C++ list::sort of classes by members

Question

How can I sort a list of classes by a certain member variable?

class Klasse {
  int _a;
  int _b;

}

...
list<Klasse> liste;
liste.sort();   // sorts by _a
liste.sort(?);  // how to sort by _b now?

Have you tried [creating a custom comparator](http://stackoverflow.com/questions/12508496/comparators-in-stl)? — tadman, Jan 14 '15 at 18:00

Bill Lynch · Accepted Answer · 2015-01-14T18:09:06.963

4

You would use a comparator object. Here's an example using a lambda.

std::list<Klasse> liste;
liste.sort([](Klasse const & lhs, Klasse const & rhs) {
    return lhs._b < rhs._b;
});

edited Jan 14 '15 at 18:09

answered Jan 14 '15 at 18:00

Bill Lynch

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C++11 or later? These things are great if your compiler supports them. – tadman Jan 14 '15 at 18:01
that's pretty C+11 :) nice! – Marcus Müller Jan 14 '15 at 18:01
By the way, lambda is not something that someone new to C++ will easily understand, so this might not be the best answer without further references. – Marcus Müller Jan 14 '15 at 18:12

milleniumbug · Answer 2 · 2015-01-14T18:06:52.437

3

See the reference.

You can write a comparison function - basically anything that can be called with two arguments of the element type of your list, and this call returns value convertible to bool. Such "anything" can be a lambda, function object, or simply just a function:

bool klasse_sort_by_b(const Klasse& l, const Klasse& r)
{
    return l._b < r._b;
}

liste.sort(klasse_sort_by_b);

edited Jan 14 '15 at 18:06

answered Jan 14 '15 at 18:00

milleniumbug

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score 1 · Answer 3 · answered Jan 14 '15 at 18:02

you need this implementation of sort:

template<typename Compare>    
void sort (Compare comp);

then pass inside a compare function like:

bool compareByA( const Klasse& first, const Klasse& second ){
 return first._a < second._a;
}

then call it:

std::list<Klasse> lst;
...
lst.sort(compareByA);
lst.sort(compareByB);

Zongor Lajos · Answer 4 · 2015-01-14T18:20:18.470

0

http://www.cplusplus.com/reference/list/list/sort/

You should write your own comparer, example and usage in the link ;)

here is the code example as promised

(thanks for the constructive criticism)

 bool compare_by_b (const Klasse& first, const Klasse& second)
    {
      return first._b < second._b ;
    }

 liste.sort(compare_by_b);

edited Jan 14 '15 at 18:20

answered Jan 14 '15 at 18:00

Zongor Lajos

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You should include code in your answer instead of linking to external resources. – Overv Jan 14 '15 at 18:01
2

he answers the question (though shortly), and gives a reference. Possibly not the most thorough explanation, but worth more than a downvote! – Marcus Müller Jan 14 '15 at 18:02
@MarcusMüller: http://meta.stackexchange.com/questions/8231/are-answers-that-just-contain-links-elsewhere-really-good-answers – Bill Lynch Jan 14 '15 at 18:05
I agree, but he did *not* just link somewhere. He said "write a comparer", which is absolutely true and might even be enough to give OP a start, if the linked site went down at one point. – Marcus Müller Jan 14 '15 at 18:09

score -1 · Answer 5 · answered Jan 14 '15 at 18:00

-1

Yes, and all you have to do is implement a comparator class, or overload the comparison Klasse::operator< operators. For reference on the sort method, see this.

answered Jan 14 '15 at 18:00

Marcus Müller

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C++ list::sort of classes by members

5 Answers5