std::shared_ptr and passing by reference

I created a threadsafe circular buffer containing shared_ptrs to openCV matrices in the following manner. I get compilation errors when I try to passing a dereferenced shared_ptr to a function that takes a constant reference.

threadsafe_circular_buffer<std::shared_ptr<cv::Mat> > buf;
std::shared_ptr<cv::Mat> ptr(buf.pop());
foo(*ptr);

where pop is defined as

template <typename T>
T threadsafe_circular_buffer::pop();

and foo is defined like

bool foo(const cv::Mat &matrix);

I get the following error:

invalid initialization of reference of type 'const cv::Mat&' from expression of type 'std::shared_ptr<cv::Mat>'

How do I deref the shared_ptr so I can pass a reference to the underlying object.