Functions in C++

Main Function
Function prototyping
Call by reference
Return by referenceReturn by reference
Inline functions
Default arguments
Const arguments
Function overloading
Friend functions and class

What is the difference between main
function in C and C++?
C does not specify the return type for theC does not specify the return type for the
main() whereas in C++ the main() returns a
value of type int.

The prototype describes the function interface to the
compiler by giving details such as the number and
type of arguments and the type of return values.
A template is used when declaring and defining aA template is used when declaring and defining a
function.
Syntax:
type function_name (argument_list);
Eg:
float volume ( int x, float y, float z)

Check whether the following function prototype
is correct or not. Give reasons.
1. float volume (int x, float y, z)
2. float volume (int, float, float)2. float volume (int, float, float)
Incorrect
Each argument variable must be declared
independently.
Correct
Variable names are optional.

C++ permits to pass parameters to the
functions by reference.
When we pass arguments by reference, the
formal arguments in the called function
become aliases to the actual arguments in
the calling function.
It is used where we would like to alter the
values of variables in the calling function.

Example:
void swap (int &a, int &b)
{
int t = a;int t = a;
a = b;
b = t;
}
Function call swap(m, n)
Values of m and n are exchanged using their
aliases a and b.

A function can also return a reference.
Example:
int & max (int & x, int & y)
{
if ( x > y )if ( x > y )
return x;
else
return y;
}
Since the return type of max() is int & the
function will return reference to x or y and not
the values.

An inline function is a function that is expanded
in line when it is invoked.
Syntax:
inline function-header
{{
function body
}
Eg:
inline double cube(double a)
{
return (a*a*a);
}

The above inline function is invoked as:
c = cube(3.0);
The speed benefits of inline functionThe speed benefits of inline function
diminish as the function grows in size.
Example

C++ allows to call a function without specifying
all its arguments.
For eg:
float amount ( float principal, int period, float
rate=0.15)
A function call:
value = amount (5000, 7); // one argument missing
value = amount ( 5000, 5, 0.12) // all arguments

In C++ an argument to a function can be
declared as const
Eg:
int strlen ( const char *p );
int length ( const string &s);
The qualifier const tells the compiler that
the function should not modify the
argument.

Function overloading means to use same
function name to create functions that
perform variety of different tasks.
The function would perform differentThe function would perform different
operations depending on the argument list in
the function call.
The correct function to be invoked is
determined by checking the number and type
of the arguments but not on the function
type.

// Declarations
int add( int a, int b); // prototype 1
int add ( int a, int b, int c); // prototype 2
double add ( double x, double y); // prototype 3
double add (int p, double q); // prototype 4double add (int p, double q); // prototype 4
double add (double p, int q); // prototype 5
//Function calls
cout << add (5, 10); // uses prototype 1
cout << add (15, 10.0); // uses prototype 4
cout << add ( 12.5, 7.5); // uses prototype 3
cout << add (5, 10, 15); // uses prototype 2
cout << add (0.75, 5); // uses prototype 5

A function call first matches the prototype having the same
number and type of arguments and then call the appropriate
function for execution.
If an exact match is not found, the compiler uses the integral
promotions to the actual arguments such as char to int, float
to double.
If either of them fails, the compiler tries to use the built in
conversations to the actual arguments and then uses the
functions whose match is unique.
long square(long n)
double square(double x)
A function call square(10) will cause an error because int
argument can be converted into either long or double there by
causing ambiguous situation.

Function overloading is used when we want
functions to perform closely related tasks.
Sometimes default arguments may be usedSometimes default arguments may be used
instead of overloading.
Example

C++ allows the common functions to be made
friendly to the classes.
For eg: consider two classes manager and
scientists and they want to use a function
income_tax().
So we make this function friendly with both the
classes, thereby allowing the function to have
access to the private data of these classes.

Declaration:
Class ABC
{
……
……
public:
……
……
friend void xyz(void); // declaration
};
The function declaration should be preceded
by the keyword friend.

