Thursday, December 2, 2010

dissect`parametrized functor declaration-cum-construction


typedef void (*WorkerFunPtr)(const std::string&);

template<typename FunT,   // The type of the function being called
         typename ParamT> // The type of its parameter

struct Adapter {
   Adapter(FunT f, ParamT& p) : // Construct this adapter and set the
      f_(f), p_(&p) {}          // members to the function and its arg

   void operator( )( ) { // This just calls the function with its arg
      f_(*p_);        
   }
private:
   FunT    f_;
   ParamT* p_;  // Use the parameter's address to avoid extra copying
};
void worker(const std::string& s) {} // a free function
int main( ) {
   boost::thread thr1(Adapter<WorkerFunPtr, std::string>(worker, someStrVar)); // this line should be decomposed as
 
   Adapter<WorkerFunPtr, std::string> // specialized type
        myAdapter(worker, someStrVar); // constructor

   boost::thread thr1(myAdapter);


Specializing a class template and then constructing an object is always a confusing process. Unfortunately we often have to do it in one line.
* specializing the Adapter template is simple enough, if we treat WorkerFunPtr as a simple type rather than a complicated typedef.
* constructing an object is simple enough
* However, the constructor is parametrized (since it's in a class template), therefore the arguments have to match the actual types specified in the template specialization.
** worker is a function but implicitly converted to a function pointer. Its type does match the actual type WorkerFunPtr
** someStrVar does match the actual type std::string
==) Therefore this line constructs an object of the specialized type, using the 2 arguments -- a func ptr and a string

Last lines passes the newly constructed myAdapter OBJECT to the thread constructor. Implicitly converted to qq/myAdapter.operator()()/ i.e. a no-arg function.
 

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