What's the simplest way of defining lexicographic comparison for elements of a class?

If I have a class that I want to be able to sort (ie support a less-than concept), and it has several data items such that I need to do lexicographic ordering then I need something like this:

struct MyData {
  string surname;
  string forename;

  bool operator<(const MyData& other) const {
    return surname < other.surname || (surname==other.surname && forename < other.forename); }
};

This becomes pretty unmanageable for anything with more than 2 data members. Are there any simpler ways of achieving it? The data members may be any Comparable class.

tuple is a good idea, but if you want to keep having names for your member variables, it might be good enough to restructure your comparison function like this:

struct MyData {
    string surname;
    string forename;
    string var;
    // ...

    bool operator<(const MyData& other) const {
        if (surname != other.surname) return surname < other.surname;
        if (forename != other.forename) return forename < other.forename;
        if (var != other.var) return var < other.var;

        // ...

        return false; //< They are equal
    }
};

Depending on your taste, you might even want a macro like #define COMPARE(field) if (field != other.field) return field < other.field; to reduce duplication. Then the function would just become a list of COMPARE-invocations.

With the advent of C++11 there's a new and concise way to achieve this using std::tie:

bool operator<(const MyData& other) const {
  return std::tie(surname, forename) < std::tie(other.surname, other.forename);
}

Mike Seymour

You could store the data in a boost::tuple, which provides lexicographic comparison, and provide named accessor functions, along the lines of:

#include <boost/tuple/tuple.hpp>
#include <boost/tuple/tuple_comparison.hpp>

struct Data {
    string &surname()  {return stuff.get<0>();}
    string &forename() {return stuff.get<1>();}

    // it would be polite to add const overloads too.

    bool operator<(const Data &other) const {return stuff < other.stuff;}

private:
    boost::tuple<string, string> stuff;
};

I believe this is also available as std::tr1::tuple, and will be std::tuple in the forthcoming standard.

Maintaining the list of accessors is probably more manageable than maintaining the comparison code.

If all members have the same type you could put them in std::vector. By default std::lexicographical_compare will be used to compare vectors.

You can use a boost::tuple or std::pair which has built-in lexigraphical comparison. Of course the disadvantage is you can't associate a method to the tuples.

来源：https://stackoverflow.com/questions/2500664/whats-the-simplest-way-of-defining-lexicographic-comparison-for-elements-of-a-c

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