual elements.
• Thus, an array of a class type is also known as an array of objects.
• An array of objects is declared in the same way as an array of any built-in data type.

Syntax:

class_name array_name [size] ;

Example:

#include <iostream> 

class MyClass { 

  int x; 

public: 

  void setX(int i) { x = i; } 

  int getX() { return x; } 

}; 

void main() 

{ 

  MyClass obs[4]; 

  int i; 

  for(i=0; i < 4; i++) 

    obs[i].setX(i); 

  for(i=0; i < 4; i++) 

    cout << "obs[" << i << "].getX(): " << obs[i].getX() << "\n"; 

  getch(); 

}

Output:

obs[0].getX(): 0
obs[1].getX(): 1
obs[2].getX(): 2
obs[3].getX(): 3

Key takeaway

• Like array of other user-defined data types, an array of type class can also be created.
• The array of type class contains the objects of the class as its individual elements.
• Thus, an array of a class type is also known as an array of objects.
• An array of objects is declared in the same way as an array of any built-in data type.

2.2 Pointers to Object

A pointer to a C++ class is done exactly the same way as a pointer to a structure and to access members of a pointer to a class you use the member access operator -> operator, just as you do with pointers to structures. Also as with all pointers, you must initialize the pointer before using it.

Let us try the following example to understand the concept of pointer to a class −

#include <iostream>

using namespace std;

class Box {

   public:

      // Constructor definition

      Box(double l = 2.0, double b = 2.0, double h = 2.0) {

         cout <<"Constructor called." << endl;

         length = l;

         breadth = b;

         height = h;

      }

      double Volume() {

         return length * breadth * height;

      }

   private:

      double length;     // Length of a box

      double breadth;    // Breadth of a box

      double height;     // Height of a box

};

int main(void) {

   Box Box1(3.3, 1.2, 1.5);    // Declare box1

   Box Box2(8.5, 6.0, 2.0);    // Declare box2

   Box *ptrBox;                // Declare pointer to a class.

   // Save the address of first object

   ptrBox = &Box1;

   // Now try to access a member using member access operator

   cout << "Volume of Box1: " << ptrBox->Volume() << endl;

   // Save the address of second object

   ptrBox = &Box2;

   // Now try to access a member using member access operator

   cout << "Volume of Box2: " << ptrBox->Volume() << endl;

   return 0;

}

When the above code is compiled and executed, it produces the following result −

Constructor called.

Constructor called.

Volume of Box1: 5.94

Volume of Box2: 102

Key takeaway

A pointer to a C++ class is done exactly the same way as a pointer to a structure and to access members of a pointer to a class you use the member access operator -> operator, just as you do with pointers to structures. Also as with all pointers, you must initialize the pointer before using it.

2.3 THIS pointer

Every object in C++ has access to its own address through an important pointer called this pointer. The this pointer is an implicit parameter to all member functions. Therefore, inside a member function, this may be used to refer to the invoking object.

Friend functions do not have a this pointer, because friends are not members of a class. Only member functions have a this pointer.

Let us try the following example to understand the concept of this pointer −

#include <iostream>

using namespace std;

class Box {

   public:

      // Constructor definition

      Box(double l = 2.0, double b = 2.0, double h = 2.0) {

         cout <<"Constructor called." << endl;

         length = l;

         breadth = b;

         height = h;

      }

      double Volume() {

         return length * breadth * height;

      }

      int compare(Box box) {

         return this->Volume() > box.Volume();

      }

   private:

      double length;     // Length of a box

      double breadth;    // Breadth of a box

      double height;     // Height of a box

};

int main(void) {

   Box Box1(3.3, 1.2, 1.5);    // Declare box1

   Box Box2(8.5, 6.0, 2.0);    // Declare box2

   if(Box1.compare(Box2)) {

      cout << "Box2 is smaller than Box1" <<endl;

   } else {

      cout << "Box2 is equal to or larger than Box1" <<endl;

   }

   return 0;

}

When the above code is compiled and executed, it produces the following result −

Constructor called.

Constructor called.

Box2 is equal to or larger than Box1

Key takeaway

Every object in C++ has access to its own address through an important pointer called this pointer. The this pointer is an implicit parameter to all member functions. Therefore, inside a member function, this may be used to refer to the invoking object.

Friend functions do not have a this pointer, because friends are not members of a class. Only member functions have a this pointer.

2.4 Type checking C++ Pointers

C++ pointers are easy and fun to learn. Some C++ tasks are performed more easily with pointers, and other C++ tasks, such as dynamic memory allocation, cannot be performed without them.

As you know every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator which denotes an address in memory. Consider the following which will print the address of the variables defined −

#include <iostream>

using namespace std;

int main () {

   int  var1;

   char var2[10];

   cout << "Address of var1 variable: ";

   cout << &var1 << endl;

   cout << "Address of var2 variable: ";

   cout << &var2 << endl;

   return 0;

}

When the above code is compiled and executed, it produces the following result −

Address of var1 variable: 0xbfebd5c0

Address of var2 variable: 0xbfebd5b6

What are Pointers?

