Sorting array of class objects using member function

Question

I'm not sure what is wrong with the following code, where I'm trying to sort an array of class objects using a member function comparator.

class Query {
    public:
        int start;
        int end;
        int index;
        bool operator<(const Query &b) {
            return this->start < b.start;
        }
};

Query query[q];

for (int i=0;i<q;++i) {
    cin>>query[i].start>>query[i].end;
    query[i].index = i;
}
sort(query,query+q);

I get the following error:

error: no matching function for call to ‘sort(main()::Query [(((unsigned int)(((int)q) + -0x00000000000000001)) + 1)], main()::Query*)’

Update: I figured out the cause of the error. I had included the class within my main. The problem was resolved when I moved the class definition outside main. I don't have a good enough understanding of C++/OOP to understand why this happens. I'd appreciate if somebody could explain or direct me to helpful resources.

Yes I have, thanks. The issue was different. All the above piece of code was within main(). But when I removed the class definition outside main, the problem was resolved. I don't understand why though. — elexhobby
– elexhobby, Commented Sep 20, 2013 at 23:26
@elexhobby You should edit your question and change it to include this information. — Bernhard Barker
– Bernhard Barker, Commented Sep 21, 2013 at 18:31
possible duplicate of Why can't a struct defined inside a function be used as functor to std::for_each? — Oswald
– Oswald, Commented Sep 21, 2013 at 19:09
You find answers here: C++ local class as functor, how to define a functor inside a function, Why does this std::sort predicate fail when the class is inside main()? — Christian Ammer
– Christian Ammer, Commented Sep 21, 2013 at 19:22

Oswald · Accepted Answer · 2013-09-21 19:13:29Z

2

Local types (i.e. types that are defined inside a function) cannot be used as template arguments in C++03 (one of the template arguments of std::sort() is the type of the objects that should be sorted). I do not know why C++03 has this restriction. C++11 does not have this restriction anymore.

edited Sep 21, 2013 at 19:13

answered Sep 21, 2013 at 19:08

Oswald

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