C++ Tutorial Pointers

C++ Tutorial

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 result something as follows:

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 variables (b) assign the address of a variable to a pointer and (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

C++ Pointers in Detail:

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:

Concept	Description
C++ Null Pointers	C++ supports null pointer, which is a constant with a value of zero defined in several standard libraries.
C++ pointer arithmetic	There are four arithmetic operators that can be used on pointers: ++, --, +, -
C++ pointers vs arrays	There is a close relationship between pointers and arrays. Let us check how?
C++ array of pointers	You can define arrays to hold a number of pointers.
C++ pointer to pointer	C++ allows you to have pointer on a pointer and so on.
Passing pointers to functions	Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function.
Return pointer from functions	C++ allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well.

C++ Tutorial Strings

C++ Tutorial

C++ Strings

C++ provides following two types of string representations:

·        The C-style character string.

·        The string class type introduced with Standard C++.

The C-Style Character String:

The C-style character string originated within the C language and continues to be supported within C++. This string is actually a one-dimensional array of characters which is terminated by a null character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a null.

The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello."

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

If you follow the rule of array initialization, then you can write the above statement as follows:

char greeting[] = "Hello";

Following is the memory presentation of above defined string in C/C++:

Actually, you do not place the null character at the end of a string constant. The C++ compiler automatically places the '\0' at the end of the string when it initializes the array. Let us try to print above-mentioned string:

#include <iostream>

using namespace std;

int main ()

{

   char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

   cout << "Greeting message: ";

   cout << greeting << endl;

   return 0;

}

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

Greeting message: Hello

C++ supports a wide range of functions that manipulate null-terminated strings:

S.N.	Function & Purpose
1	strcpy(s1, s2); Copies string s2 into string s1.
2	strcat(s1, s2); Concatenates string s2 onto the end of string s1.
3	strlen(s1); Returns the length of string s1.
4	strcmp(s1, s2); Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2.
5	strchr(s1, ch); Returns a pointer to the first occurrence of character ch in string s1.
6	strstr(s1, s2); Returns a pointer to the first occurrence of string s2 in string s1.

Following example makes use of few of the above-mentioned functions:

#include <iostream>

#include <cstring>

using namespace std;

int main ()

{

   char str1[10] = "Hello";

   char str2[10] = "World";

   char str3[10];

   int  len ;

   // copy str1 into str3

   strcpy( str3, str1);

   cout << "strcpy( str3, str1) : " << str3 << endl;

   // concatenates str1 and str2

   strcat( str1, str2);

   cout << "strcat( str1, str2): " << str1 << endl;

   // total lenghth of str1 after concatenation

   len = strlen(str1);

   cout << "strlen(str1) : " << len << endl;

   return 0;

}

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

strcpy( str3, str1) : Hello

strcat( str1, str2): HelloWorld

strlen(str1) : 10

The String Class in C++:

The standard C++ library provides a string class type that supports all the operations mentioned above, additionally much more functionality. We will study this class in C++ Standard Library but for now let us check following example:

At this point, you may not understand this example because so far we have not discussed Classes and Objects. So can have a look and proceed until you have understanding on Object Oriented Concepts.

#include <iostream>

#include <string>

using namespace std;

int main ()

{

   string str1 = "Hello";

   string str2 = "World";

   string str3;

   int  len ;

   // copy str1 into str3

   str3 = str1;

   cout << "str3 : " << str3 << endl;

   // concatenates str1 and str2

   str3 = str1 + str2;

   cout << "str1 + str2 : " << str3 << endl;

   // total lenghth of str3 after concatenation

   len = str3.size();

   cout << "str3.size() :  " << len << endl;

   return 0;

}

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

str3 : Hello

str1 + str2 : HelloWorld

str3.size() :  10

C++ Tutorial Arrays

C++ Tutorial

C++ Arrays

C++ provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays:

To declare an array in C++, the programmer specifies the type of the elements and the number of elements required by an array as follows:

type arrayName [ arraySize ];

This is called a single-dimension array. The arraySize must be an integer constant greater than zero and type can be any valid C++ data type. For example, to declare a 10-element array called balance of type double, use this statement:

double balance[10];

Initializing Arrays:

You can initialize C++ array elements either one by one or using a single statement as follows:

double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};

The number of values between braces { } can not be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array:

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write:

double balance[] = {1000.0, 2.0, 3.4, 17.0, 50.0};

You will create exactly the same array as you did in the previous example.

balance[4] = 50.0;

The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representaion of the same array we discussed above:

Accessing Array Elements:

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example:

double salary = balance[9];

The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above-mentioned three concepts viz. declaration, assignment and accessing arrays:

#include <iostream>

using namespace std;

#include <iomanip>

using std::setw;

int main ()

{

   int n[ 10 ]; // n is an array of 10 integers

   // initialize elements of array n to 0          

   for ( int i = 0; i < 10; i++ )

   {

      n[ i ] = i + 100; // set element at location i to i + 100

   }

   cout << "Element" << setw( 13 ) << "Value" << endl;

   // output each array element's value                      

   for ( int j = 0; j < 10; j++ )

   {

      cout << setw( 7 )<< j << setw( 13 ) << n[ j ] << endl;

   }

   return 0;

}

This program makes use of setw() function to format the output. When the above code is compiled and executed, it produces the following result:

Element        Value

      0          100

      1          101

      2          102

      3          103

      4          104

      5          105

      6          106

      7          107

      8          108

      9          109

C++ Arrays in Detail:

Arrays are important to C++ and should need lots of more detail. There are following few important concepts, which should be clear to a C++ programmer:

Concept	Description
Multi-dimensional arrays	C++ supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.
Pointer to an array	You can generate a pointer to the first element of an array by simply specifying the array name, without any index.
Passing arrays to functions	You can pass to the function a pointer to an array by specifying the array's name without an index.
Return array from functions	C++ allows a function to return an array.