Copyright © 2008 Pearson Addison-Wesley. All rights reserved. Chapter 16 Exception Handling.

Copyright © 2008 Pearson Addison-Wesley. All rights reserved.

Chapter 16Chapter 16Exception HandlingException Handling

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OverviewOverview

16.1 Exception-Handling Basics16.1 Exception-Handling Basics

16.2 Programming Techniques for 16.2 Programming Techniques for

Exception Handling Exception Handling

Copyright © 2008 Pearson Addison-Wesley. All rights reserved.

16.116.1Exception-Handling BasicsException-Handling Basics

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Exception Handling BasicsException Handling Basics

It is often easier to write a program by firstIt is often easier to write a program by firstassuming that nothing incorrect will happenassuming that nothing incorrect will happen

Once it works correctly for the expected Once it works correctly for the expected cases, add code to handle the exceptional cases, add code to handle the exceptional casescases

Exception handling is commonly used to Exception handling is commonly used to handle error situationshandle error situations Once an error is handled, it is no longer an Once an error is handled, it is no longer an

errorerror

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Functions and Functions and Exception HandlingException Handling

A common use of exception handling:A common use of exception handling: Functions with a special case that is Functions with a special case that is

handled in different ways depending on handled in different ways depending on how the function is usedhow the function is used

If the function is used in different If the function is used in different programs, each program may require a programs, each program may require a different action when the special case different action when the special case occurs occurs

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Exception Handling MechanismException Handling Mechanism

In C++, exception handling proceeds by:In C++, exception handling proceeds by: Some library software or your code signals Some library software or your code signals

that something unusual has happenedthat something unusual has happened This is called This is called throwing an exceptionthrowing an exception At some other place in your program you At some other place in your program you

place the code that deals with the place the code that deals with the exceptional caseexceptional case

This is called This is called handling the exceptionhandling the exception

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A Toy ExampleA Toy Example

Exception handling is meant to be used Exception handling is meant to be used sparingly in situations that are generally sparingly in situations that are generally not reasonable introductory examplesnot reasonable introductory examples

For this example:For this example: Suppose milk is so important that we Suppose milk is so important that we

almost never run outalmost never run out We still would like our program to handle We still would like our program to handle

the situation of running out of milkthe situation of running out of milk

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The Milk Example (cont.)The Milk Example (cont.)Code to handle the normal situations involvingCode to handle the normal situations involvingmilk, might be:milk, might be:

cout << "Enter number of donuts:\n"; cout << "Enter number of donuts:\n"; cin >> donuts; cin >> donuts; cout << "Enter number of glasses of milk:\n"; cout << "Enter number of glasses of milk:\n"; cin >> milk; cin >> milk; dpg = donuts /static_cast<double>(milk); dpg = donuts /static_cast<double>(milk); cout << donuts << " donuts.\n" cout << donuts << " donuts.\n" << milk << " glasses of milk.\n" << milk << " glasses of milk.\n" << "You have " << dpg << "You have " << dpg << " donuts per glass of milk.\n"; << " donuts per glass of milk.\n";

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If there is no milk, the code on the previous If there is no milk, the code on the previous slide results in a division by zeroslide results in a division by zero We could add a test case for this We could add a test case for this

situationsituation The next slide shows the program with The next slide shows the program with

the test casethe test case The two slides after that shows the The two slides after that shows the

program rewritten using a C++ exceptionprogram rewritten using a C++ exception

The No Milk ProblemThe No Milk Problem

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The try BlockThe try BlockThe program of Display 16.2 replaces the testThe program of Display 16.2 replaces the testcase in the if-else statement with case in the if-else statement with if(milk <= 0)if(milk <= 0) throw donuts; throw donuts;

This if-else code is found in the try blockThis if-else code is found in the try block try try {{ Some_Code Some_Code

}}

which encloses the code to handle the normal, which encloses the code to handle the normal, non error situationsnon error situations

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The Try Block OutlineThe Try Block OutlineThe try block encloses code that you want to The try block encloses code that you want to "try" but that could cause a problem"try" but that could cause a problem

The basic outline of a try block is:The basic outline of a try block is:

try try {{

Code_To_TryCode_To_Try Possibly_Throw_An_ExceptionPossibly_Throw_An_Exception

More_CodeMore_Code}}

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The ExceptionThe Exception

To throw an exception, a throw-statement To throw an exception, a throw-statement is used to throw a valueis used to throw a value In the milk example:In the milk example:

throw donuts; throw donuts;throws an throws an integerinteger value. value.

