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POWhy are boost::scoped_ptr or std::unique_ptr non-copyable?
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copied!<p>In <a href="http://www.boost.org/doc/libs/1_54_0/libs/smart_ptr/scoped_ptr.htm" rel="nofollow">boost::scoped_ptr</a>, says "It supplies a basic "resource acquisition is initialization" facility, without shared-ownership or transfer-of-ownership semantics." It is done through some non-copyable mechanism. </p> <p>My question is why is there requirement for no shared-ownership?</p> <p>Here is what I mean:</p> <pre><code> template <typename T> class my_scoped_ptr { public: // constructor implemented not shown here // operator*() implemented not shown here // operator -> implemented not shown here ~my_scoped_ptr() { delete my_p; my_p = NULL; } private: T * my_p; }; void foo() { my_scoped_ptr<someclass> p (new someclass()); my_scoped_ptr<someclass> q (p); // both p & q point to same my_p object // may be an exception is raised } </code></pre> <p>Ok now , regardless whether or not exception is raised the my_p will be deleted. so, when the code get out of foo's scope ...my_scope_ptr p destructor is called, deleting my_p and setting my_p to null. Now my_scope_ptr q destructor is called again deleting my_p , which is null at this point. It seems at destruction time I could care less whether a remaining copied pointer is pointing to a valid object. </p> <p>So, why would I need to care that my_scoped_ptr should not be copyable? I don't see any harm of a ptr being copyable, if the ptr takes care of deleting the object pointed to , once it exits the scope. ??!!</p>

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