Absolute C++ (4th Edition) part 76 potx

18

Exception Handling
18.1 EXCEPTION HANDLING BASICS 759
A Toy Example of Exception Handling 759
Defining Your Own Exception Classes 768
Multiple Throws and Catches 768
Pitfall: Catch the More Specific Exception First 772
Tip: Exception Classes Can Be Trivial 773
Throwing an Exception in a Function 773
Exception Specification 775
Pitfall: Exception Specification in Derived Classes 777
18.2 PROGRAMMING TECHNIQUES FOR EXCEPTION
HANDLING 779
When to Throw an Exception 779
Pitfall: Uncaught Exceptions 781
Pitfall: Nested

try-catch
Blocks 781
Pitfall: Overuse of Exceptions 782
Exception Class Hierarchies 782
Testing for Available Memory 782
Rethrowing an Exception 783
CHAPTER SUMMARY 783
ANSWERS TO SELF-TEST EXERCISES 784
PROGRAMMING PROJECTS 785

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18

Exception Handling

It’s the exception that proves the rule.

Common maxim
INTRODUCTION

One way to write a program is to first assume that nothing unusual or incor-
rect will happen. For example, if the program takes an entry off a list, you
might assume that the list is not empty. Once you have the program working
for the core situation where things always go as planned, you can then add
code to take care of the exceptional cases. C++ has a way to reflect this
approach in your code. Basically, you write your code as if nothing very
unusual happens. After that, you use the C++ exception handling facilities to
add code for those unusual cases.
Exception handling is commonly used to handle error situations, but per-
haps a better way to view exceptions is as a way to handle exceptional situa-
tions. After all, if your code correctly handles an “error,” then it no longer is an
error.
Perhaps the most important use of exceptions is to deal with functions that
have some special case that is handled differently depending on how the func-
tion is used. Perhaps the function will be used in many programs, some of
which will handle the special case in one way, while others will handle it in
some other way. For example, if there is a division by zero in the function,
then it may turn out that for some invocations of the function the program
should end, but for other invocations of the function something else should
happen. You will see that such a function can be defined to throw an excep-
tion if the special case occurs; that exception will allow the special case to be

handled outside the function. Thus, the special case can be handled differently
for different invocations of the function.
In C++, exception handling proceeds as follows: Either some library soft-
ware, or your code, provides a mechanism that signals when something
unusual happens. This is called

throwing an exception

. At another place in your
program you place the code that deals with the exceptional case. This is called

handling the exception

. This method of programming makes for cleaner code.
Of course, we still need to explain the details of how you do this in C++.
Most of this chapter only uses material from Chapters 1 through 9. How-
ever, the sections “Exception Specification in Derived Classes” and “Exception
Class Hierarchies” use material from Chapter 14. The section “Testing for
Available Memory” uses material from Chapter 17. Any or all of these listed

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Exception Handling Basics 759

sections may be omitted without hurting the continuity of the chapter. The section
“Exception Specification” has one paragraph that refers to derived classes (Chapter 14),
but that paragraph may be omitted.

Exception Handling Basics

Well, the program works for most cases. I didn’t know it had to work for

that



case.

Computer Science Student,

Appealing a grade

Exception handling is meant to be used sparingly and in situations that are more
involved than what is reasonable to include in a simple introductory example. So, we
will teach you the exception handling details of C++ by means of simple examples that
would not normally use exception handling. This makes a lot of sense for learning
about the exception handling details of C++, but do not forget that these first examples
are toy examples; in practice, you would not use exception handling for anything that
simple.

