Microsoft KB Archive/123603

= DOC: Smart Pointers Example Is Incorrect =

Article ID: 123603

Article Last Modified on 12/9/2003

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APPLIES TO


 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 4.1 Subscription
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 5.0 Professional Edition

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This article was previously published under Q123603



SUMMARY
The Smart Pointers sample code uses C++ templates to create generic wrappers to encapsulate pointers of almost any type. The sample encapsulates pointers and overrides pointer operators to add new functionality to pointer operations. However, the Smart Pointers sample code in the documentation contains the following errors:


 * stdio.h is not included and TRACE is not defined.
 * Operator Ptr& operator=(Ptr &p_) is missing.
 * Implementation for operator Ptr& operator=(T* p_) is incorrect.

The Smart Pointers sample code may be found in "Templates" chapter of "Programming Techniques" included in the Visual C++ Books Online. Searching for "Smart Pointers" in search plus of the Books Online also brings you to the sample.



MORE INFORMATION
The following version of the Smart Pointers sample has been corrected and enhanced to better demonstrate the concept of Smart Pointers: #include   #define TRACE printf class RefCount { int crefs; public: RefCount(void) { crefs = 0; } ~RefCount { TRACE("goodbye(%d)\n", crefs); } void upcount(void) { ++crefs; TRACE("up to %d\n", crefs);} void downcount(void) {       if (--crefs == 0) {        delete this; }       else TRACE("downto %d\n", crefs); }  };   class Sample : public RefCount { public: void doSomething(void) { TRACE("Did something\n");} };  template  class Ptr { T* p;  public: Ptr(T* p_) : p(p_) { p->upcount; } ~Ptr(void) { p->downcount; } operator T*(void) { return p; } T& operator*(void) { return *p; } T* operator->(void) { return p; } Ptr& operator=(Ptr &p_)   // This is newly added {return operator=((T *) p_);} Ptr& operator=(T* p_) { p->downcount; p = p_; p->upcount; return *this; // This is corrected }  };

int main { Ptr p = new Sample; // sample #1 Ptr p2 = new Sample; // sample #2 p = p2; // #1 will have 0 crefs, so it is destroyed; // #2 will have 2 crefs. p->doSomething; return 0; // As p2 and p go out of scope, their destructors call // downcount. The cref variable of #2 goes to 0, so #2 is      // destroyed }

Additional query words: 9.00 template

Keywords: kbdocerr KB123603

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