Chapter – 3
Using Functions and Classes
This chapter describes how to
- Declare and call standard functions and
- Use standard classes.
This includes using standard header files. In addition, we will be working with string variables, i.e. objects belonging to the standard class string for the first time.
Functions and classes that you define on your own will not be introduced until later in the book.
■ DECLARING FUNCTIONS
Exampleofafunction prototype
The prototype above yields the following information to the compiler:
- func is the function name
- the function is called with two arguments: the first argument is of type int, the second of type double
- the return value of the function is of type long.
Mathematical standard functions
□ Declarations
Each name (identifier) occurring in a program must be known to the compiler or it will cause an error message. That means any names apart from keywords must be declared, i.e. introduced to the compiler, before they are used.
Each time a variable or a function is defined it is also declared. But conversely, not every declaration needs to be a definition. If you need to use a function that has already been introduced in a library, you must declare the function but you do not need to rede- fine it.
□ Declaring Functions
A function has a name and a type, much like a variable. The function’s type is defined by its return value, that is, the value the function passes back to the program. In addition, the type of arguments required by a function is important. When a function is declared, the compiler must therefore be provided with information on
- The name and type of the function and
- The type of each argument.
This is also referred to as the function prototype.
Examples:
int toupper(int); double pow(double, double);
This informs the compiler that the function toupper() is of type int, i.e. its return value is of type int, and it expects an argument of type int. The second function pow() is of type double and two arguments of type double must be passed to the function when it is called. The types of the arguments may be followed by names, how- ever, the names are viewed as a comment only.
Examples:
int toupper(int c); double pow(double base, double exponent);
Examples:
int toupper(int c); double pow(double base, double exponent);
From the compiler’s point of view, these prototypes are equivalent to the prototypes in the previous example. Both junctions are standard junctions.
Standard function prototypes do not need to be declared, nor should they be, as they have already been declared in standard header files. If the header file is included in the program’s source code by means of the #include directive, the function can be used immediately.
Example: #include <cmath>
Following this directive, the mathematical standard functions, such as sin(), cos(), and pow(), are available. Additional details on header files can be found later in this chapter.
■ FUNCTION CALLS
Sample program
Screenoutput
2.5 raised to the power 3 yields: 15.625 2 + (5 raised to the power 2.5) yields: 57.9017
□ Function Calls
A function call is an expression of the same type as the function and whose value corre- sponds to the return value. The return value is commonly passed to a suitable variable.
Example: y = pow( x, 3.0);
In this example the function pow()is first called using the arguments x and 3.0, and the result, the power x3, is assigned to y.
As the function call represents a value, other operations are also possible. Thus, the function pow() can be used to perform calculations for double values.
Example: cout << 2.0 + pow( 5.0, x);
This expression first adds the number 2.0 to the return value of pow(5.0,x), then outputs the result using cout.
Any expression can be passed to a function as an argument, such as a constant or an arithmetical expression. However, it is important that the types of the arguments corre- spond to those expected by the function.
The compiler refers to the prototype to check that the function has been called cor- rectly. If the argument type does not match exactly to the type defined in the prototype, the compiler performs type conversion, if possible.
Example: y = pow( x, 3); // also ok!
The value 3 of type int is passed to the function as a second argument. But since the function expects a double value, the compiler will perform type conversion from int to double.
If a function is called with the wrong number of arguments, or if type conversion proves impossible, the compiler generates an error message. This allows you to recognize and correct errors caused by calling functions at the development stage instead of causing runtime errors.
Example: float x = pow(3.0 + 4.7); // Error!
The compiler recognizes that the number of arguments is incorrect. In addition, the compiler will issue a warning, since a double, i.e. the return value of pow(), is assigned to a float type variable.
■ TYPE void FOR FUNCTIONS
Sample program
NOTE: The statement cin >> seed; reads an integer from the keyboard, because seed is of the unsigned int type.
Sample screen output
--- Random Numbers --- To initialize the random number generator, please enter an integer value: 7777 Three random numbers: 25435 6908 14579
□ Functions without Return Value
You can also write functions that perform a certain action but do not return a value to the function that called them. The type void is available for functions of this type, which are also referred to as procedures in other programming languages.
Example: void srand( unsigned int seed );
The standard function srand() initializes an algorithm that generates random num- bers. Since the function does not return a value, it is of type void. An unsigned value is passed to the function as an argument to seed the random number generator. The value is used to create a series of random numbers.
