Re: std::sort (AGAIN)

Hi,

I know that std::sort() is a frequent topic here, and I know it is covered
multiple times in the FAQ, but I can't find the answer to something that
should be simple. It was trivial in C using qsort(), at least, but I got
the crazy idea in my head that it would be a good idea to start using STL.

I want to sort a vector of structs. The catch is that the user can select
which fields in the struct are used for the sort. That alone I can do, but
what I can't do is make it work with member functions. From what I can
gather in the FAQ, there are a few possibilities:

1. Make the compare function static. I can't do this because the
comparison is dependent on what the user selected. In other words, the
compare function must access private members of the class doing the sort.

2. Create a global instance of the class and a wrapper function that calls
the compare member, then pass that wrapper function to std::sort(). I
can't do this because I need the *actual* object to sort its own data, not
some generic global object. I might even have multiple objects doing sorts
and they each might select different options so that they each perform the
sort differently.

3. Use a functor. This is ugly and hacky and I admit that I don't really
get it. But looking at examples of functors, I don't understand how the
function could access the internals of the class doing the sorting.