Re: Overloading operator== for pair<>

From:
Paul Bibbings <paul.bibbings@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 01 Jun 2010 19:28:42 +0100
Message-ID:
<87wrui4nfp.fsf@gmail.com>
"Tom Sz." <tom.usenet.73@gmail.com> writes:

Hello.
I was experimenting a bit and wrote something but the result is not
what I expected.
Can someone tell me why the code below doesn't display "Found".

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

bool operator== (const pair<int, int>& l, const pair <int, int>& r)
{
  return l.first == r.first;
}


Here you are providing a custom op== for a standard library type,
std::pair. You should first of all be aware, if you are not already,
that one such op== already exists, declared as:

   namespace std {
      template <class T1, class T2>
      bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
   }

As this operator is declared in namespace std together with std::find,
it will be found and used by the latter and your own custom operator
will not be considered.

(To illustrate this, wrap your own custom operator in namespace std,
whereupon /it/ will be preferred as a non-template function. However,
this is by way of illustration only and should not be considered as a
solution to your problem.)

int main()
{
   vector<pair<int, int> > vec;
   vec.push_back(pair<int, int>(0, 0));
   vec.push_back(pair<int, int>(0, 1));
   vec.push_back(pair<int, int>(1, 1));
   vec.push_back(pair<int, int>(2, 1));

   pair<int, int> p(2, 2);

   vector<pair<int, int> >::iterator it = find(vec.begin(), vec.end(),
p);
   if (it != vec.end()) cout << "Found" << endl;
}


What you need here is probably a predicate and std::find_if. Something
like:

   template<typename T1, typename T2>
   class eq_first {
   public:
      eq_first(std::pair<T1, T2> p)
         : p_(p)
      { }
      bool operator()(std::pair<T1, T2> p)
      { return p.first == p_.first; }
   private:
      std::pair<T1, T2> p_;
   };

   // ...

   std::vector<std::pair<int, int> >::iterator it =
      std::find_if(vec.begin(), vec.end(), eq_first<int, int>(p));

Regards

Paul Bibbings

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