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Exception: Exception

Overview

Descendants of class Exception are used to communicate between Kernel.raise and rescue statements in begin ... end blocks. Exception objects carry information about the exception – its type (the exception's class name), an optional descriptive string, and optional traceback information. Exception subclasses may add additional information like NameError#name.

Programs may make subclasses of Exception, typically of ::StandardError or ::RuntimeError, to provide custom classes and add additional information. See the subclass list below for defaults for raise and rescue.

When an exception has been raised but not yet handled (in rescue, ensure, at_exit and END blocks) the global variable $! will contain the current exception and $@ contains the current exception's backtrace.

It is recommended that a library should have one subclass of ::StandardError or ::RuntimeError and have specific exception types inherit from it. This allows the user to rescue a generic exception type to catch all exceptions the library may raise even if future versions of the library add new exception subclasses.

For example:

class MyLibrary
  class Error < RuntimeError
  end

  class WidgetError < Error
  end

  class FrobError < Error
  end

end

To handle both WidgetError and FrobError the library user can rescue MyLibrary::Error.

The built-in subclasses of Exception are:

Class Method Summary

Instance Method Summary

Constructor Details

.new(msg = nil) ⇒ Exception

Construct a new Exception object, optionally passing in a message.

Class Method Details

.exception(string) ⇒ Exception

With no argument, or if the argument is the same as the receiver, return the receiver. Otherwise, create a new exception object of the same class as the receiver, but with a message equal to string.to_str.

Instance Method Details

#==(obj) ⇒ Boolean

Equality—If obj is not an Exception, returns false. Otherwise, returns true if exc and obj share same class, messages, and backtrace.

#backtrace ⇒ Array

Returns any backtrace associated with the exception. The backtrace is an array of strings, each containing either “filename:lineNo: in `method''' or “filename:lineNo.''

def a
  raise "boom"
end

def b
  a()
end

begin
  b()
rescue => detail
  print detail.backtrace.join("\n")
end

produces:

prog.rb:2:in `a'
prog.rb:6:in `b'
prog.rb:10

#backtrace_locations ⇒ Array

Returns any backtrace associated with the exception. This method is similar to #backtrace, but the backtrace is an array of ::Thread::Backtrace::Location.

Now, this method is not affected by Exception#set_backtrace().

#cause ⇒ Exception?

Returns the previous exception ($!) at the time this exception was raised. This is useful for wrapping exceptions and retaining the original exception information.

#exception(string) ⇒ Exception

With no argument, or if the argument is the same as the receiver, return the receiver. Otherwise, create a new exception object of the same class as the receiver, but with a message equal to string.to_str.

#inspect ⇒ String

Return this exception's class name and message

#message ⇒ String

Returns the result of invoking exception.to_s. Normally this returns the exception's message or name.

#set_backtrace(backtrace) ⇒ Array

Sets the backtrace information associated with exc. The #backtrace must be an array of ::String objects or a single ::String in the format described in #backtrace.

#to_s ⇒ String

Returns exception's message (or the name of the exception if no message is set).