question about using bind2nd to convert a binary predicate to unary predicate

In STL, "bind2nd" is used to convert a binary prediate to unary predicate. I try to declare a unary predicate by bind2nd, but it does not work.
Please tell me the reason.


#include<functional>
#include<algorithm>
#include<vector>

using namespace std;
int main()
{
      vector<int> v;
      typedef vector<int>::iterator iterator;

      v.push_back(1);      
      v.push_back(2);      
      v.push_back(3);      

      // this part work well
      iterator it1 = find_if(v.begin(), v.end(), bind2nd(equal_to<int>(), 3));

      // however, the following doesn't work
      unary_function<int,bool> p = bind2nd(equal_to<int>(), 3);      
      iterator it2 = find_if(v.begin(), v.end(), p);//when compile: error C2064: term does not evaluate to a function

      return 0;
}
zma64Asked:
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PhilipNicolettiCommented:
    binder2nd<equal_to<int> > p = bind2nd(equal_to<int>(),3);

Not really sure why. I saw this on the web some time ago and just
put it in my notes.

This also works:

     binder1st<equal_to<int> > p = bind1st(equal_to<int>(),3);
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efnCommented:
Good answer from PhilipNicoletti.  Here's why it works.

find_if wants a function object as its third parameter.  A function object is an object of a class that has an operator().  unary_function is just a structure with a couple of typedefs.  It doesn't have an operator() itself.  It's intended as a base class for function objects so they can inherit the typedefs.  So the compiler complained because it couldn't call p(), because p didn't have an operator ().

What you want as the type of p is something this is derived from unary_function, but has an operator ().  That's exactly what binder2nd is, and conveniently enough, a binder2nd is what bind2nd returns.  The binder2nd has a function object and a second parameter as members, and its operator() calls the function object.

--efn
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PhilipNicolettiCommented:
That makes sense,

when I tried the original code on Intel's cpp compiler, it gave
an error message about needing operator () or function
pointer (I'm not sure of the exact wording - I don't have
access to the compiler right now).
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tinchosCommented:
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