Smart pointer: set by reference, reset, set null, null-check or reset-check

Smart pointer: set by reference, reset, set null, null-check or reset-check

For the first time, I'm using smart pointers in C++. I've some question
about std::shared_ptr:
Set pointer by reference:
MyToy mytoy_1, mytoy_2;
set_mytoy(mytoy_1, some_data);
set_mytoy(mytoy_2, some_data);
shared_ptr<MyToy> ptr_mytoy(mytoy_1);
Reset and new assignment:
ptr_mytoy.reset(mytoy_2);
Reset without assignment:
ptr_mytoy.reset();
Set NULL (?):
ptr_mytoy(null_ptr);
Are these examples right?
How can I check if a smart pointer is "empty" (for instance, after
.reset()) or if is NULL?