DOC: Smart Pointers Example Is Incorrect
ID: Q123603
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The information in this article applies to:
-
Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0, 4.1, 5.0
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<T> &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 <stdio.h>
#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 T> 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<T> &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<Sample> p = new Sample; // sample #1
Ptr<Sample> 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 kbVC200 kbVC210 kbVC400 kbVC410 kbVC500
Version : winnt:2.0,2.1,4.0,4.1,5.0
Platform : winnt
Issue type :