Re: Problem with functor, maybe I need operator== for my struct?

From:

Saeed Amrollahi <amrollahi.saeed@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 25 Jul 2011 05:14:21 -0700 (PDT)

Message-ID:

<45e0fa70-4d95-4b0a-bd68-6373efc649d1@bl1g2000vbb.googlegroups.com>

On Jul 25, 2:25 pm, Fencer <no.i.d...@want.mail.from.spammers.invalid>
wrote:

Hello, I am very rusty in C++ and I'm having some problems: I have a
std::set<SomeClass> and I need to look through the set to find if any of
the instances of SomeClass that it contains has a particular value for
one of its data members. I know I could simply iterate through the set
myself and check every SomeClass instance if any of them is a match, but
I tried to do it using functors instead.
The following simplified test program does not build, and it seems to be
that operator== is missing for the struct s1. Could you gurus help me
explain why my test program does not build and how I should fix it?

#include <algorithm>
#include <functional>
#include <set>

using namespace std;

struct s1
{
    s1(int u, int v) : u(u), v(v) {}

    int u, v;

};

struct OnV : public unary_function<s1, bool>
{
    explicit OnV(int v) : v(v) {}

    bool operator()(s1 const& inst) const
    {
       return v == inst.v;
    }

    int v;

};

int main()
{
    set<s1> a_set;

    find(a_set.begin(), a_set.end(), OnV(4711));

}

$ g++ -Wall -Wextra -std=c++98 -pedantic functor.cpp -o functor.exe
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h: In
function =91_InputIterator std::__find(_InputIterator, _InputIterator,
const _Tp&, std::input_iterator_tag) [with _InputIterator =
std::_Rb_tree_const_iterator<s1>, _Tp = OnV]':
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h:3814:
instantiated from =91_IIter std::find(_IIter, _IIter, const _Tp&) [with
_IIter = std::_Rb_tree_const_iterator<s1>, _Tp = OnV]'
functor.cpp:30: instantiated from here
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h:151:
error: no match for =91operator==' in
=91__first.std::_Rb_tree_const_iterator<_Tp>::operator* [with _Tp = s1]=

()

== __val'

Thank you for reading and thanks for any help! Even though I could use
the alternative solution I mentioned above that does not involve
functors, I wanted to know what I did wrong with my functor and
hopefully learn something from your replies.

- Fencer

Hi

You need to special function objects called predicate
and find_if generic function:

struct Pair
{
Pair(int u, int v) : u(u), v(v) {}
int u, v;
};

// Just for illustration: Somehow inkorect and incomplt
// Set relies on a comparison operation (by default <)
bool operator<(const Pair& p1, const Pair& p2)
{
  return (p1.u < p2.u && p1.v < p2.v) ? true : false;
}

struct CheckV { // predicate
  explicit CheckV(int v) : v(v) {}
  bool operator()(Pair const& inst) const
  {
     return v == inst.v;
  }
  int v;
};

struct CheckU { // predicate
  explicit CheckU(int u) : u(u) {}
  bool operator()(Pair const& inst) const
  {
     return u == inst.u;
  }
  int u;
};

#include <set>
#include <algorithm>
#include <iostream>

int main()
{
  using namespace std;
  set<Pair> MySet;
  MySet.insert(Pair(0, 1));
  MySet.insert(Pair(1, 2));
  MySet.insert(Pair(2, 4711));

  set<Pair>::const_iterator cit =
        find_if(MySet.begin(), MySet.end(), CheckV(4711));
  if (cit != MySet.end())
   cout << "(" << cit->u << ", " << cit->v << ")\n";

  return 0;
}

HTH,
  -- Saeed Amrollahi