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noatom

Help with weird code

3 posts in this topic


#include <iostream>
#include <vector>

using namespace std;

class Obj {
  static int i, j;
public:
  void f() const { cout << i++ << endl; }
  void g() const { cout << j++ << endl; }
};

// Static member definitions:
int Obj::i = 47;
int Obj::j = 11;

// Container:
class ObjContainer {
  vector<Obj*> a;
public:
  void add(Obj* obj) { a.push_back(obj); }
  friend class SmartPointer;
};

class SmartPointer {
  ObjContainer& oc;
  int index;
public:
  SmartPointer(ObjContainer& objc) : oc(objc) {
    index = 0;
  }
  // Return value indicates end of list:
  bool operator++() { // Prefix
    if(index >= oc.a.size()) return false;
    if(oc.a[++index] == 0) return false;
    return true;
  }
  bool operator++(int) { // Postfix
    return operator++(); // Use prefix version
  }
  Obj* operator->() const {
   
    return oc.a[index];
  }
};

int main() {
  const int sz = 10;
  Obj o[sz];
  ObjContainer oc;
  for(int i = 0; i < sz; i++)
    oc.add(&o[i]); // Fill it up
  SmartPointer sp(oc); // Create an iterator
  do {
    sp->f(); // Pointer dereference operator call
    sp->g();
  } while(sp++);
} ///:~

 

The code compiles,in case you want to know.Now,the problem:

when the function for the overloaded -> is called,it returns a pointer.I don't understand how an UNDEREFERENCED pointer can acces a function.

 

sp->g();

right there,the -> function is called,it returns a pointer and it immediatly has acces to functions,why? Shouldn't it be dereferenced first? Or it got dereferenced somewhere else,but I can't see where?

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Did you get this code from here? If so, the answer is right at the top. If not, let me quote:

 

The dereferencing operator-> can be defined as a unary postfix operator. That is, given a class:

 

class Ptr{
//...
X * operator->();
};
 

 

Objects of class Ptr can be used to access members of class X in a very similar manner to the way pointers are used. For example:

 

void f(Ptr p )
{
p->m = 10 ; // (p.operator->())->m = 10
}
 

The statement p->m is interpreted as (p.operator->())->m. Using the same concept, following example explain how a class access operator -> can be overloaded.

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When the -> operator is overloaded the returned value is then used as the left hand side of another application of -> with the previous right hand side. This is why -> is overloaded as a unary operator rather than a binary operator. So your smart pointer returns a pointer and that pointer then has -> applied to it with f() or g() on the right hand side.
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ok,this is the point where i say: i'm on the weird side of the internet again :)

 

it's kinda weird,the return value of -> is then used with -> ...oh

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