Purpose of const boost::shared_ptr<T>& as function argument?

Question

I'm using a certain large and well-maintained open-source C++ library and came across a class definition having a constructor of the form

class SomeClass {
    SomeClass( const boost::shared_ptr<SomeOtherClass>& );
}

My question is: what's the point of passing a const boost::shared_ptr<T> by reference? Is there really a non-negligible amount of overhead associated with passing a boost::shared_ptr<T> by value, or is there some other kind of danger to passing a boost::shared_ptr<T> by value that I'm not aware of?

Answer 1

Passing this by value is going to copy it which results in a reference count increment, synchronized across all threads. Definitely non-negligible.