Explicit unique Handle Modern c++

I am following along a course about modern C++. I get an error with explicit unique_handle function. It says m_value is unknown override specifier? I listed the value above it shows still be in variable space.

explicit unique_handle(pointer value = Traits::invalid()) throw()
        m_value {   value }
    {

    }

http://ift.tt/2f6ZRw6