strong reference and weak reference

Cyclic References

Reference counting is a convenient resource management mechanism, it has one fundamental drawback though: cyclic references are not freed automatically, and are hard to detect by the computer. The simplest example is this:

struct CDad;struct CChild;typedef boost::shared_ptr<CDad>   CDadPtr;typedef boost::shared_ptr<CChild> CChildPtr;struct CDad : public CSample{   CChildPtr myBoy;};struct CChild : public CSample{  CDadPtr myDad;};// a "thing" that holds a smart pointer to another "thing":CDadPtr   dad(new CDad); CChildPtr child(new CChild);// deliberately create a circular reference:dad->myBoy = child; child->myDad = dad;// resetting one ptr...child.reset();

dad still references the CDad object, which itself references theCChild. The whole thing looks like this:

If we now call dad.reset(), we lose all "contact" with the two objects. But this leaves both with exactly one reference, and the shared pointers see no reason to delete either of them! We have no access to them anymore, but they mutually keep themselves "alive". This is a memory leak at best; in the worst case, the objects hold even more critical resources that are not released correctly.

The problem is not solvable with a "better" shared pointer implementation (or at least, only with unacceptable overhead and restrictions). So you have to break that cycle. There are two ways:

1. Manually break the cycle before you release your last reference to it
2. When the lifetime of Dad is known to exceed the lifetime of Child, the child can use a normal (raw) pointer to Dad.
3. Use a boost::weak_ptr to break the cycle.

Solutions (1) and (2) are no perfect solutions, but they work with smart pointer libraries that do not offer aweak_ptr like boost does. But let's look at weak_ptr in detail:

Using weak_ptr to break cycles

Strong vs. Weak References:

A strong reference keeps the referenced object alive (i.e., as long as there is at least one strong reference to the object, it is not deleted).boost::shared_ptr acts as a strong reference. In contrast, a weak reference does not keep the object alive, it merely references it as long as it lives.

Note that a raw C++ pointer in this sense is a weak reference. However, if you have just the pointer, you have no ability to detect whether the object still lives.

boost::weak_ptr<T> is a smart pointer acting as weak reference. When you need it, you can request a strong (shared) pointer from it. (This can beNULL if the object was already deleted.) Of course, the strong pointer should be released immediately after use. In the above sample, we can decide to make one pointer weak:

struct CBetterChild : public CSample{  weak_ptr<CDad> myDad;  void BringBeer()  {    shared_ptr<CDad> strongDad = myDad.lock(); // request a strong pointer    if (strongDad)                      // is the object still alive?      strongDad->SetBeer();    // strongDad is released when it goes out of scope.    // the object retains the weak pointer  }};

See the Sample 5 for more.

 

reference: http://www.codeproject.com/KB/stl/boostsmartptr.aspx#Cyclic%20References