Question

For a rules engine developed in C++, one of the core features is the value type. What I have so far is a bit like a COM-style VARIANT – each value knows its type. There are some rules for type conversion but it’s a bit messy.

I wondered if there are nice drop-in value classes I could use which solve this, without requiring me to use a whole pre-built system. For instance maybe boost has something?

Answer 1

Looking for boost::any or boost::variant?

There are basically three types of variant implementations:

1. A type that can be freely casted between types (think untyped languages) — boost::lexical_cast is your friend here, or boost::variant…
2. A type that can hold any type, but is typesafe — e.g. initialized with an int, stays an int and doesn’t allow to be treated implicitly like anything else — this is the boost::any type
3. The evil allow anything type — cast to what you want without error checking, no type information held — think void*