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Overloading operators

Overloading operators. Why overload operators? (==, =, <, >>, +=, …) notational convenience match user expectations because we can (except :: and . And .*) C++ heuristic: avoid behind-the-scenes conversion Cpstring v. standard string string filename=“sesame.txt”; ifstream input;

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Overloading operators

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  1. Overloading operators • Why overload operators? (==, =, <, >>, +=, …) • notational convenience • match user expectations • because we can (except :: and . And .*) • C++ heuristic: avoid behind-the-scenes conversion • Cpstring v. standard string string filename=“sesame.txt”; ifstream input; input.open(filename); // CPstring input.open(filename.c_str()); // standard string • vectors Vector<int> a(20); a[0] = 1; a = 2; // avoid with explicit keyword • www.cs.duke.edu/csed/cplus and Stroustrup Chapter 11

  2. motivation for Membersort<...> class Alphasort : public Compare { public: virtual int operator()(const DirEntry & lhs, const DirEntry & rhs) { if (lhs.name() == rhs.name()) return 0; else if (lhs.name() < rhs.name()) return -1; else return 1; } } class Sizesort : public Compare { public: virtual int operator()(const DirEntry & lhs, const DirEntry & rhs) { if (lhs.size() == rhs.size()) return 0; else if (lhs.size() < rhs.size()) return -1; else return 1; } } • what about sorting by time? DirEntry::time()

  3. Factor out common code, why use a template? class Alphasort : public Compare { public: virtual int operator()(const DirEntry & lhs, const DirEntry & rhs) { if (lhs.name() == rhs.name()) return 0; else if (lhs.name() < rhs.name()) return -1; else return 1; } } template <class Kind> class MemberSort : public Compare { public: typedef Kind (DirEntry::* MemPtr)() const; MemberSort(MemPtr ptr) : myPtr(ptr){} virtual int operator()(const DirEntry & lhs, const DirEntry & rhs) { if ((lhs.*myPtr)() == (rhs.*myPtr)()) return 0;

  4. Alternatives to makeIterator() • return a vector Scandir scanner; Vector<DirEntry> vde; .... scanner.get(vde); • return a C-style array Scandir scanner; DirEntry * varray = scanner.get(); • other options? • advantages/disadvantages compared to iterator? • problems with iterator, why IterProxy? • where is the concrete iterator implemented, why? • is friendship required?

  5. IterProxy • Proxy aka surrogate (GOF 207) • need: more versatile or sophisticated reference to an object than a simple pointer • smart pointer, virtual proxy/web proxy PointerProxy<Iterator<DirEntry> > iter(scanner.makeIterator()); for( iter->first(); ! iter.isDone(); iter->next() )... • note: iter is allocated on the stack, not on the heap • cleans itself up with destructor • acts like a pointer, overload operator-> • potential problem with assignment of proxies

  6. Assignment problems IterProxy<DirEntry> iter(scanner.makeIterator()); IterProxy<DirEntry> copy = iter; .... // both go out of scope • options available to stop double deletion • disable assignment, copy --- how? • transfer ownership on assign or copy, but what to leave behind? • other proxy uses include lazy/on-demand creation, copy on write, e.g., reference counted strings

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