Should I release cv::Ptr before reassigning?

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An doc says :

Template class for smart pointers with shared ownership.

A Ptr<t> pretends to be a pointer to an object of type T. Unlike an ordinary pointer, however, the object will be automatically cleaned up once all Ptr instances pointing to it are destroyed.

Ptr is similar to boost::shared_ptr that is part of the Boost library (http://www.boost.org/doc/libs/release...) and std::shared_ptr from the C++11 standard.

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and to modify parameters you can use method setVarThreshold

You don't need to release object because opencv will do it