Question

There is my program that illustrates predicate template that could be used as sorting predicate for STL containers instantiation:

#include <iostream>
#include <set>
#include <iterator>
#include <algorithm>
#include <functional>
#include <string>

using namespace std;

template<typename T, template <typename> class comp = std::less> struct ComparePtr: public binary_function<T const *, T const *, bool> {
  bool operator()(T const *lhs, T const *rhs) const {
    comp<T> cmp;
    return (cmp(*lhs, *rhs));
  }
};

int wmain() {
  string sa[] = {"Programmer", "Tester", "Manager", "Customer"};
  set<string *, ComparePtr<string>> ssp;
  for (int i(0) ; i < sizeof sa/sizeof sa[0] ; ++i)
    ssp.insert(sa + i);

  for_each(ssp.begin(), ssp.end(), [](string *s){ cout << s->c_str() << endl; });

  return 0;
}

Please, focus on predicate: is it written correctly?
Is it good to instantiate comp? Is there a way that allows not to instantiate comp predicate?

Answer 1

Why use a template template at all and not just add a simple wrapper around general binary comparators?

template<typename P>
struct PointerPred {
  P p;
  template<typename T>
  bool operator()(const T& x, const T& y) { return p(*x, *y); }
};

Boost Pointer Containers could also make this a lot easier.

Ildjarn showed how to implement your functor correctly:

template<template<typename> class comp = std::less>
struct ComparePtr {
  template<typename T>
  bool operator()(T const *lhs, T const *rhs) const {
    return comp<T>()(*lhs, *rhs);
  }
};

This way it isn’t necessary to specify the type of the arguments anymore.

Edit: There used to be std::forward and rvalue references in my code, which made absolutely no sense.