□ Functions without Arguments
If a function does not expect an argument, the function prototype must be declared as
void or the braces following the function name must be left empty.
Example: int rand( void ); // or int rand();
The standard function rand() is called without any arguments and returns a random number between 0 and 32767. A series of random numbers can be generated by repeating the function call.
□ Usage of srand() and rand()
The function prototypes for srand() and rand() can be found in both the cstdlib
and stdlib.h header files.
Calling the function rand() without previously having called srand() creates the same sequence of numbers as if the following statement would have been proceeded:
srand(1);
If you want to avoid generating the same sequence of random numbers whenever the program is executed, you must call srand() with a different value for the argument whenever the program is run.
It is common to use the current time to initialize a random number generator. See Chapter 6 for an example of this technique.
■ HEADER FILES
Usingheaderfiles
□ Using Header Files
Header files are text files containing declarations and macros. By using an #include directive these declarations and macros can be made available to any other source file, even in other header files.
Pay attention to the following points when using header files:
- Header files should generally be included at the start of a program before any other declarations
- You can only name one header file per #include directive
- The file name must be enclosed in angled brackets < … > or double quotes ” … “.
□ Searching for Header Files
The header files that accompany your compiler will tend to be stored in a folder of their own—normally called include. If the name of the header file is enclosed by angled brackets < … >, it is common to search for header files in the include folder only. The current directory is not searched to increase the speed when searching for header files.
C++ programmers commonly write their own header files and store them in the cur- rent project folder. To enable the compiler to find these header files, the #include directive must state the name of the header files in double quotes.
Example: #include "project.h"
The compiler will then also search the current folder. The file suffix .h is normally used for user-defined header files.
□ Standard Class Definitions
In addition to standard function prototypes, the header files also contain standard class definitions. When a header file is included, the classes defined and any objects declared in the file are available to the program.
Example:
#include <iostream> using namespace std;
Following these directives, the classes istream and ostream can be used with the cin and cout streams. cin is an object of the istream class and cout an object of the ostream class.
■ STANDARD HEADER FILES
HeaderfilesoftheC++standardlibrary
NOTE: Some IDE’s put the old-fashionediostream.h andiomanip.h header files at your disposal. Within these header files the identifiers ofiostreamandiomanipare not contained in the std namespace but are declared globally.
Header files of the C standard library
The C++ standard library header files are shown opposite. They are not indicated by the file extension .h and contain all the declarations in their own namespace, std. Name- spaces will be introduced in a later chapter. For now, it is sufficient to know that identi- fiers from other namespaces cannot be referred to directly. If you merely stipulate the directive
Example: #include <iostream>
the compiler would not be aware of the cin and cout streams. In order to use the iden- tifiers of the std namespace globally, you must add a using directive.
Example:
#include <iostream> #include <string> using namespace std;
You can then use cin and cout without any additional syntax. The header file string has also been included. This makes the string class available and allows user- friendly string manipulations in C++. The following pages contain further details on this topic.
□ Header Files in the C Programming Language
The header files standardized for the C programming language were adopted for the C++ standard and, thus, the complete functionality of the standard C libraries is available to C++ programs.
Example: #include <math.h>
Mathematical functions are made available by this statement.
The identifiers declared in C header files are globally visible. This can cause name conflicts in large programs. For this reason each C header file, for example name.h, is accompanied in C++ by a second header file, cname, which declares the same identifiers in the std namespace. Including the file math.h is thus equivalent to
Example: #include
using namespace std;
The string.h or cstring files must be included in programs that use standard func- tions to manipulate C strings. These header files grant access to the functionality of the C string library and are to be distinguished from the string header file that defines the string class.
Each compiler offers additional header files for platform dependent functionalities. These may be graphics libraries or database interfaces.
■ USING STANDARD CLASSES
Sampleprogramusingclassstring
NOTE: Both the operators + and += for concatenation and the relational operators <, <=, >, >=, ==, and != are defined for objects of class string. Strings can be printed with cout and the operator <<. The class string will be introduced in detail later on.
Sample screen output
What is your name: Rose Summer --------------------------------- Hello Rose Summer Your name is 11 characters long! ---------------------------------
Several classes are defined in the C++ standard library. These include stream classes for input and output, but also classes for representing strings or handling error conditions.
