Chaining of ordering predicates (e.g. for std::sort)

Question

You can pass a function pointer, function object (or boost lambda) to std::sort to define a strict weak ordering of the elements of the container you want sorted.

However, sometimes (enough that I've hit this several times), you want to be able to chain "primitive" comparisons.

A trivial example would be if you were sorting a collection of objects that represent contact data. Sometimes you will want to sort by

last name, first name, area code

. Other times

first name, last name

- yet other times

age, first name, area code

... etc

Now, you can certainly write an additional function object for each case, but that violates the DRY principle - especially if each comparison is less trivial.

It seems like you should be able to write a hierarchy of comparison functions - the low level ones do the single, primitive, comparisons (e.g. first name < first name), then higher level ones call the lower level ones in succession (probably chaining with && to make use of short circuit evaluation) to generate the composite functions.

The trouble with this approach is that std::sort takes a binary predicate - the predicate can only return a bool. So if you're composing them you can't tell if a "false" indicates equality or greater than. You can make your lower level predicates return an int, with three states - but then you would have to wrap those in higher level predicates before they could be used with std::sort on their own.

In all, these are not insurmountable problems. It just seems harder than it should be - and certainly invites a helper library implementation.

Therefore, does anyone know of any pre-existing library (esp. if it's a std or boost library) that can help here - of have any other thoughts on the matter?

[Update]

As mentioned in some of the comments - I've gone ahead and written my own implementation of a class to manage this. It's fairly minimal, and probably has some issues with it in general. but on that basis, for anyone interested, the class is here:

http://pastebin.com/f52a85e4f

And some helper functions (to avoid the need to specify template args) is here:

http://pastebin.com/fa03d66e

Answer 1

You could build a little chaining system like so:

struct Type {
  string first, last;
  int age;
};

struct CmpFirst {
  bool operator () (const Type& lhs, const Type& rhs) { return lhs.first < rhs.first; }
};

struct CmpLast {
  bool operator () (const Type& lhs, const Type& rhs) { return lhs.last < rhs.last; }
};

struct CmpAge {
  bool operator () (const Type& lhs, const Type& rhs) { return lhs.age < rhs.age; }
};

template <typename First, typename Second>
struct Chain {
  Chain(const First& f_, const Second& s_): f(f_), s(s_) {}

  bool operator () (const Type& lhs, const Type& rhs) {
    if(f(lhs, rhs))
      return true;
    if(f(rhs, lhs))
      return false;

    return s(lhs, rhs);
  }

  template <typename Next>
  Chain <Chain, Next> chain(const Next& next) const {
     return Chain <Chain, Next> (*this, next);
  }

  First f;
  Second s;
};

struct False { bool operator() (const Type& lhs, const Type& rhs) { return false; } };

template <typename Op>
Chain <False, Op> make_chain(const Op& op) { return Chain <False, Op> (False(), op); }

Then to use it:

vector <Type> v;  // fill this baby up

sort(v.begin(), v.end(), make_chain(CmpLast()).chain(CmpFirst()).chain(CmpAge()));

The last line is a little verbose, but I think it's clear what's intended.