A class for exchanging parameters

From: Joe Gottman (joegottman_at_[hidden])
Date: 2000-05-26 20:38:57

    Suppose you want to sort some objects using a custom ordering, so you create a binary comparison function myCompare.  Later, you want to sort some more objects using the same comparison, but in the opposite order.  The naive way of doing this is the following:

sort(begin(), end(), not2(myCompare));

Unfortunately, this will probably cause your computer to crash, because a predicate pred used in sorting must satisfy !(pred(x,y)) for all x and y.  Obviously, if your predicate does this, not of your predicate will not.  The correct way of reversing a comparison is to evaluate pred(y,x) instead of pred(x,y), see the canonical definition of x > y as y < x for an example.

Here is a class that does just that.  It simply takes a binary predicate and exchanges the order of the parameters.  Please tell me what you think.

#define EXCHANGE_PARAMETERS_H
#include <functional>
//This class reverses the order of parameters of a binary_function
//object. Its most obvious use is to reverse the direction of a
//comparison for sorting or STL sets, maps, etc.
namespace boost {
template <class OP>
class exchange_parameters_t
    : public std::binary_function<typename OP::second_argument_type,
    typename OP::first_argument_type,
    typename OP::result_type>
{
    private:
    OP op;

    public:
    exchange_parameters_t() : op() {};
    exchange_parameters_t(const OP &in_op) : op(in_op) {};
        typename OP::result_type operator()(
        const typename OP::second_argument_type &x1,
        const typename OP::first_argument_type &x2) const
        {return op(x2, x1);}
};

template <class OP>
exchange_parameters_t<OP> exchange_parameters(const OP &op)
{
    return exchange_parameters_t<OP>(op);
}

#endif

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