■
A TOY EXAMPLE OF EXCEPTION HANDLING

For this example, suppose that milk is such an important food in our culture that peo-
ple almost never run out of it, but still we would like our programs to accommodate
the very unlikely situation of running out of milk. The basic code, which assumes we
do not run out of milk, might be as follows:

cout << "Enter number of donuts:\n";
cin >> donuts;
cout << "Enter number of glasses of milk:\n";
cin >> milk;

dpg = donuts/static_cast<double>(milk);
cout << donuts << " donuts.\n"
<< milk << " glasses of milk.\n"
<< "You have " << dpg
<< " donuts for each glass of milk.\n";

If there is no milk, then this code will include a division by zero, which is an error.
To take care of this special situation where we run out of milk, we can add a test. The
complete program with this added test for the special situation is shown in Display
18.1. The program in Display 18.1 does not use exception handling. Now, let’s see how
this program can be rewritten using the C++ exception handling facilities.
In Display 18.2, we have rewritten the program from Display 18.1 using an excep-
tion. This is only a toy example, and you would probably not use an exception in this
case. However, it does give us a simple example to work with. Although the program as
18.1

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760 Exception Handling
Display 18.1 Handling a Special Case without Exception Handling
1 #include <iostream>
2 using std::cin;
3 using std::cout;
4 int main( )
5 {
6 int donuts, milk;
7 double dpg;
8 cout << "Enter number of donuts:\n";
9 cin >> donuts;
10 cout << "Enter number of glasses of milk:\n";
11 cin >> milk;

12 if (milk <= 0)
13 {
14 cout << donuts << " donuts, and No Milk!\n"
15 << "Go buy some milk.\n";
16 }
17 else
18 {
19 dpg = donuts/static_cast<double>(milk);
20 cout << donuts << " donuts.\n"
21 << milk << " glasses of milk.\n"
22 << "You have " << dpg
23 << " donuts for each glass of milk.\n";
24 }
25 cout << "End of program.\n";
26 return 0;
27 }
S
AMPLE
D
IALOGUE
Enter number of donuts:
12
Enter number of glasses of milk:
0
12 donuts, and No Milk!
Go buy some milk.
End of program.

18_CH18.fm Page 760 Monday, August 18, 2003 1:23 PM
Exception Handling Basics 761

Display 18.2 Same Thing Using Exception Handling
(part 1 of 2)
1
2 #include <iostream>
3 using std::cin;
4 using std::cout;
5 int main( )
6 {
7 int donuts, milk;
8 double dpg;
9 try
10 {
11 cout << "Enter number of donuts:\n";
12 cin >> donuts;
13 cout << "Enter number of glasses of milk:\n";
14 cin >> milk;
15
16 if (milk <= 0)
17 throw donuts;
18 dpg = donuts/static_cast<double>(milk);
19 cout << donuts << " donuts.\n"
20 << milk << " glasses of milk.\n"
21 << "You have " << dpg
22 << " donuts for each glass of milk.\n";
23 }
24 catch(int e)
25 {
26 cout << e << " donuts, and No Milk!\n"
27 << "Go buy some milk.\n";
28 }

29 cout << "End of program.\n";
30 return 0;
31 }
S
AMPLE
D
IALOGUE
1
Enter number of donuts:
12
Enter number of glasses of milk:
6
12 donuts.
6 glasses of milk.
You have 2 donuts for each glass of milk.
End of program.
This is just a toy example to learn C++ syntax.
Do not take it as an example of good typical
use of exception handling.

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762 Exception Handling

a whole is not simpler, at least the part between the words

try

and

catch

is cleaner,
which hints at the advantage of using exceptions. Look at the code between the words

try

and

catch

. It is basically the same as the code in Display 18.1, except that rather
than the big

if-else

statement (highlighted in Display 18.1), this new program has
the following smaller

if

statement (plus some simple nonbranching statements):

if (milk <= 0)
throw donuts;

This

if

statement says that if there is no milk, then do something exceptional. That

something exceptional is given after the word

catch

. The idea is that the normal situa-
tion is handled by the code following the word

try

, and that exceptional situations are
handled by the code following the word

catch

. Thus, we have separated the normal
case from the exceptional case. In this toy example that does not really buy us too
much, but in other situations it will prove to be very helpful. Let’s look at the details.
The basic way of handling exceptions in C++ consists of the

try-throw-catch

threesome. A

try

block

has the following syntax:

try

{

Some_Code

}

This

try

block contains the code for the basic algorithm that tells what to do when
everything goes smoothly. It is called a

try

block because you are not 100% sure that
all will go smoothly, but you want to “give it a try.”
If something unusual does happen, you want to throw an exception, which is a way
of indicating that something unusual happened. So the basic outline, when we add a

throw

, is as follows:

try
{
Display 18.2 Same Thing Using Exception Handling
(part 2 of 2)
S
AMPLE

D
IALOGUE
2
Enter number of donuts:
12
Enter number of glasses of milk:
0
12 donuts, and No Milk!
Go buy some milk.
End of program.
try block

