smart pointers: shared ownership: non-intrusive/intrusive strategy

Question

We know that shared ownership has two underlying strategies:

• non-intrusive and
• intrusive

Please help me to understand what do these strategies mean? What are implementation details?

Answer 1

Non-intrusive is when the smart pointer object provides reference counting. Ex.: std::shared_ptr, boost::shared_ptr

Intrusive is when the smart pointer relies on stored object embedded reference counting (i.e. stored object is responsible for reference counting). Ex.: boost::intrusive_ptr

Note: intrusive pointer needs the "glue" code to increment and decrement reference counter inside the stored object. In boost library you must provide 2 functions to the intrusive_ptr object to perform reference counter increment and decrement.

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smart pointers (boost) explained There is no direct answer on my question here but is very helpful as a quick overview of smart pointers semantics.