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.

strong pointer免除 memory leaks的問題

理由如下：
Since the destruction of the last strong reference to an object will destruct the object, one cannot 'forget' to deallocate the memory.

weak pointer免除cycle reference問題

理由如下：
所以我們可以使用一個weak pointer和一個strong pointer指向一個物件，當strong pointer被釋放時，且當weak pointer被釋放了，該儲存物件的記憶體也會被釋放。

weak pointer 和strong pointer可免除dangling pointers

理由如下：
We have eliminated dangling pointers. If you have a strong pointer to an object, you can be sure it will not be destructed before your pointer is. If you have a weak pointer to an object, and the object is destructed, your weak pointer will be valued null, which is easily tested for.

reference:

http://www.codeproject.com/KB/stl/boostsmartptr.aspx
http://code.google.com/p/mist/wiki/SmartPointers