Here are some of the things I've used run-time reflection of some kind for, in various languages:
- When how to perform an operation depends on the type of more than one thing. For example, if you have a Shape class and want to implement Shape::intersection to compute the intersection of two shapes. Dispatching based on one of the shapes is easy; $shape1->intersection($shape2) will find the type of $shape1 and dispatch appropriately. But unless the other shape's type is known at compile-time, there will need to be some kind of run-time type information available to know how to compute the intersection. If $shape2 is coming from a database or the network, its type certainly won't be available at compile-time.
- Loading things at run-time. For example, I wrote a streaming database system in Java that started by reading a configuration file, then loaded the classes named there. It was best to look at the class as soon as it was loaded and make sure it was a type I could deal with, so more information was available for error reporting.
- Automatically providing information for things like GUI editors and SOAP interface generators. It is convenient for these systems to be able to walk a class's methods to see what it can do, to make that functionality available over a network connection or a GUI.
- Displaying information. I have a program that manages several different types of objects. I just added on a GUI displayer, and the easiest way to do that was to write a little display module, and have it say "if this is type 1, display it this way; if it's type 2, display it this way; etc...". It could have been done through subclassing, but it would have been much more complicated.
- Working around quirky behavior. I may know that a particular class will handle a particular situation incorrectly, so I can detect that class at runtime and try to work around the problem. For example, if you know a particular class has trouble with unicode, maybe you strip it out before sending it to that class.
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