C++ bind2nd question

Question

This was one of the questions showed up on my Final exam. I can't figure out what I'm supposed to do. I know BindSecArg requires a () operator, but not sure what goes inside.

In this question you are required to implement something similar to std::bind2nd. For simplicity main is written using a ”for”-loop, but it may be rewritten with ”for each” and STL containers.

class Functor1 {
  public:
     int operator()(const int & i, const int & j) const {
      return i+j;
     }
};

class Functor2 {
   public:
    int operator()(const int & i, const int & j) const {
       return i*j;
     }
};

template <typename T>
class BindSecArg

};

int main () {
  Functor1 f1;
  for (int i=0; i<10; ++i) std::cout << f1(i,i) << " "; //0 2 4 6 8 10
  std::cout << std::endl;

  Functor2 f2;
  for (int i=0; i<10; ++i) std::cout << f2(i,i) << " "; //0 1 4 9 16 25
  std::cout << std::endl;

  BindSecArg<Functor1> b1(4); //bind second argument of Functor1 to 4
  for (int i=0; i<10; ++i) std::cout << b1(i) << " "; //4 5 6 7 8 9
  std::cout << std::endl;

  BindSecArg<Functor2> b2(4); //bind second argument of Functor2 to 4
  for (int i=0; i<10; ++i) std::cout << b2(i) << " "; //0 4 8 12 16 20
  std::cout << std::endl;
  }

Extra credit question: your implementation most probably doesn’t work (which is OK!) with

 class Functor3 {
    public:
      std::string operator()(const std::string & i, const std::string & j) const {
        return i+j;
      }
 };

how does STL solve this problem?

Answer 1

probably there are better implementations:

template <typename T>
class BindSecArg
{
public:
   BindSecArg(int value2) : m_value2(value2){ };
   int operator()(int value1) { return T()(value1, m_value2);}
private:
   int m_value2;
};

int the link I posted in the comment to your question you can find the stl code.