Basic program constructs in C++

Basic program constructs in C++

Following is a sample program in C++ that prints a string on the string.

#include<iostream.h>

int main()

{

            cout<<”Hi Everybody”;

            return 0;

}

Like in C, functions are the basic building block in C++. The above example consists of a single function called main ().

When a C++ program executes, the first statement that executes will be the first statement in main () function. The main function calls member functions of various classes (using objects) to carry out the real work. It may also call other stand-alone functions.

In C++, the return type of the main function is ‘int’. So, it returns one integer value to the operating system. Since the return type int is default, the keyword ‘int’ in main () is optional. So,

            main ()

            {

                …….// also valid

            }

Compilers generate error if no value is returned. Many operating systems test the return values. If the exit value is zero (0), the operating system will understand that the program ran successfully. If the returned value (exit value) is non zero, it would mean that there was problem.

Comment syntax

In C++, comments start with a double slash (//) symbol and end at the end of the line. A comment may start at the beginning of a line or anywhere in the line and whatever follows till the end of that line is ignored. There is no closing symbol. If we need to comment multiple lines, we can write as

// this is an

// example

// of multi line comments

The C comment style /*………….*/ may also be used for multi line comments.

The output operator

The statement – cout<<”Hi Everybody”; in the above example prints the phrase in quotation marks on the screen.

Here, the identifier ‘cout’, pronounced as see out, is a predefined object of standard stream in C++. The operator << is called ‘insertion’ or ‘put to’ operator. It inserts the content on its right to the object on its left.

            cout<<a

In the above case, the statement will display the content of the variable ‘a’.

The input operator

A statement

                        cin>>a;

is an input statement.  This causes the program to wait for the user to type and give some input. The given input is stored in the variable a.

Here, the identifier ‘cin’, pronounced as ‘see in’ is an object of standard input stream. The operator ‘>>’ is called ‘extraction’ or ‘get from’ operator. It extracts or gets value from keyboard and assigns it to the variable on its right.

Cascading I/O operators

The i/o operators can be used repeatedly in a single i/o statements as follows.

            cout<<a<<b<<c;

            cin>>x>>y>>z;

These are perfectly legal. The above cout statement first sends the value of ‘a’ to cout, then sends the value of b and then sends the value of c. Similarly, the cin statement first reads a value and stores in x, then reads again and stores in y and then in z. The multiple uses of i/o operators in one statement is called cascading.

The iostream header file

The directive ‘#include<iostream.h>’ causes the preprocessor to add the contents of iostream.h file to the program. It contains the declarations of identifiers cout, cin and the operators << and >>. So, the header file iostream should always be included at the beginning if we need to use cin, cout, << and >> operators in our program.

Tokens: Tokens are the smallest individual units in a program. Keywords, identifiers, constants, strings and operators are tokens in C++.

Keywords: Keywords are explicitly reserved identifiers and can not be used as names for the program variables or other user-defined program elements. Some keywords are int, auto, switch, case, do, else, public, default, continue etc.

Functions

A function is a single comprehensive unit that performs a specified task. This specified task is repeated each time the function is called. Functions break large programs into smaller tasks. They increase the modularity of the programs and reduce code redundancy.

Like in C, C++ programs also should contain a main function, where the program always begins execution. The main function may call other functions, which in turn will again call other functions.

When a function is called, control is transferred to the first statement of the function body. Once the statements of the function get executed (when the last closing bracket is encountered) the program control return to the place from where this function was called.

Data types in C++

Fig. Hierarchy of C++ Data types

Enumerated Data Types

Like structures, enumerated data type is another user defined data type. Enumerated means that all the values are listed. They are used when we know a finite list of values that a data type can take on or it is an alternative way for creating symbolic constants. The ‘enum’ keyword automatically lists a list of words and assign them values 0,1,2…

Eg.  enum shape  {circle, square, triangle};

        enum Boolean  {true, false};

        enum switch  {on, off};

The above example is equivalent to

            const  circle = 0;

            const square = 1;

            const triangle = 2;

We can even use standard arithmetic operator on enum types. We can also use relational operators when suitable. This is because, the enum data types are internally treated as integers.

Once we specify the data type, we need to define variables of that type.

            Eg. shape s1,s2;

Now the variables s1, s2 can hold only the members of ‘shape’ data type (those are circle, square and triangle) and can not hold anything except these values. If other values are given, error will be generated.

An example

#include<iostream.h>
#include<conio.h>
enum days {sun, mon, tue, wed, thur, fri, sat};

void main()

{

            days d1,d2;

            d1 = sun;

            d2 = thur;

            int diff = d2-d1;  // using arithmetic operator

            cout<<”Days between”<<diff<<endl;

            if(d1<d2)   // using relational operator

                        cout<<”d1 comes first”;

            getch();

}

Reference variables

Reference variables are new type of variable introduced in C++. It provides an alias (another name) for a previously defined variable. A syntax to create a reference variable is

            data-type  &reference-name = variable name;

eg-

            float total = 100;

            float &sum = total; // creating reference variable for ‘total’.

In the above example, we are creating a reference variable ‘sum’ for an existing variable ‘total’. Now these can be used interchangeably. Both of these names refer to same data object in memory. If the value is manipulated and changed using one name then it will change for another also. Eg- the statement

            total = total + 200; will change value of ‘total’ to 300. And it will also change for ‘sum’. So the statements

cout<<sum;

cout<<total; both will print 300. This is because both the variables use same data object in memory.

-          A reference variable must be initialized at the time of declaration, since this will establish correspondence between the reference and the data object which it names.

-          The symbol & is not an address operator here. The notation int & means reference to integer type data.

-          References can also be created for user defined data types like structures and classes.

-          Another application of reference variable is in passing arguments to function.

In general, arguments are passed by value. The called function creates a new value of the same type as the argument and copies the argument value into it. The function does not access the actual value. Although this provides security to the actual data, it is not suitable if we need to modify actual data. For such situations, we can use reference. Instead of value, a reference to the original variable is passed to the called function. This is called calling function by reference. The advantage is that the called function can use actual variable and not its copy only. Likewise, we can also return values using reference. (Example- swapping values)

Manipulators

The manipulators are operators used with insertion operator “<<’ to format or manipulate the data display. ‘endl’ and ‘setw’ are most common manipulators.

endl manipulator causes a linefeed to be inserted into the output stream. i.e the cursor moves to next line. It is similar to ‘\n’ character.

Eg-

                        cout<<”Kathmandu”<<endl;

                        cout<<”Nepal”;

See output

setw manipulator specify a field width to a number or string that follows it and force them to be printed right justified. The field width is given as an argument to this manipulator.

Eg-

                        x = 456; y = 40;

                        cout<<setw(5)<<x<<setw(5)<<y;

The manipulator will specify a field 5 for printing the value of x. The value is right justified within the fields as shown below. For y, it will specify again space of width 5, right justifies and prints.