C++ set unique and order

Question

I want to make unique and order in set<Foo, FooComp> .

In below code, I want a to unique and order by b and c. So, no same foo.a and order by foo.b and foo.c .

How can I do this?

struct Foo {
    int a, b, c;
    Foo(int a, int b, int c) : a(a), b(b), c(c) {}
}

struct FooComp {
    bool operator() (const Foo& f, const Foo& s) const {
        if (f.pattern == s.pattern) {
            return false;
        }
        if (f.start == s.start) {
            return f.length < s.length;
        }
        return f.start < s.start;
    }
}

or am I use other STL or data structures?

Answer 1

This is not possible using the standard library set.

The comparison operator is tightly coupled with the ordering.

Although a bit worse solution in terms of performance, you could have a set that contains all your objects, sorted by 'a' only using:

struct Foo {
    int a, b, c;
    Foo(int a, int b, int c) : a(a), b(b), c(c) {}
    bool operator<(const Foo& rhs) const {
        return a < rhs.a;
    }
    friend ostream& operator<<(ostream&, const Foo&);
};

And then, whenever you want to sort it using your unique algorithm, just copy it into a vector and sort it according to your needs:

vector<Foo> v;
std::copy(s.begin(), s.end(), std::back_inserter(v));
std::sort(v.begin(), v.end(), [](const Foo& lhs, const Foo& rhs){ return (lhs.b == rhs.b) ? lhs.c > rhs.c : lhs.b > rhs.b; });

Edited

This implements the logic you used in your Pastebin example. Entire sample here

Answer 2

There's a ready-made library for this kind of thing in boost called boost.multi_index.

It allows the declaration of a container that satisfies multiple indexes and their constraints.

It's a little archaic and could do with some love but it does the job.

You can start with something like this:

struct Foo {
    int a, b, c;
    Foo(int a, int b, int c) : a(a), b(b), c(c) {}
};

#include <tuple>
#include <type_traits>
#include <utility>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/ordered_index.hpp>


struct get_a
{
    using result_type = int const&;
    result_type operator()(Foo const& l) const {
        return l.a;
    }
};

struct get_bc
{
    using result_type = std::tuple<int const&, int const&>;

    result_type operator()(Foo const& l) const {
        return std::tie(l.b, l.c);
    }
};

namespace foo {
    using namespace boost;
    using namespace boost::multi_index;

    struct by_a {};
    struct by_bc {};

    using FooContainer = 
multi_index_container
<
    Foo,
    indexed_by
    <
        ordered_unique<tag<by_a>, get_a>,
        ordered_non_unique<tag<by_bc>, get_bc>
    >
>;
}

int main()
{
    foo::FooContainer foos;

    foos.insert(Foo{ 1, 2,3 });
    foos.insert(Foo{ 2, 2,4 });

}