should it be allowed to change the method signature in a non statically typed language

Hypothetic and academic question.

pseudo-code:

<pre><code>
class Book{
 read(theReader)
}

class BookWithMemory extends Book {
  read(theReader, aTimestamp = null)
}
</pre></code>

Assuming:开发者_如何学编程

• an interface (if supported) would prohibit it

• default value for parameters are supported

Notes:

• PHP triggers an strict standards error for this.

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I'm not surprised that PHP strict mode complains about such an override. It's very easy for a similar situation to arise unintentionally in which part of a class hierarchy was edited to use a new signature and a one or a few classes have fallen out of sync.

To avoid the ambiguity, name the new method something different (for this example, maybe readAt?), and override read to call readAt in the new class. This makes the intent plain to the interpreter as well as anyone reading the code.

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The actual behavior in such a case is language-dependent -- more specifically, it depends on how much of the signature makes up the method selector, and how parameters are passed.

If the name alone is the selector (as in PHP or Perl), then it's down to how the language handles mismatched method parameter lists. If default arguments are processed at the call site based on the static type of the receiver instead of at the callee's entry point, when called through a base class reference you'd end up with an undefined argument value instead of your specified default, similarly to what would happen if there was no default specified.

If the number of parameters (with or without their types) are part of the method selector (as in Erlang or E), as is common in dynamic languages that run on JVM or CLR, you have two different methods. Create a new overload taking additional arguments, and override the base method with one that calls the new overload with default argument values.

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If I am reading the question correctly, this question seems very language specific (as in it is not applicable to all dynamic languages), as I know you can do this in ruby.

class Book
  def read(book)
    puts book
  end
end

class BookWithMemory < Book
  def read(book,aTimeStamp = nil)
    super book
    puts aTimeStamp
  end
end

I am not sure about dynamic languages besides ruby. This seems like a pretty subjective question as well, as at least two languages were designed on either side of the issue (method overloading vs not: ruby vs php).