A First Look at Exceptions

​Exceptions

1. Indicate that something unexpected has occurred or been detected

2. Allow program to deal with the problem in a controlled manner

3. Can be as simple or complex as program design requires

Terminology

• Exception: object or value that signals an error

• Throw an exception: send a signal that an error has occurred

• Catch/Handle an exception: process the exception; interpret the signal

Key Words

• throw – followed by an argument, is used to throw an exception

• try – followed by a block { }, is used to invoke code that throws an exception

• catch – followed by a block { }, is used to detect and process exceptions thrown in preceding try block. Takes a parameter that matches the type thrown.

Flow of Control

1. A function that throws an exception is called from within a try block

2. If the function throws an exception, the function terminates and the try block is immediately exited. A catch block to process the exception is searched for in the source code immediately following the try block.

3. If a catch block is found that matches the exception thrown, it is executed. If no catch block that matches the exception is found, the program terminates.

Example 1

// function that throws an exception
int totalDays(int days, int weeks) {
    if ((days < 0) || (days > 7))
    throw string("invalid number of days“);
    // the argument to throw is the character string
    else
    return (7 * weeks + days);
}

Example 2

try // block that calls function {
    totDays = totalDays(days, weeks);
    cout << "Total days: " << days;
} catch (string msg) // interpret exception {
    cout << "Error: " << msg;
}

What Happens

1. try block is entered. totalDays function is called

2. If 1st parameter is between 0 and 7, total number of days is returned and catch block is skipped over (no exception thrown)

3. If exception is thrown, function and try block are exited, catch blocks are scanned for 1st one that matches the data type of the thrown exception. catch block executes

Exceptions and Objects

• An exception class can be defined in a class and thrown as an exception by a member function

• An exception class may have:

  • no members: used only to signal an error

  • members: pass error data to catch block

• A class can have more than one exception class

What Happens After catch Block?

Once an exception is thrown, the program cannot return to throw point. The function executing throw terminates (does not return), other calling functions in try block terminate, resulting in unwinding the stack