Each class is a type with certain properties and capacities. As previously mentioned, the properties of a class are defined by its data members and the class’s capacities are defined by its methods. Methods are functions that belong to a class and cooperate with the members to perform certain operations. Methods are also referred to as member func- tions.
□ Creating Objects
An object is a variable of a class type, also referred to as an instance of the class. When an object is created, memory is allocated to the data members and initialized with suitable values.
Example: string s("I am a string");
In this example the object s, an instance of the standard class string (or simply a string), is defined and initialized with the string constant that follows. Objects of the string class manage the memory space required for the string themselves. In general, there are several ways of initializing an object of a class. A string can thus be initialized with a certain number of identical characters, as the example on the oppo- site page illustrates
□ Calling Methods
All the methods defined as public within the corresponding class can be called for an object. In contrast to calling a global function, a method is always called for one particular object. The name of the object precedes the method and is separated from the method by a period.
Example: s.length(); // object.method();
The method length() supplies the length of a string, i.e. the number of characters in a string. This results in a value of 13 for the string s defined above.
□ Classes and Global Functions
Globally defined functions exist for some standard classes. These functions perform certain operations for objects passed as arguments. The global function getline(), for exam- ple, stores a line of keyboard input in a string.
Example: getline(cin, s);
The keyboard input is terminated by pressing the return key to create a new-line charac- ter, ‘\n’, which is not stored in the string.
□ EXERCISES
Screenoutputforexe
Listing for exercise 2
// A program containing errors!
# include <iostream>, <string>
# include <stdlib>
# void srand( seed);
int main()
{
string message "\nLearn from your mistakes!";
cout << message << endl;
int len = length( message);
cout << "Length of the string: " << len << endl;
// And a random number in addition:
int a, b;
a = srand(12.5);
b = rand( a );
cout << "\nRandom number: " << b << endl;
return 0;
}Exercise 1
Create a program to calculate the square roots of the numbers
4 12.25 0.0121
and output them as shown opposite.Then read a number from the keyboard and output the square root of this number.
To calculate the square root, use the function sqrt(), which is defined by the following prototype in the math.h (or cmath ) header file:
double sqrt( double x);
The return value of the sqrt() function is the square root of x.
Exercise 2
The program on the opposite page contains several errors! Correct the errors and ensure that the program can be executed.
Exercise 3
Create a C++ program that defines a string containing the following character sequence:
I have learned something new again!
and displays the length of the string on screen.
Read two lines of text from the keyboard. Concatenate the strings using ” * ” to separate the two parts of the string. Output the new string on screen.
□ SOLUTIONS
Exercise 1
// Compute square roots
#include <iostream>
#include <cmath> using namespace std;
int main()
{
double x1 = 4.0, x2 = 12.25, x3 = 0.0121;
cout << "\n Number \t Square Root" << endl;
cout << "\n " << x1 << " \t " << sqrt(x1)
<< "\n " << x2 << " \t " << sqrt(x2)
<< "\n " << x3 << " \t " << sqrt(x3) << endl;
cout << "\nType a number whose square root is to be"
" computed. ";
cin >> x1;
cout << "\n Number \t Square Root" << endl;
cout << "\n " << x1 << " \t " << sqrt(x1) << endl;
return 0;
}Exercise 2
// The corrected program:
#include <iostream> // Just one header file in a line
#include <string>
#include <cstdlib> // Prototypes of functions
// void srand( unsigned int seed);
// int rand(void);
// or:
// #include <stdlib.h>
using namespace std; // Introduces all names of namespace
// std into the global scope.
int main()
{
string message = "\nLearn from your mistakes!";...// =
cout << message << endl;
int len = message.length();
// instead of: length(message);
cout << "Length of the string: " << len << endl;
// And another random number:
int b; // Variable a is not needed.
srand(12); // instead of: a = srand(12.5);
b = rand(); // instead of: b = rand(a);
cout << "\nRandom number: " << b << endl;
return 0;
}Exercise 3
#include <iostream> // Declaration of cin, cout
#include <string> // Declaration of class string using namespace std;
int main()
{
string message("I have learned something new again!\n"),
prompt("Please input two lines of text:"),
str1, str2, sum;
cout << message << endl; // Outputs the message
cout << prompt << endl; // Request for input
getline( cin, str1); // Reads the first
getline( cin, str2); // and the second line of text
sum = str1 + " * " + str2; // Concatenates, assigns
cout << sum << endl; // and outputs strings.
return 0;
}