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[edit]

[edit] Intent

Adding reference counting to an object to which a reference count cannot directly be added.

[edit] Also Known As

• Reference Counting (non-intrusive)
• Detached Counted Body

[edit] Motivation

Some times Counted Body idiom can't be applied to certain classes when their sources are not available. We might only have object code and a header file. Detached Counted Body or non-intrusive reference counting can be used in such cases to prevent expensive copies of bodies.

[edit] Solution and Sample Code

Associate both a shared count, and a separate shared body, with each instance of a common handle abstraction:

class String {
public:
	String(): rep(new char[1]), count(new int(1)) {
		rep[0] = '\0';
	}
	String(const String &s): rep(s.rep), count(s.count) {
		(*count)++;
	}
	String &operator=(const String &s) {
		String temp(s);
                this->swap(temp);
                return *this;
	}
        void swap (String & s) throw () {
           std::swap (this->count, s.count);
           std::swap (this->rep, s.rep);
        }
	~String() {
		if(--*count <= 0) {
			delete [] rep;
			delete count;
		}
	}
	String(const char *s): count(new int(1)),
			rep(new char[strlen(s)+1]) {
		strcpy(rep,s);
	}
	. . . .
private:
	char *rep;
	int *count;
};
 
int main() {
	String a = "hello", b = "world";
	a = b;
	return 0;
}

[edit] Consequences

Handles are slightly more expensive to copy than in Counted Body, memory fragmentation may increase, and initial construction overhead is higher because we are allocating multiple blocks.

[edit] Known Uses

• boost::shared_ptr
• ACE_Refcounted_Auto_Ptr

[edit] Related Idioms

• Counted Body
• Handle Body

[edit] References

http://users.rcn.com/jcoplien/Patterns/C++Idioms/EuroPLoP98.html#DetachedCountedBody

[edit] Copyright

Copyright ©1998 Lucent Technologies, Bell Labs Innovations. All rights reserved. Permission granted to reprint verbatim for non-commercial use provided this copyright notice appears. (Contents modified and enhanced for Wikibooks)