Iterators

1. Iterators Andy Wang Data Structures, Algorithms, and Generic Programming

2. Iterators Overview • Provide access to container elements • Provide interface between generic algorithms and containers • Proper type • Uniform public interface across variety of containers

3. Iterators Overview (2) • Classify iterators by public interface functionality • input and output iterators • forward iterators • bidirectional iterators • random access iterators • adaptor iterators • Specialize iterators to containers with implementation

4. Forward Iterators // comparison logic bool operator==(const Iterators &) const; bool operator!=(const Iteartor&) const; // element access value_type& operator*() const; // motion Iterator& operator++(); Iterator& operator++(int); // proper type Iterator(); ~Iterator(); Iterator& Iterator(const Iterator&); Iteartor& operator=(const Iterator&);

5. Forward Iterators • Minimum public interface Container::Iterator I; for (I = C.Begin(); I != C.End(); ++I) { cout << *I; }

6. Bidirectional Iterators • Forward iterator with more motion // motion Iterator& operator++(); Iterator& operator++(int); Iterator& operator--(); Iterator& operator--(int);

7. Random Access Iterators • Bidirectional iterator with random accesses // bracket operator T& operator[] (unsigned int i) const; // pointer arithmetics long operator-(const Iterator I2) const; // n = I – I2 Iterator operator+(long n) const; // I2 = I + n Iterator& operator+=(long n); Iterator& operator-=(long n); // similar declarations of last three for all other integral types

8. Ok for v = *I; not ok for *I = v; Input Iterators • Specialized read-only forward iterator // comparison logic bool operator==(const Iterators &) const; bool operator!=(const Iteartor&) const; // element access const value_type& operator*() const; // motion Iterator& operator++(); Iterator& operator++(int); // proper type Iterator(); ~Iterator(); Iterator& Iterator(const Iterator&); Iteartor& operator=(const Iterator&); // no assignment operator

9. Example • g_copy template <class I, class J> void g_copy(I source_begin, I source_end, J dest_begin) { while (source_begin != source_end) { *dest_begin++ = *source_begin++; } }

10. Output Iterators • Specialized write-only forward iterator // no comparison logic // write-only element access Iterator& operator*() const; // motion Iterator& operator++(); Iterator& operator++(int); // proper type Iterator(); ~Iterator(); Iterator& Iterator(const Iterator&); // no assignment operator

11. The Iterator Hierarchy • Enforced by discipline in design • Not inheritance Input Iterators Output Iterators Forward Iterators Bindirectional Iterators Random Access Iterators

12. Iterator Adaptors • Insert iterators • Adaptee: a container class • Adaptor: an output iterator • Utility: replace overwrite with insert in generic algorithms • Stream iterators • Adaptee: an istream/ostream class • Adaptor: an input/output iterator • Utility: read/write directly from/to istream/ostream in generic algorithms

13. Iterator Adaptors (2) • Reverse iterators • Adaptee: an iterator class • Adaptor: an iterator of the same category • Utility: normal direction of increment/decrement is reversed—applies generic algorithms in reverse direction

14. Insert Iterators template <class C> class PushBackIterator { public: explicit PushBackIterator(C& x):Cptr(&x) {} PushBackIterator<C>& operator=(const typename C::value_type & t) { Cptr->PushBack(t); return *this; } PushBackIterator<C>& operator*() { return *this; } PushBackIterator<C>& operator++() { return *this; } PushBackIterator<C>& operator++(int) {return *this; } protected: C* Cptr; }; // usage TList<char> L; PushBackIterator <TList <char> > Litr(L); g_copy(V.Begin(), V.End(), Litr);