Question

Recently I was talking to a co-worker about C++ and lamented that there was no way to take a string with the name of a class field and extract the field with that name; in other words, it lacks reflection. He gave me a baffled look and asked when anyone would ever need to do such a thing.

Off the top of my head I didn’t have a good answer for him, other than ‘hey, I need to do it right now’. So I sat down and came up with a list of some of the things I’ve actually done with reflection in various languages. Unfortunately, most of my examples come from my web programming in Python, and I was hoping that the people here would have more examples. Here’s the list I came up with:

1. Given a config file with lines like
   x = ‘Hello World!’
   y = 5.0
   dynamically set the fields of some config object equal to the values in that file. (This was what I wished I could do in C++, but actually couldn’t do.)

2. When sorting a list of objects, sort based on an arbitrary attribute given that attribute’s name from a config file or web request.

3. When writing software that uses a network protocol, reflection lets you call methods based on string values from that protocol. For example, I wrote an IRC bot that would translate
   !some_command arg1 arg2
   into a method call actions.some_command(arg1, arg2) and print whatever that function returned back to the IRC channel.

4. When using Python’s __getattr__ function (which is sort of like method_missing in Ruby/Smalltalk) I was working with a class with a whole lot of statistics, such as late_total. For every statistic, I wanted to be able to add _percent to get that statistic as a percentage of the total things I was counting (for example, stats.late_total_percent). Reflection made this very easy.

So can anyone here give any examples from their own programming experiences of times when reflection has been helpful? The next time a co-worker asks me why I’d ‘ever want to do something like that’ I’d like to be more prepared.

Answer 1

I can list following usage for reflection:

• Late binding
• Security (introspect code for security reasons)
• Code analysis
• Dynamic typing (duck typing is not possible without reflection)
• Metaprogramming

Some real-world usages of reflection from my personal experience:

• Developed plugin system based on reflection
• Used aspect-oriented programming model
• Performed static code analysis
• Used various Dependency Injection frameworks
• …

Reflection is good thing 🙂