SPL Improvements: Exceptions

• Version: 1.0
• Date: 2012-02-24
• Author: Levi Morrison levim@php.net
• Status: Work-in-progress
• Special thanks: NikiC, Anthony Ferrara

The exceptions defined in the SPL are not very well documented, and their meanings are unclear. This RFC covers known problems and provides some solutions.

• There are no examples that show when to throw the specific exception. Dawid Krysiak lamented this in a comment.
• Descriptions are too short.

A LogicException is currently documented as: “Exception that represents error in the program logic. This kind of exceptions should directly lead to a fix in your code.”

Known subclasses:

• BadFunctionCallException
• DomainException
• InvalidArgumentException
• LengthException
• OutOfRangeException

A RuntimeException is currently documented as: “Exception thrown if an error which can only be found on runtime occurs.”

Known subclasses:

• OutOfBoundsException
• OverflowException
• RangeException
• UnderflowException
• UnexpectedValueException

It is not always clear when each should be used. Consider the following situation where we define a function that does integer division:

function intDivide($dividend, $divisor) {
    //add error handling
 
    return $dividend / $$divisor;
} 
 
intDivide(1, new StdClass()); // incorrect type
intDivide(1, 0); //correct type, incorrect value

The first error has to deal with incorrect type, and the second is a correct type but the value is not valid for division. These are clearly different errors. Which is a DomainException and which is an InvalidArgumentException? Are they both DomainExceptions? InvalidArgumentExceptions?

In the overview for SPL exceptions, the table of contents lists all of the exceptions defined in the SPL. This should be changed to list the exception hierarchy:

Exception
  LogicException
    BadFunctionCallException
      BadMethodCallException
    DomainException
    InvalidArgumentException
    LengthException
    OutOfRangeException
  RuntimeException
    OutOfBoundsException
    OverflowException
    RangeException
    UnderflowException
    UnexpectedValueException
    

Fortunately, Dawid Krysiak submitted it as a comment on 02 Feb 2011. It would be helpful to also list the subclasses of each exception on their individual pages.

Every exception needs an example of how to use it, giving preference to examples taken from the SPL data-structures. This will also increase the awareness of the SPL data-structures and where they throw exceptions.