A Friend function possesses certain special
characteristics:
It is not in the scope of the class to which it has been
declared as friend.
It cannot be called using the object of the class.(as it
doesn’t come under the scope of the class)
It can be invoked like a normal function without theIt can be invoked like a normal function without the
help of any object.
Unlike member functions, it cannot access the
member names directly and has to use an object
name and dot membership operator with each
member name.
It can be declared either in public or private part of
the class without affecting its meaning.
Usually, it has the objects as arguments.

Class sample
{
int a;
int b;
public:
void setvalue() { a = 25; b = 40; }
friend float mean(sample s);
};
float mean(sample s)
{
return float(s.a + s.b)/2.0;
}
int main()
{
Sample X; //Object X
X.setValue();
cout << “Mean value = “ << mean(X) << “n”;
return 0;
}

Member functions of one class can be friend
functions of another class.
class X
{
……
…………
int fun1(); // member function of X
……
};
class Y
{
……
……
friend int X :: fun1(); // fun1() of X is friend of Y
……
};

class ABC; // forward declaration
class XYZ
{
int x;
public:
void setvalue(int i ) { x = i ; }void setvalue(int i ) { x = i ; }
friend void max(XYZ, ABC);
};
class ABC
{
int a;
public:
void setvalue(int i ) { a = i; }
friend void max(XYZ, ABC);
};

void max(XYZ m, ABC n ) // definition of friend
{
if ( m.x >= n.a)
cout << m.x;
else
cout << n.a;
}}
int main()
{
ABC abc;
abc.setvalue(10);
XYZ xyz;
xyz.setvalue(20);
max(xyz, abc);
return 0;
}

A friend function can access and alter the values
of private members of the class, thus violating
the data hiding principle of OOPs.
This can be done by calling friend function byThis can be done by calling friend function by
reference.
Here the local copies of the objects are not
made instead a pointer to the address of the
object is passed and the called function directly
works on the actual object used in the call.

class class_2;
class class_1
{
int value1;
public:
void indata(int a) { value1 = a; }
void display() { cout<< value1 << “n”; }void display() { cout<< value1 << “n”; }
friend void exchange(class_1 &, class_2 &);
};
class class_2
{
int value2;
public:
void indata(int a) { value2 = a; }
void display() { cout<< value2 << “n”; }
friend void exchange(class_1 &, class_2 &);
};

void exchange(class_1 & x, class_2 & y) // object x and y are aliases
{ of C1 and C2
int temp = x.value1; // directly modifying the values of
x.value1 = y.value2; value1 and value2 of class_1 and
y.value2 = temp; class_2
}
int main()
{
class_1 c1;
class_2 c2;
c1.indata(100);c1.indata(100);
c2.indata(200);
cout << “values before exchange” << “n”;
c1.display();
c2.dispaly();
exchange( c1, c2);
cout << “values after exchange” << “n”;
c1.display();
c2.dispaly();
return 0;
}

We can declare all the member functions of
a class as a friend function of another class.
In such cases the class is known as a friend
class.
Syntax:Syntax:
class Z
{
……….
friend class X;
};

State true or false:
Class members are public by default.
Friend functions have access to only public members
of class.
An entire class can be made a friend of another class.
Functions cannot return class objects.
A function designed as private is accessible only to
member functions of that class.
A function designed as public can be accessed like any
other ordinary functions.

When will you make a function inline? Why?
When a function is small and is likely to be called many time we make it
as inline. Inline expansion makes a program run faster because the
overhead of a function call and return is eliminated.
How does an inline function differ from a pre-processor macro?
The compiler replaces the function code when the inline functions areThe compiler replaces the function code when the inline functions are
called and hence perform type checking whereas macro are simply the
substitution of code by the pre-processor before compilation and hence
no type checking or error checking.
What is the significance of an empty parenthesis in a function
declaration?
Empty parenthesis indicates that the function does not contains any
arguments.

What is the main advantages of passing arguments by
reference?
When the arguments are passed by reference then we can
alter the original data instead of its local copies.
What do you meant by overloading of a function? When doWhat do you meant by overloading of a function? When do
we use this concept?
Function overloading means to use same function name to
create functions that perform variety of different tasks.
Function overloading is used perform different operations
depending on the argument list in the function call. It is also
used for handling class objects.

Object Oriented Programming with C++ by E.
Balagurusamy.

Functions in C++

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