How do I sort a set by the dereferenced values?

Question

Suppose I have a set std::set<*int> and I want the elements to be sorted by the integer to which they point rather than the pointer type; is there some standard comp function I can use from the std library? If not, how would I declare such a set?

I'm guessing I would have to do define my own comparison function but how does that look in practice?

@JohnZwinck I wouldn't, I actually have a set of pointers to classes with their own `<` operators, but I thought this would be a simplified analogy to my problem. — quant, Oct 15 '13 at 12:09
Given all that, this is a nearly exact duplicate of the question I just linked. You'll find its solution works for you too. — John Zwinck, Oct 15 '13 at 12:11
@JohnZwinck that's similar, but it applies to a list which is sorted by `std::sort()`. In my case I need to declare an object of type `std::set<*int,Comp>`, and I don't know how to do that! — quant, Oct 15 '13 at 12:13
Please don't mark this to close; that question is not related to this one, at least not as far as I can tell. If it is, please tell me why as I would love to know! — quant, Oct 15 '13 at 12:14

score 3 · Accepted Answer · edited May 23 '17 at 11:50

3

Using the solution from Sort a std::list<myclass*> with myclass::operator<(myclass &other) it goes like this:

template <typename T>
struct PComp
{
    bool operator ()(const T* a, const T* b) const
    {
       return *a < *b;
    }
};

std::set<int*, PComp<int> > my_set;

edited May 23 '17 at 11:50

Community

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answered Oct 15 '13 at 12:20

John Zwinck

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Cool, I would never have guess that, thanks! – quant Oct 15 '13 at 12:28
You're welcome. No need to guess--just think about what the requirements are on the template parameters. Using a custom comparator like this is fairly commonplace, and this usage is pretty vanilla. :) – John Zwinck Oct 15 '13 at 12:29
Hmm, I'm getting a `Error: Function template PComp is not a type name` error... – quant Oct 15 '13 at 12:57
1

Yeah, I had posted a function template rather than a functor. I've fixed it now. – John Zwinck Oct 15 '13 at 13:04
@JohnZwinck : is it possible to do this inline in an anonymous way? – xmoex Oct 15 '13 at 13:11
It might be "possible," but it's not reasonable. I've never tried, and I wouldn't suggest it. – John Zwinck Oct 15 '13 at 13:36
1

@xmoex: [No](http://ideone.com/o1xP1D) – MSalters Oct 15 '13 at 13:43
@xmoex nice solution when you only want to declare it once. – quant Oct 16 '13 at 00:47

How do I sort a set by the dereferenced values?

1 Answers1