The value thrown is sometimes called an The value thrown is sometimes called an exceptionexception

You can throw a value of any typeYou can throw a value of any type

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The catch-blockThe catch-blockSomething that is thrown goes from one placeSomething that is thrown goes from one placeto anotherto another

A C++ throw causes the flow of control to go A C++ throw causes the flow of control to go to another placeto another place When an exception is thrown, the try block When an exception is thrown, the try block

stops executing and the catch-block begins stops executing and the catch-block begins executionexecution

This is This is catching catching oror handling the exception handling the exception

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The Milk catch-blockThe Milk catch-blockThe catch-block from the milk example looksThe catch-block from the milk example lookslike, but is not, a function definition with a like, but is not, a function definition with a parameter:parameter: catch(int e) //catch parameter is e catch(int e) //catch parameter is e

{{ cout << e << donuts, and no milk!\n" cout << e << donuts, and no milk!\n" << "Go buy some milk.\n"; << "Go buy some milk.\n"; } } If no exception is thrown, the catch-block is If no exception is thrown, the catch-block is

ignored during program executionignored during program execution

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The catch-block ParameterThe catch-block Parameter

The catch-block parameter, (recall that the The catch-block parameter, (recall that the catch-block is not a function) does two catch-block is not a function) does two things:things: The type of the catch-block parameter The type of the catch-block parameter

identifies the kind of value the catch-identifies the kind of value the catch-block can catchblock can catch

The catch-block parameter provides a The catch-block parameter provides a name for the value caught so you can name for the value caught so you can write code using the value that is caughtwrite code using the value that is caught

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try-blocks and if-elsetry-blocks and if-elsetry-blocks are very similar to if-else statementstry-blocks are very similar to if-else statements If everything is normal, the entire try-block is If everything is normal, the entire try-block is

executedexecuted else, if an exception is thrown, the catch-block else, if an exception is thrown, the catch-block

is executedis executed

A big difference between try-blocks and if-elseA big difference between try-blocks and if-elsestatements is the try-block's ability to send a statements is the try-block's ability to send a message to one of its branchesmessage to one of its branches

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try-throw-catch Reviewtry-throw-catch Review

This is the basic mechanism for throwing This is the basic mechanism for throwing and catching exceptionsand catching exceptions The try-block includes a throw-statementThe try-block includes a throw-statement If an exception is thrown, the try-block If an exception is thrown, the try-block

ends and the catch-block is executedends and the catch-block is executed If no exception is thrown, then after the If no exception is thrown, then after the

try-block is completed, execution try-block is completed, execution continues with the code following the continues with the code following the catch-block(s)catch-block(s)

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Defining an Exception ClassDefining an Exception Class

Because a throw-statement can throw a value of Because a throw-statement can throw a value of

any type, it is common to define a class whose any type, it is common to define a class whose objects can carry the kind of information youobjects can carry the kind of information youwant thrown to the catch-blockwant thrown to the catch-block

A more important reason for a specialized A more important reason for a specialized exception class is so you can have a different exception class is so you can have a different type to identify each possible kind of exceptionaltype to identify each possible kind of exceptionalsituationsituation

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An exception class is just a class that An exception class is just a class that happens to be used as an exception classhappens to be used as an exception class

See the next example of a program with a See the next example of a program with a programmer defined exception class programmer defined exception class

The Exception ClassThe Exception Class

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Throwing a Class TypeThrowing a Class TypeThe program in Display 16.3 uses the The program in Display 16.3 uses the throw-statementthrow-statement throw NoMilk(donuts); throw NoMilk(donuts); This invokes a constructor for the class NoMilkThis invokes a constructor for the class NoMilk The constructor takes a single argument of The constructor takes a single argument of

type inttype int The NoMilk object is what is thrownThe NoMilk object is what is thrown The catch-block then uses the statementThe catch-block then uses the statement

e.get_donuts( ) e.get_donuts( )to retrieve the number of donuts to retrieve the number of donuts

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A try-block can throw any number of exceptions A try-block can throw any number of exceptions of different typesof different types In any one execution, only one exception can In any one execution, only one exception can

be thrownbe thrown Each catch-block can catch only one Each catch-block can catch only one

exceptionexception Multiple catch-blocks may be usedMultiple catch-blocks may be used

A parameter is not required in a catch-blockA parameter is not required in a catch-block

A sample program with two catch-blocks is A sample program with two catch-blocks is found in next program.found in next program.