A pointer is a variable whose value is the address of another variable. Like any variable or constant, you must declare a pointer before you can work with it. The general form of a pointer variable declaration is −

type *var-name;

Here, type is the pointer's base type; it must be a valid C++ type and var-name is the name of the pointer variable. The asterisk you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration −

int    *ip;    // pointer to an integer

double *dp;    // pointer to a double

float  *fp;    // pointer to a float

char   *ch     // pointer to character

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

Using Pointers in C++

There are few important operations, which we will do with the pointers very frequently. (a) We define a pointer variable. (b) Assign the address of a variable to a pointer. (c) Finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations −

#include <iostream>

using namespace std;

int main () {

   int  var = 20;   // actual variable declaration.

   int  *ip;        // pointer variable

   ip = &var;       // store address of var in pointer variable

   cout << "Value of var variable: ";

   cout << var << endl;

   // print the address stored in ip pointer variable

   cout << "Address stored in ip variable: ";

   cout << ip << endl;

   // access the value at the address available in pointer

   cout << "Value of *ip variable: ";

   cout << *ip << endl;

   return 0;

}

When the above code is compiled and executed, it produces result something as follows −

Value of var variable: 20

Address stored in ip variable: 0xbfc601ac

Value of *ip variable: 20

Pointers in C++

Pointers have many but easy concepts and they are very important to C++ programming. There are following few important pointer concepts which should be clear to a C++ programmer −

Key takeaway

C++ pointers are easy and fun to learn. Some C++ tasks are performed more easily with pointers, and other C++ tasks, such as dynamic memory allocation, cannot be performed without them.

As you know every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator which denotes an address in memory.

2.5 Pointers to Derived types

Data types are means to identify the type of data and associated operations of handling it. There are three types of data types:

1. Derived Data Types
2. Pre-defined Data Types
3. User-defined Data Types

Fig 1 – DataTypes

In this article, the Derived Data Type is explained:

Derived Data Types

The data-types that are derived from the primitive or built-in datatypes are referred to as Derived Data Types. These can be of four types namely:

• Function
• Array
• Pointers
• References

Let’s briefly understand each of the following derived datatypes:

1. Function: A function is a block of code or program-segment that is defined to perform a specific well-defined task. A function is generally defined to save the user from writing the same lines of code again and again for the same input. All the lines of code are put together inside a single function and this can be called anywhere required. main() is a default function that is defined in every program of C++.

Syntax:

FunctionType FunctionName(parameters)

Example:

Output:

m is 20

2.     Array: An array is a collection of items stored at continuous memory locations. The idea of array is to represent many instances in one variable.

Fig 2 – Example

Syntax:

DataType ArrayName[size_of_array];

Example:

Output:

5 2 -10 5

3.     Pointers: Pointers are symbolic representation of addresses. They enable programs to simulate call-by-reference as well as to create and manipulate dynamic data structures. It’s general declaration in C/C++ has the format:

Syntax:

datatype *var_name;

Example:

int *ptr;  

ptr points to an address

which holds int data

Example:

Output:

Value at ptr = 0x7ffc10d7fd5c

Value at var = 20

Value at *ptr = 20

4.     Reference: When a variable is declared as reference, it becomes an alternative name for an existing variable. A variable can be declared as reference by putting ‘&’ in the declaration.

Example:

Output:

x = 20

ref = 30

Key takeaway

Data types are means to identify the type of data and associated operations of handling it. There are three types of data types:

1. Derived Data Types
2. Pre-defined Data Types
3. User-defined Data Types

2.6 Pointers to Class members

A pointer to a C++ class is done exactly the same way as a pointer to a structure and to access members of a pointer to a class you use the member access operator -> operator, just as you do with pointers to structures. Also as with all pointers, you must initialize the pointer before using it.

Let us try the following example to understand the concept of pointer to a class –

#include <iostream>

using namespace std;

class Box {

   public:

      // Constructor definition

      Box(double l = 2.0, double b = 2.0, double h = 2.0) {

         cout <<"Constructor called." << endl;

         length = l;

         breadth = b;

         height = h;

      }

      double Volume() {

         return length * breadth * height;

      }

   private:

      double length;     // Length of a box

      double breadth;    // Breadth of a box

      double height;     // Height of a box

};

int main(void) {

   Box Box1(3.3, 1.2, 1.5);    // Declare box1

   Box Box2(8.5, 6.0, 2.0);    // Declare box2

   Box *ptrBox;                // Declare pointer to a class.

   // Save the address of first object

   ptrBox = &Box1;

   // Now try to access a member using member access operator

   cout << "Volume of Box1: " << ptrBox->Volume() << endl;

   // Save the address of second object

   ptrBox = &Box2;

   // Now try to access a member using member access operator

   cout << "Volume of Box2: " << ptrBox->Volume() << endl;

   return 0;

}

When the above code is compiled and executed, it produces the following result −

Constructor called.

