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.

Howeve

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 
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 
  Chain  chain(const Next& next) const {
     return Chain  (*this, next);
  }

  First f;
  Second s;
};

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

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

Then to use it:

vector  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.