Multiple Throws and CatchesMultiple Throws and Catches

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A Default catch-blockA Default catch-blockWhen catching multiple exceptions, write the When catching multiple exceptions, write the catch-blocks for the most specific exceptions catch-blocks for the most specific exceptions firstfirst Catch-blocks are tried in order and the first one Catch-blocks are tried in order and the first one

matching the type of exception is executedmatching the type of exception is executed

A default (and last) catch-block to catch any A default (and last) catch-block to catch any exception can be made using "…" as the exception can be made using "…" as the catch-block parametercatch-block parameter catch(…) catch(…) { <the catch block code> } { <the catch block code> }

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Exception Class DivideByZeroException Class DivideByZeroIn the last program, exception class In the last program, exception class DivideByZero was defined as DivideByZero was defined as class DivideByZero class DivideByZero { } ; { } ; This class has no member variables or This class has no member variables or

member functionsmember functions This is a trivial exception class This is a trivial exception class DivideByZero is used simply to activate the DivideByZero is used simply to activate the

appropriate catch-blockappropriate catch-block There is nothing to do with the catch-block There is nothing to do with the catch-block

parameter so it can be omitted as shown in the parameter so it can be omitted as shown in the last program.last program.

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Exceptions In FunctionsExceptions In FunctionsIn some cases, an exception generated in a In some cases, an exception generated in a function is not handled in the functionfunction is not handled in the function It might be that some programs should end, It might be that some programs should end,

while others might do something else, so while others might do something else, so within the function you might not know how to within the function you might not know how to handle the exceptionhandle the exception

In this case, the program places the function In this case, the program places the function invocation in a try block and catches the invocation in a try block and catches the exception in a following catch-blockexception in a following catch-block

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Function safe_divideFunction safe_divideThe next program includes a function that The next program includes a function that throws, but does not catch an exceptionthrows, but does not catch an exception In function safe_divide, the denominator is In function safe_divide, the denominator is

checked to be sure it is not zero. If it is checked to be sure it is not zero. If it is zero, an exception is thrown:zero, an exception is thrown: if (bottom == 0) if (bottom == 0) throw DivideByZero( ); throw DivideByZero( );

The call to function safe_divide is found in The call to function safe_divide is found in the try-block of the programthe try-block of the program

If a function does not catch an exception it should If a function does not catch an exception it should warn programmers than an exception might be warn programmers than an exception might be thrown by the functionthrown by the function An exception specification, also called a throw An exception specification, also called a throw

list, appears in the function declaration and list, appears in the function declaration and definition:definition:

double safe_divide(int n, int d) throw double safe_divide(int n, int d) throw (DivideByZero);(DivideByZero); if multiple exceptions are thrown and not caught if multiple exceptions are thrown and not caught

by a function:by a function: double safe_divide(int n, int d) double safe_divide(int n, int d) throw (DivideByZero, OtherException); throw (DivideByZero, OtherException);

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Exception SpecificationException Specification

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Exceptions Not ListedExceptions Not Listed

If an exception is not listed in an exception If an exception is not listed in an exception specification and not caught by the function:specification and not caught by the function: The program ends The program ends

If there is no exception specification at all, it is If there is no exception specification at all, it is the same as if all possible exceptions are listedthe same as if all possible exceptions are listed These exceptions will be treated "normally" These exceptions will be treated "normally"

An empty exception specification list means An empty exception specification list means that no exceptions should be thrown and not that no exceptions should be thrown and not caughtcaught

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Sample Sample Exception SpecificationsException Specifications

void some_function ( ) throw ( );void some_function ( ) throw ( );//empty exception list; so all exceptions not //empty exception list; so all exceptions not // caught by the function end the program// caught by the function end the program

void some_function( ) throw(DivideByZero, void some_function( ) throw(DivideByZero, OtherException);OtherException);//Exceptions DivideByZero and //Exceptions DivideByZero and //OtherException//OtherException//treated normally. All others end the //program //treated normally. All others end the //program

Sample Sample Exception SpecificationsException Specifications

void some_function( );void some_function( );// All exceptions of all types treated // All exceptions of all types treated //normally//normally// If not caught by a catch-block, the // If not caught by a catch-block, the //program ends//program ends

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Type ConversionType Conversion

No automatic type conversions are done No automatic type conversions are done with exceptionswith exceptions if double is in the exception specification, if double is in the exception specification,

an int cannot be thrown unless int is an int cannot be thrown unless int is also in the exception specificationalso in the exception specification

No automatic conversion is performedNo automatic conversion is performed

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Section 16.1 ConclusionSection 16.1 ConclusionCan youCan you

List the three components of exception List the three components of exception handling?handling?

Write code to catch an exception of type Write code to catch an exception of type char?char?

Create an exception specification for a Create an exception specification for a function?function?

Create an exception class?Create an exception class?

Copyright © 2008 Pearson Addison-Wesley. All rights reserved.

