If you create a struct in C++, you must initialize it by hand. If the struct contains classes, their constructors will not be called automatically, so they will be filled with random garbage.
It is important to know this and apply this knowledge lest your application crash in interesting fashion.
Happily, I already knew this.
Update: And you can have a constructor for your struct in C++, so if you bother to write the constructor, you can do the initialization trivially, because then the default constructor for your classes will be called. I learn something new every day. :)
You still need to actually write the constructor though...