18_CH18.fm Page 762 Monday, August 18, 2003 1:23 PM
Exception Handling Basics 763


Code_To_Try


Possibly_Throw_An_Exception


More_Code

}

The following is an example of a

try

block with a

throw

statement included (copied
from Display 18.2):

try
{
cout << "Enter number of donuts:\n";
cin >> donuts;
cout << "Enter number of glasses of milk:\n";
cin >> milk;
if (milk <= 0)
throw donuts;
dpg = donuts/static_cast<double>(milk);
cout << donuts << " donuts.\n"
<< milk << " glasses of milk.\n"
<< "You have " << dpg
<< " donuts for each glass of milk.\n";
}

The following statement

throws

the

int

value

donuts

:

throw donuts;

The value thrown (in this case,

donuts

) is sometimes called an

exception

; the execu-
tion of a

throw statement is called throwing an exception. You can throw a value of
any type. In this case, an
int value is thrown.
When something is “thrown,” something goes from one place to another place. In
C++ what goes from one place to another is the flow of control (as well as the value
thrown). When an exception is thrown, the code in the surrounding
try block stops
executing and another portion of code, known as a
catch block, begins execution. Exe-
cuting the
catch block is called catching the exception or handling the exception.

When an exception is thrown, it should ultimately be handled by (caught by) some
catch block. In Display 18.2, the appropriate catch block immediately follows the try
block. We repeat the catch block in what follows:
catch(int e)
{
cout << e << " donuts, and No Milk!\n"
<< "Go buy some milk.\n";
}
throw
statement
exception
throwing
an
exception
catch
block
handling
the
exception
18_CH18.fm Page 763 Monday, August 18, 2003 1:23 PM
764 Exception Handling
This catch block looks very much like a function definition that has a parameter of
type
int. It is not a function definition, but in some ways a catch block is like a func-
tion. It is a separate piece of code that is executed when your program encounters (and
executes) the following (within the preceding
try block):
throw
Some_int
;

So, this throw statement is similar to a function call, but instead of calling a func-
tion, it calls the
catch block and says to execute the code in the catch block. A catch
block is often referred to as an exception handler, which is a term that suggests that a
catch block has a function-like nature.
What is that identifier
e in the following line from a catch block?
catch(int e)
That identifier e looks like a parameter and acts very much like a parameter. In fact, the
identifier, such as
e, in the catch-block heading is called the catch-block parameter.
Each
catch block can have at most one catch-block parameter. The catch-block
parameter does two things:
1. The
catch-block parameter is preceded by a type name that specifies what kind of
thrown value the
catch block can catch.
2. The
catch-block parameter gives you a name for the thrown value that is caught, so
you can write code in the
catch block that does things with that value.
We will discuss these two functions of the
catch-block parameter in reverse order. This
subsection discusses using the
catch-block parameter as a name for the value that was
thrown and is caught. The subsection entitled “Multiple Throws and Catches,” later in
this chapter, discusses which
catch block (which exception handler) will process a value
throw

S
TATEMENT
S
YNTAX
throw
Expression_for_Value_to_Be_Thrown
;
When the throw statement is executed, the execution of the enclosing try block is stopped. If
the
try block is followed by a suitable catch block, then flow of control is transferred to the
catch block. A throw statement is almost always embedded in a branching statement, such as
an
if statement. The value thrown can be of any type.
E
XAMPLE
if (milk <= 0)
throw donuts;
exception
handler
catch-block
parameter
18_CH18.fm Page 764 Monday, August 18, 2003 1:23 PM
Exception Handling Basics 765
that is thrown. Our current example has only one catch block. A common name for a
catch-block parameter is e, but you can use any legal identifier in place of e.
Let’s see how the
catch block in Display 18.2 works. When a value is thrown, exe-
cution of the code in the
try block ends and control passes to the catch block (or
blocks) that is placed right after the

