Correct Way to Define a Predicate Function in C++

I'm trying to write predicate function for use with STL algorithms. I see that they are two ways to define a predicate:

(1) Use a simple function as below:

bool isEven(unsigned int i)   
{ return (i%2 == 0); }

std::find_if(itBegin, itEnd, isEven); 

(2) Use the operator() function as below:

class checker {  
public:  
  bool operator()(unsigned int i)  
  { return (i%2 == 0); }  
}; 

std::find_if(itBegin, itEnd, checker); 

I have more use for the second type as I usually would like to create a predicate object with some members in it and use them in the algorithm. When I add the same isEven function inside checker and use it as a predicate, I get an error:
3. Syntax which gives error:

class checker { 
    public: 
       bool isEven(unsigned int i) 
       { return (i%2 == 0); }
}; 

checker c; 
std::find_if(itBegin, itEnd, c.isEven); 

Calling c.isEven gives an error during compilation saying undefined reference to some function. Can someone explain why 3. is giving error? Also, I would appreciate any pointers to read about predicate and iterator basics.