|
|
Next: Functions with parameters and
Up: Introduction to User-defined functions
Previous: Introduction to User-defined functions
Functions with no parameters are of limited use. Usually they will not return a value but carry out some operation. For example consider the following function which skips three lines on output.
void skipthree(void)
// skips three lines on output
{
cout << endl << endl << endl;
}
Download program.Note that the function-type has been given as
void, this
tells the compiler that this function does not return any value.
Because the function does not take any parameters the
parameter-list is empty, this is indicated by the void
parameter-list. No local variables are required by this
function and the function implementation only requires the sending of
three successive end of line characters to the output stream
cout. Note the introductory comment that describes what the
function does. All functions should include this information as minimal
comment.
Since this function does not return a value it cannot be used in an expression and is called by treating it as a statement as follows:
skipthree();
Even though there are no parameters the empty parameter list ()
must be inserted.
When a function is called the C++ compiler must insert appropriate
instructions into the object code to arrange to pass the actual
parameter values to the function code and to obtain any values
returned by the function. To do this correctly the compiler must know
the types of all parameters and the type of any return value. Thus
before processing the call of a function it must already know how the
function is defined. This can be done by defining any functions that
are used in the main program before the main program, for example the
function
skipthree could be incorporated in a program as follows:
#include <iostream.h>
void skipthree(void)
// Function to skip three lines
{
cout << endl << endl << endl;
}
void main()
{
int ....;
float ....;
cout << "Title Line 1";
skipthree();
cout << "Title Line 2";
.
.
}
However this has disadvantages, namely:
The way round both the problems above is to use Function
prototypes. A function prototype supplies information about the
return type of a function and the types of its parameters. This
function prototype is then placed before the main program that uses
the function. The full function definition is then placed after the
main program or may be contained in a separate file that is compiled
separately and linked to the main program later. The function
prototype is merely a copy of the function heading. Thus the function
prototype for the function skipthree is:
void skipthree(void);which would be included in the program file as follows:
#include <iostream.h>
void skipthree(void); // function prototype
void main()
{
int ....;
float ....;
cout << "Title Line 1";
skipthree();
cout << "Title Line 2";
.
.
}
// Now the function definition
void skipthree(void)
// Function to skip three lines
{
cout << endl << endl << endl;
}
Download program.
In fact when using functions from the stream libraries and the
mathematical libraries prototypes are required for these functions.
This is handled by including the files iostream.h and
math.h which, among other things, contain the function
prototypes.