C++ operations on vector of struct

I am getting error "no match for operator==in__first.
Here is the code:

header file:

#include 
#include 

#include 
#include 
#include 

using namespace std;

struct rankingElement {
   string url;
   int rank;
};


class RankingCreator {
public:
   static const int MAX_QUERY_SIZE = 20;
   RankingCreator();
   virtual ~RankingCreator();
   bool checkPageRank(rankingElement rElement, vector ranking);
   void insertIntoRanking(rankingElement rElement);
};

And source file :

#include "RankingCreator.h"

bool RankingCreator::checkPageRank(rankingElement rElement,
                                   vector ranking)
{
   if (ranking.size() < MAX_QUERY_SIZE) {
      // enough space for another page in ranking

      return true;
   } else {
      if (find(ranking.begin(), ranking.end(), rElement.url) != ranking.end()) {
         // url is already in ranking
         return false;
      } else {

      }
   }
   return true;

}

I tried commenting some blocks of code in source file and the line with find() function seems to generate the errors. It is function from class algorithm used to check whether a vector already contains certain element. I found out that it is wrong, because I try to compare struct with string in find function. I can handle that by copying urls from ranking to another vector of string and then use that vector of strings in find function, but when I tried to do that I can't access elements in 'ranking' vector - for instance ranking[i].url doesn't work and I don't know why.. and help would be appreciated.

Answer

find(ranking.begin(), ranking.end(), rElement.url)

you are telling find to search for rElement.url (which is of type string) in [ranking.begin(), ranking.end()) (which has elements of type rankingElements). When not explicitely giving std::find a comparison function it tries to use operator== to compare the elements with the searched for elements. So it tries to invoke operator==(rankingElement, string) which obviously doesn't exist. Imo the best way to make something like that work is to use find_if which accepts a predicate:

struct functor  {
    string url;
    functor(const string& str):url(str){}
    bool operator()(const rankingElement& val) { return val.url == this->url; }
};
...
if(find_if(ranking.begin(), ranking.end(), functor(rElement.url)) != ranking.end() )

otherwise you can write an operator==:

bool operator==(const rankingElement& elem, const string& url) 
{ return elem.url == url; }

this will work with find