C++ operations on vector of struct

Question

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

header file:

#include <stdio.h>
#include <iostream>
#include <string.h>
#include <vector>
#include <algorithm>

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<rankingElement> ranking);
   void insertIntoRanking(rankingElement rElement);
};

And source file :

#include "RankingCreator.h"

bool RankingCreator::checkPageRank(rankingElement rElement,
                                   vector<rankingElement> 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 1

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

Answer 2

There is no way the compiler knows how to compare an object of type rankingElement with an object of type string (this is what is happeneing in std::find).

In the std::find you are doing the test:

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

// Inside this you are iterating through a vector of `rankingElement`
// But the element you ae testing against is: `rElement.url` which is a std::string


// Sort of like this:
ranking[0] == rElement.url
              ^^^^^^^^^^^^  Object of type string
^^^^^^^^^^ Object of type rankingElement

There are a couple of solutions:

1. Provide an appropriate compare function
   • bool operator==(rankingElement const& lhs, std::string const& rhs)
2. Use the version of find that allows you to pass a function/functor
   • std::find_if(ranking.begin(), ranking.end(), test_Ranking_against_url)

Important Point:

1. Never put using namespace std; in the header file.

2. You can put it using namespace std; the source file.
   But I would even discourage that.
   Prefer to use the long form of all members of std. (ie. std::vector (not vector)).

3. Pass by reference any large objects (like vectors).
   Use const reference if you don't want the original changed.
   Then you don't incur the cost of a copy operation.

Answer 3

When you use UDF types as the types of STL containers you must before provide valid:

1. copy ctor;
2. copy assignment operator ('operator=')
3. and at least operator== and operator<