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Defensive Programming

Code Complete

Author : Steven C. McConnell.

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Defensive Programming

You take responsibility for protecting yourself

even when it might be the others fault.

Eg: if a routine is passed bad data it won’t be hurt, even if the bad data is another

routine’s fault

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Protecting Your Program

From Invalid Inputs

“Garbage in, garbage out.” A good program uses “garbage in, nothing

out”; “garbage in, error message out”; or “no garbage allowed in” instead.

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Contd….

three general ways to handle garbage in

1. Check the values of all data from external sources

2. Check the values of all routine input

parameters

3. Decide how to handle bad inputs

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Assertions

An assertion is code - usually a routine or macro.

an assertion is True - means everything is operating as

expected.

False - means it has detected an unexpected error in the code.

An assertion takes two arguments: 1. a Boolean expression that describes the

assumption that’s supposed to be true 2. a message to display if it isn’t.

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Contd…

assert denominator != 0 : "denominator is unexpectedly equal to 0.";

denominator != 0, is a boolean expression that evaluates to True or False.

The second argument is a message to print if the first argument is False—that is, if the assertion is false.

C++ Example of an Assertion Macro

#define ASSERT( condition, message ) { \ if ( !(condition) ) { \

fprintf( stderr, "Assertion %s failed: %s\n", \

#condition, message ); \

exit( EXIT_FAILURE ); \

} \

}

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Guidelines for Using Assertions

Use error handling code for conditions you expect to occur; use assertions for conditions that should never occur

Avoid putting executable code in assertions

– Visual Basic Example of a Dangerous Use of an Assertion

Debug.Assert( PerformAction() ) ' Couldn't perform action

– Visual Basic Example of a Safe Use of an Assertion actionPerformed = PerformAction()

Debug.Assert( actionPerformed ) ' Couldn't perform action

Use assertions to document preconditions and postconditions

For highly robust code, assert, and then handle the error anyway

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Error Handling Techniques

Return a neutral value

Substitute the next piece of valid data

Return the same answer as the previous time

Substitute the closest legal value

Log a warning message to a file

Return an error code

Call an error processing routine/object

Display an error message wherever the error is encountered

Handle the error in whatever way works best locally

Shutdown

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Exceptions

Exceptions are a code can pass along errors or

exceptional events to the code that called it.

Code that has no sense of the context of an error can

return control to other parts of the system that might

have a better ability to interpret the error and do

something useful about it.

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Exceptions

Benefits of exceptions and avoiding the difficulties often associated with them.

Use exceptions to notify other parts of the program about errors that should not be ignored

Throw an exception only for conditions that are truly exceptional

Don’t use an exception to pass the buck

Avoid throwing exceptions in constructors and destructors unless you catch them in the same place

Throw exceptions at the right level of abstraction

Include all information that led to the exception in the exception message

Avoid empty catch blocks

Know the exceptions your library code throws

Consider building a centralized exception reporter

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Contd….

Bad Java Example of a Class That Throws an Exception at an Inconsistent Level of Abstraction class Employee {

...

public TaxId getTaxId() EOFException {

...

}

... }

Good Java Example of a Class That Throws an Exception at a Consistent Level of Abstraction class Employee {

...

public TaxId getTaxId() throws EmployeeDataNotAvailable { ...

}

...

}

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Barricade Your Program to Contain the

Damage Caused by Errors

Barricades used to be called “firewalls,” but the term “firewall” now commonly refers to port blocking. Check data crossing the boundaries of a safe

area for validity and respond sensibly if the data isn’t valid

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Relationship between Barricades and

Assertions

Routines that are outside the barricade should use error handling because it isn’t safe to make any assumptions about the data.

Routines inside the barricade should use assertions, because the

data passed to them is supposed to be sanitized before it’s passed across the barricade.

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Debugging Aids

Introduce Debugging Aids Early

Use Offensive Programming

Use version control and build tools like make

Use a built-in preprocessor

Use debugging stubs

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Determining How Much Defensive Programming

to Leave in Production Code

Leave in code that checks for important errors

Remove code that checks for trivial errors

Remove code that results in hard crashes

Leave in code that helps the program crash gracefully

Log errors for your technical support personnel

See that the error messages you leave in are friendly