16.216.2Programming Techniques Programming Techniques

for Exception-Handlingfor Exception-Handling

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Programming TechniquesProgramming Techniquesfor Exception Handlingfor Exception Handling

A guideline for exception handling is to A guideline for exception handling is to separate throwing an exception and catching separate throwing an exception and catching an exception into separate functions an exception into separate functions Place the throw-statement in one function Place the throw-statement in one function

and list the exception in the exception and list the exception in the exception specificationspecification

Place the function invocation and catch-Place the function invocation and catch-clause in a try-block of a different functionclause in a try-block of a different function

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try and throw…Againtry and throw…Again

Here is a general approach to useHere is a general approach to usein using throw:in using throw:

void functionA( ) throw void functionA( ) throw (MyException)(MyException)

{{ … … throw MyException(<an argument?>); throw MyException(<an argument?>); } }

next ==>

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catch…againcatch…againUsing FunctionA from the previous slide, hereUsing FunctionA from the previous slide, hereis how to catch MyException:is how to catch MyException: void functionB( )void functionB( ){ …{ …trytry { … { … functionA( ); functionA( ); … …

}} catch(MyException e) catch(MyException e) { < handle the exception> { < handle the exception> } }}}

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When to Throw An ExceptionWhen to Throw An ExceptionThrowing exceptions is generally reserved forThrowing exceptions is generally reserved forthose cases when handling the exceptional those cases when handling the exceptional case depends on how and where the function case depends on how and where the function was invokedwas invoked In these cases it is usually best to let the In these cases it is usually best to let the

programmer calling the function handle the programmer calling the function handle the exceptionexception

An uncaught exception ends your programAn uncaught exception ends your program

If you can easily write code to handle the If you can easily write code to handle the problem do not throw an exceptionproblem do not throw an exception

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Nested try-catch BlocksNested try-catch BlocksAlthough a try-block followed by its catch-blockAlthough a try-block followed by its catch-blockcan be nested inside another try-blockcan be nested inside another try-block It is almost always better to place the nested It is almost always better to place the nested

try-block and its catch-block inside a function try-block and its catch-block inside a function definition, then invoke the function in the outerdefinition, then invoke the function in the outertry-blocktry-block

An error thrown but not caught in the innerAn error thrown but not caught in the innertry-catch-blocks is thrown to the outer try-blocktry-catch-blocks is thrown to the outer try-blockwhere it might be caughtwhere it might be caught

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Overuse of ExceptionsOveruse of Exceptions

Throwing an exception allows you to transfer Throwing an exception allows you to transfer flow of control to almost any place in your flow of control to almost any place in your programprogram

Such un-restricted flow of control is generallySuch un-restricted flow of control is generallyconsidered poor programming style as it makesconsidered poor programming style as it makesprograms difficult to understandprograms difficult to understand

Exceptions should be used sparingly and only Exceptions should be used sparingly and only when you cannot come up with an alternative when you cannot come up with an alternative that produces reasonable codethat produces reasonable code

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Exception Class HierarchiesException Class HierarchiesIt can be useful to define a hierarchy of It can be useful to define a hierarchy of exception classes.exception classes. You might have an ArithmeticError You might have an ArithmeticError

exception class with DivideByZeroError as exception class with DivideByZeroError as a derived classa derived class

Since a DivideByZeroError object is also an Since a DivideByZeroError object is also an

ArithmeticError object, every catch-block ArithmeticError object, every catch-block for an ArithmeticError will also catch a for an ArithmeticError will also catch a DivideByZeroErrorDivideByZeroError

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Checking For Available MemoryChecking For Available MemoryThe new operator allocates memory from the The new operator allocates memory from the freestore: NodePtr pointer = new Node;freestore: NodePtr pointer = new Node; What if there is no memory available?What if there is no memory available? bad_alloc is a predefined exception and can be bad_alloc is a predefined exception and can be

used in this way since new throws a bad_alloc used in this way since new throws a bad_alloc exception:exception:

try try { NodePtr pointer = new Node; { NodePtr pointer = new Node; } } catch(bad_alloc) catch(bad_alloc) { cout << "Ran out of memory!"; { cout << "Ran out of memory!"; } }

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Rethrowing an ExceptionRethrowing an Exception

The code within a catch-block can throw The code within a catch-block can throw an exceptionan exception This feature can be used to pass the This feature can be used to pass the

same or a different exception up the same or a different exception up the chain of exception handling blockschain of exception handling blocks

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Section 16.2 ConclusionSection 16.2 Conclusion

Can youCan you

Describe what happens if an exception is Describe what happens if an exception is never caught?never caught?

Write code that nests a try-block inside Write code that nests a try-block inside another try-block?another try-block?

Describe the type of situations in which Describe the type of situations in which exceptions should not be thrown?exceptions should not be thrown?

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Chapter 16 -- EndChapter 16 -- End