try block. The catch block from Display 18.2 is
reproduced here:
catch(int e)
{
cout << e << " donuts, and No Milk!\n"
<< "Go buy some milk.\n";
}
When a value is thrown, the thrown value must be of type int in order for this particu-
lar
catch block to apply. In Display 18.2, the value thrown is given by the variable
donuts; because donuts is of type int, this catch block can catch the value thrown.
Suppose the value of
donuts is 12 and the value of milk is 0, as in the second sample
dialogue in Display 18.2. Since the value of
milk is not positive, the throw statement
within the
if statement is executed. In that case the value of the variable donuts is
thrown. When the
catch block in Display 18.2 catches the value of donuts, the value
of
donuts is plugged in for the catch-block parameter e and the code in the catch
block is executed, producing the following output:
12 donuts, and No Milk!
Go buy some milk.
If the value of donuts is positive, the throw statement is not executed. In this case
the entire
try block is executed. After the last statement in the try block is executed,
the statement after the
catch block is executed. Note that if no exception is thrown, the
catch block is ignored.

This discussion makes it sound like a
try-throw-catch setup is equivalent to an if-
else
statement. It almost is equivalent, except for the value thrown. A try-throw-
catch
setup is like an if-else statement with the added ability to send a message to one of
the branches. This does not sound much different from an
if-else statement, but it
turns out to be a big difference in practice.
To summarize in a more formal tone, a
try block contains some code that we are
assuming includes a
throw statement. The throw statement is normally executed only
in exceptional circumstances, but when it is executed, it throws a value of some type.
When an exception (a value such as
donuts in Display 18.2) is thrown, the try block
ends. All the rest of the code in the
try block is ignored and control passes to a suitable
catch block. A catch block applies only to an immediately preceding try block. If the
exception is thrown, then that exception object is plugged in for the
catch-block
parameter, and the statements in the
catch block are executed. For example, if you look
at the dialogues in Display 18.2, you will see that as soon as the user enters a nonposi-
tive number, the
try block stops and the catch block is executed. For now, we will
18_CH18.fm Page 765 Monday, August 18, 2003 1:23 PM
766 Exception Handling
assume that every try block is followed by an appropriate catch block. We will later
discuss what happens when there is no appropriate

catch block.
If no exception (no value) is thrown in the
try block, then after the try block is
completed, program execution continues with the code after the
catch block. In other
words, if no exception is thrown, the
catch block is ignored. Most of the time when
the program is executed, the
throw statement will not be executed, and so in most cases
the code in the
try block will run to completion and the code in the catch block will
be ignored completely.
catch-
B
LOCK
P
ARAMETER
The catch-block parameter is an identifier in the heading of a catch block that serves as a
placeholder for an exception (a value) that might be thrown. When a suitable value is thrown in
the preceding
try block, that value is plugged in for the catch-block parameter. (In order for
the
catch block to be executed the value throw must be of the type given for its catch-block
parameter.) You can use any legal (nonreserved-word) identifier for a
catch-block parameter.
E
XAMPLE
catch(int e)
{
cout << e << " donuts, and No Milk!\n"

<< "Go buy some milk.\n";
}
e
is the catch-block parameter.
try-throw-catch
The basic mechanism for throwing and catching exceptions is a try-throw-catch sequence.
The
throw statement throws the exception (a value). The catch block catches the exception (the
value). When an exception is thrown, the
try block ends and then the code in the catch block is
executed. After the
catch block is completed, the code after the catch block or blocks is exe-
cuted (provided the
catch block has not ended the program or performed some other special
action).
(The type of the thrown exception must match the type listed for the catch-block parameter or
else the exception will not be caught by that
catch block. This point is discussed further in the
subsection “Multiple Throws and Catches.”)
If no exception is thrown in the try block, then after the try block is completed, program execu-
tion continues with the code after the
catch block or blocks. (In other words, if no exception is
thrown, the
catch block or blocks are ignored.)
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