CIS 250 - Handling exceptions

Objectives

• use assertions to detect problems
• use validation to detect and correct problems
• use exceptions to detect and correct problems
• create custom exceptions
• throw exceptions

Assertions

• #include <cassert>
• do NOT use to validate user input
• assertions are intended for detecting internal application errors that should not happen
• assertions cause programs to terminate with an error message
• the error message generally includes the expression that failed, along with the filename and line number in the source code of the assertion
• example: assert (divisor != 0);
• example: assert (obj != NULL);

Validation

• errors can be avoided by doing data validation
• validation gets cumbersome in many circumstances
• it is difficult to write validation tests for all possibilities
• validation is best used to check user input

Exceptions

• exceptions are useful for catching any errors in a block of code
• the try block contains the code with potential errors
• the catch blocks contain code to be run when particular errors occur
• Example:

        try {
           // statements with possible exceptions
        } catch (datatype1 identifier1) {
           // statements run if a datatype1 exception is encountered above
        } catch (datatype2 identifier2) {
           // statements run if a datatype2 exception is encountered above
        } catch (...) {
           // statements run if exception other than those listed is encountered
        }

• only the first matching catch block will be run when an exception is encountered
• you can throw an exception with any data type
• you can create your own classes for exceptions
• it is common to add a string object message to an exception class
• include a getMessage() function to an exception class with a message
• include a constructor that accepts a message argument when creating an exception class that includes a message
• example of throwing an exception: throw 5;
• example of throwing an exception: throw divisionByZero();
• you can rethrow an exception within a catch block using: throw;
• C++ base exception class is: exception
• exceptions in C++ are divided into logic_error and runtime_error exceptions
• logic_error exceptions include things such as invalid_argument and out_of_range
• runtime_error exceptions include overflow_error and underflow_error