Question

I got a piece of code as follows:

 template <typename T>
 class Ordered_list{
 public:
     Ordered_list(bool (*less_than_function)(const T&, const T&)): less_than(less_than_function){}

 private:
     bool (*less_than) (const T&, const T&);
 };

 bool less_than_f(const int& i1, const int& i2){
     return i1 < i2;
 }   

 class Collection{
 public:
     Ordered_list<int> list(less_than_f);
 };

The code doesn’t compile, and I got the error:

test.cpp:16: error: 'less_than_f' is not a type

I was trying to create a object for the Ordered_list class, which I need to pass in a function pointer. I tried to google for the reason, but didn’t get much. Can anybode tell me what should I do to make it work?

Answer 1

class Collection{
  public:
    Ordered_list<int> list(less_than_f);
 };

Here you are declaring a function with return Ordered_list<int> and take a nonamed parametr of type less_than_f (Opss! no type), while you want to have a member “list“. You need to initializate it in the initialization list of the constructor (because Ordered_list dont have a default constructor) [hmm.. not sure about C++11 new options to initialize, but not now in VC2012].

 class Collection{
  public:
    Ordered_list<int> list;
    Collection (): list(less_than_f){} 
    explicit Collection (   bool (*less_than_function)(const int& i1, const int& i2)   )
       : list(less_than_function)  {}
   };