A nested iterator
It’s more common to see a
“smart pointer” or “iterator” class nested within the
class that it services. The previous example can be rewritten to nest
SmartPointer inside ObjContainer like this:
//: C12:NestedSmartPointer.cpp
#include <iostream>
#include <vector>
#include "../require.h"
using namespace std;
class Obj {
static int i, j;
public:
void f() { cout << i++ << endl; }
void g() { 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); }
class SmartPointer;
friend class SmartPointer;
class SmartPointer {
ObjContainer& oc;
unsigned 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 {
require(oc.a[index] != 0, "Zero value "
"returned by SmartPointer::operator->()");
return oc.a[index];
}
};
// Function to produce a smart pointer that
// points to the beginning of the ObjContainer:
SmartPointer begin() {
return SmartPointer(*this);
}
};
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
ObjContainer::SmartPointer sp = oc.begin();
do {
sp->f(); // Pointer dereference operator call
sp->g();
} while(++sp);
} ///:~
Besides the actual nesting of the class,
there are only two differences here. The first is in the
declaration
of the class so that it can be a friend:
class SmartPointer;
friend SmartPointer;
The compiler must first know that the
class exists before it can be told that it’s a
friend.
The second difference is in the
ObjContainer member function begin( ), which produces a
SmartPointer that points to the beginning of the ObjContainer
sequence. Although it’s really only a convenience, it’s valuable
because it follows part of the form used in the Standard C++
Library.
