C++ Tutorial
C++ Functions
A function
is a group of statements that together perform a task. Every C++ program has at
least one function, which is main(),
and all the most trivial programs can define additional functions.
You can
divide up your code into separate functions. How you divide up your code among
different functions is up to you, but logically the division usually is so each
function performs a specific task.
A function declaration tells the compiler about a function's
name, return type, and parameters. A function definition provides the actual body of the
function.
The C++
standard library provides numerous built-in functions that your program can
call. For example, function strcat() to concatenate two strings, function memcpy() to copy one memory location to another
location and many more functions.
A function
is knows as with various names like a method or a sub-routine or a procedure
etc.
Defining a Function:
The
general form of a C++ function definition is as follows:
return_type function_name( parameter list )
{
body of the function
}
A C++
function definition consists of a function header and a function body. Here are
all the parts of a function:
·
Return Type: A
function may return a value. The return_type is the data type of the value the
function returns. Some functions perform the desired operations without
returning a value. In this case, the return_type is the keyword void.
·
Function Name: This is the actual name of the function. The function name and the
parameter list together constitute the function signature.
·
Parameters: A parameter is like a placeholder. When a function is invoked, you
pass a value to the parameter. This value is referred to as actual parameter or
argument. The parameter list refers to the type, order, and number of the
parameters of a function. Parameters are optional; that is, a function may
contain no parameters.
·
Function Body: The function body contains a collection of statements that define
what the function does.
Example:
Following
is the source code for a function called max().
This function takes two parameters num1 and num2 and returns the maximum
between the two:
// function returning the max between two numbers
int max(int num1, int num2)
{
// local variable declaration
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Function Declarations:
A function declaration tells the compiler about a function
name and how to call the function. The actual body of the function can be
defined separately.
A function
declaration has the following parts:
return_type function_name( parameter list );
For the
above defined function max(), following is the function declaration:
int max(int num1, int num2);
Parameter
names are not importan in function declaration only their type is required, so
following is also valid declaration:
int max(int, int);
Function
declaration is required when you define a function in one source file and you
call that function in another file. In such case, you should declare the
function at the top of the file calling the function.
Calling a Function:
While
creating a C++ function, you give a definition of what the function has to do.
To use a function, you will have to call or invoke that function.
When a
program calls a function, program control is transferred to the called
function. A called function performs defined task and when its return statement
is executed or when its function-ending closing brace is reached, it returns
program control back to the main program.
To call a
function, you simply need to pass the required parameters along with function
name, and if function returns a value, then you can store returned value. For
example:
#include <iostream>
using namespace std;
// function declaration
int max(int num1, int num2);
int main ()
{
// local variable declaration:
int a = 100;
int b = 200;
int ret;
// calling a function to get max value.
ret = max(a, b);
cout << "Max value is : " << ret << endl;
return 0;
}
// function returning the max between two numbers
int max(int num1, int num2)
{
// local variable declaration
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
I kept
max() function along with main() function and compiled the source code. While
running final executable, it would produce the following result:
Max value is : 200
Function Arguments:
If a
function is to use arguments, it must declare variables that accept the values
of the arguments. These variables are called the formal parameters of the function.
The formal
parameters behave like other local variables inside the function and are
created upon entry into the function and destroyed upon exit.
While
calling a function, there are two ways that arguments can be passed to a
function:
|
Call
Type
|
Description
|
|
Call by value
|
This method copies the actual value of an
argument into the formal parameter of the function. In this case, changes
made to the parameter inside the function have no effect on the argument.
|
|
Call by pointer
|
This method copies the address of an argument
into the formal parameter. Inside the function, the address is used to access
the actual argument used in the call. This means that changes made to the
parameter affect the argument.
|
|
Call by reference
|
This method copies the reference of an
argument into the formal parameter. Inside the function, the reference is
used to access the actual argument used in the call. This means that changes
made to the parameter affect the argument.
|
By
default, C++ uses call by
value to pass arguments. In
general, this means that code within a function cannot alter the arguments used
to call the function and above mentioned example while calling max() function
used the same method.
Default Values for
Parameters:
When you
define a function, you can specify a default value for each of the last
parameters. This value will be used if the corresponding argument is left blank
when calling to the function.
This is
done by using the assignment operator and assigning values for the arguments in
the function definition. If a value for that parameter is not passed when the
function is called, the default given value is used, but if a value is
specified, this default value is ignored and the passed value is used instead.
Consider the following example:
#include <iostream>
using namespace std;
int sum(int a, int b=20)
{
int result;
result = a + b;
return (result);
}
int main ()
{
// local variable declaration:
int a = 100;
int b = 200;
int result;
// calling a function to add the values.
result = sum(a, b);
cout << "Total value is :" << result << endl;
// calling a function again as follows.
result = sum(a);
cout << "Total value is :" << result << endl;
return 0;
}
When the
above code is compiled and executed, it produces the following result:
Total value is :300
Total value is :120
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