A design issue with a tree of information in C++.

Question

Sorry in advance for the lengthy explanation!

I have a C++ application that uses a hash_map to store a tree of information that was parsed from a text file. The values in the map are either a child hash_map or a string. These values were parsed from a text file and then stored into the map.

I wanted to avoid having to send the strings and maps as a copy to the hash map assignment function, so when file was parsed, I created a pointer to new string() or a new hash_map() and stored the value into the map as "arbitrary" data (pointer to a void).

However, this poses a pretty big problem when it comes to clean-up, as deleting a void doesn't behave like one would want it to (and it makes sense). I looked for an easy solution by just creating an Object class and made child classes called StringObj and HashMap, which stored their respective data, and the appropriate destructor was called since the hash_map value type was changed to a pointer to an Object.

Is there an easier way to solve this? I looked into dynamic casting and thought it might work well, since I could catch the exception from the failed cast, and treat it appropriately, but I can't help but feel there might be a simpler solution, or that I'm over-complicating it a bit.

Suggestions?

Thanks in advance, Jengerer

Answer 1

One other option is that you could store a std::pair<hash_map*, string*> for each entry in the hash map. Obviously set the unused pointer in each pair to NULL so you can tell which is used and which isn't.

Debatable whether that's neater than your approach or not, although I would hazard that it's less code since you don't need definitions of Object, StringObj and HashMap.

Answer 2

Use boost::variant (which is equivalent to a C++ union for user-defined types), C++ union (applicable in this case as you're working with only pointers) or boost::any (which can store any type) to store a pointer to either hash_map or string.