Re: Operater < overloading for struct problem

From:
"mlimber" <mlimber@gmail.com>
Newsgroups:
comp.lang.c++
Date:
1 Mar 2007 08:03:47 -0800
Message-ID:
<1172765027.287246.180610@p10g2000cwp.googlegroups.com>
On Mar 1, 10:45 am, "Frank" <f.f.nee...@gmail.com> wrote:

Hello everyone,

I am having trouble overloading the < operator for an assignment. I
use a struct that contains information and I would like to sort this
structure using STL sort with my own criteria of sorting. Basically, I
would like to sort on visitor count of the Attraction structure.
However, it never uses the < overloaded operator with my code.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

struct Attraction {
        string name;
        int visitors;

};

class Handler { //Define the class Handler

        private:
                vector<Attraction*> attractions ;
                vector<Attraction*>::iterator p ;

        public: //Public functions
                ~Handler();
                void addAttraction(string name, int visitors);
                void printAttractions();

};

#endif

and in the Handler.cpp I have:

Handler.cpp - snippet:

bool operator<(const Attraction& a,const Attraction& b){
                return a.visitors < b.visitors;
        }

Here is the function that performs the sort after adding a value:

void Handler::addAttraction(string name, int visitors){

                Attraction *attr;
                attr=new Attraction();

                attr->name=name;
                attr->visitors=visitors;

                attractions.push_back(attr);
                sort(attractions.begin(),attractions.end());

        }

However, whatever I do, it will never use the overloaded < operator
for sorting. What am I doing wrong? If I add the overloaded function
in the header it starts complaining because it will also be inserted
into the main program which is confusing since I have a guard around
the header file.


I suspect the fundamental problem is that you are sorting pointers
rather than objects. In other words, std::sort works on the value_type
of the container, which in your case is Attraction*, not Attraction.

So, do you need to operator on the Attraction object polymorphically?
If not, don't use pointers; just allocate it on the stack, and let
vector make a copy (assuming attractions is now of type
vector<Attraction>:

 void Handler::addAttraction( const string& name, const int visitors )
 {
   const Attraction attr = { name, visitors };
   attractions.push_back(attr);
   sort(attractions.begin(),attractions.end());
 }

Note also that I changed the function parameters to const (see
http://www.parashift.com/c++-faq-lite/const-correctness.html) and the
string to a reference so it doesn't make an additional copy (see
http://www.parashift.com/c++-faq-lite/references.html).

If you do need to use it polymorphically and to store it in a vector,
prefer smart pointers such as std::tr1::shared_ptr (aka
boost::shared_ptr) or one of the smart pointers found in this FAQ and
those following:

http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.22

BTW, you could put just the function prototype for your operator< in
the header or you could declare it "inline" to get rid of the
duplication problem.

Cheers! --M

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