Constructor called.

Volume of Box1: 5.94

Volume of Box2: 102

Key takeaway

A pointer to a C++ class is done exactly the same way as a pointer to a structure and to access members of a pointer to a class you use the member access operator -> operator, just as you do with pointers to structures. Also as with all pointers, you must initialize the pointer before using it.

2.7 Dynamic Allocation Pointers:-New & Delete Operator

A good understanding of how dynamic memory really works in C++ is essential to becoming a good C++ programmer. Memory in your C++ program is divided into two parts −

• The stack − All variables declared inside the function will take up memory from the stack.
• The heap − This is unused memory of the program and can be used to allocate the memory dynamically when program runs.

Many times, you are not aware in advance how much memory you will need to store particular information in a defined variable and the size of required memory can be determined at run time.

You can allocate memory at run time within the heap for the variable of a given type using a special operator in C++ which returns the address of the space allocated. This operator is called new operator.

If you are not in need of dynamically allocated memory anymore, you can use delete operator, which de-allocates memory that was previously allocated by new operator.

new and delete Operators

There is following generic syntax to use new operator to allocate memory dynamically for any data-type.

new data-type;

Here, data-type could be any built-in data type including an array or any user defined data types include class or structure. Let us start with built-in data types. For example we can define a pointer to type double and then request that the memory be allocated at execution time. We can do this using the new operator with the following statements −

double* pvalue  = NULL; // Pointer initialized with null

pvalue  = new double;   // Request memory for the variable

The memory may not have been allocated successfully, if the free store had been used up. So it is good practice to check if new operator is returning NULL pointer and take appropriate action as below −

double* pvalue  = NULL;

if( !(pvalue  = new double )) {

   cout << "Error: out of memory." <<endl;

   exit(1);

}

The malloc() function from C, still exists in C++, but it is recommended to avoid using malloc() function. The main advantage of new over malloc() is that new doesn't just allocate memory, it constructs objects which is prime purpose of C++.

At any point, when you feel a variable that has been dynamically allocated is not anymore required, you can free up the memory that it occupies in the free store with the ‘delete’ operator as follows −

delete pvalue;        // Release memory pointed to by pvalue

Let us put above concepts and form the following example to show how ‘new’ and ‘delete’ work −

#include <iostream>

using namespace std;

int main () {

   double* pvalue  = NULL; // Pointer initialized with null

   pvalue  = new double;   // Request memory for the variable

   *pvalue = 29494.99;     // Store value at allocated address

   cout << "Value of pvalue : " << *pvalue << endl;

   delete pvalue;         // free up the memory.

   return 0;

}

If we compile and run above code, this would produce the following result −

Value of pvalue : 29495

Dynamic Memory Allocation for Arrays

Consider you want to allocate memory for an array of characters, i.e., string of 20 characters. Using the same syntax what we have used above we can allocate memory dynamically as shown below.

char* pvalue  = NULL;         // Pointer initialized with null

pvalue  = new char[20];       // Request memory for the variable

To remove the array that we have just created the statement would look like this −

delete [] pvalue;             // Delete array pointed to by pvalue

Following the similar generic syntax of new operator, you can allocate for a multi-dimensional array as follows −

double** pvalue  = NULL;      // Pointer initialized with null

pvalue  = new double [3][4];  // Allocate memory for a 3x4 array

However, the syntax to release the memory for multi-dimensional array will still remain same as above −

delete [] pvalue;            // Delete array pointed to by pvalue

Dynamic Memory Allocation for Objects

Objects are no different from simple data types. For example, consider the following code where we are going to use an array of objects to clarify the concept −

#include <iostream>

using namespace std;

class Box {

   public:

      Box() {

         cout << "Constructor called!" <<endl;

      }

      ~Box() {

         cout << "Destructor called!" <<endl;

      }

};

int main() {

   Box* myBoxArray = new Box[4];

   delete [] myBoxArray; // Delete array

   return 0;

}

If you were to allocate an array of four Box objects, the Simple constructor would be called four times and similarly while deleting these objects, destructor will also be called same number of times.

If we compile and run above code, this would produce the following result −

Constructor called!

Constructor called!

Constructor called!

Constructor called!

Destructor called!

Destructor called!

Destructor called!

Destructor called!

Key Takeaway

A good understanding of how dynamic memory really works in C++ is essential to becoming a good C++ programmer. Memory in your C++ program is divided into two parts −

• The stack − All variables declared inside the function will take up memory from the stack.
• The heap − This is unused memory of the program and can be used to allocate the memory dynamically when program runs.

REFERENCE 

1 Object-Oriented Programming with C++ by E. Balaguruswamy. (Tata McGraw-Hill) 6th Edition and onwards

2. Object oriented Programming with C++- by SouravSahay (Oxford) 2ndedition