Questions on usages of shared_ptr - C++

I have few questions on the best practices of using shared_ptr.

Question 1

Is copying shared_ptr cheap? Or do I need to pass it as reference to my own helper functions and return as value? Something like,

void init_fields(boost::shared_ptr<foo>& /*p_foo*/);
void init_other_fields(boost::shared_ptr<foo>& /*p_foo*/);

boost::shared_ptr<foo> create_foo()
{
    boost::shared_ptr<foo> p_foo(new foo);
    init_fields(p_foo);
    init_other_fields(p_foo);
}

Question 2

Should I use boost::make_shared to construct a shared_ptr? If yes, what advantages it offers? And how can we use make_shared when T doesn't have a parameter-less constructor?

Question 3

How to use const foo*? I have found two approaches for doing this.

void take_const_foo(const foo* pfoo)
{

}

int main()
{
    boost::shared_ptr<foo> pfoo(new foo);
    take_const_foo(pfoo.get());
    return 0;
}

OR

typedef boost::shared_ptr<foo> p_foo;
typedef const boost::shared_ptr<const foo> const_p_foo;

void take_const_foo(const_p_foo pfoo)
{

}

int main()
{
     boost::shared_ptr<foo> pfoo(new foo);
     take_const_foo(pfoo);
     return 0;
}

Question 4

How can I return and check for NULL on a shared_ptr object? Is it something like,

boost::shared_ptr<foo> get_foo()
{
     boost::shared_ptr<foo> null_foo;
     return null_foo;
}

int main()
{
     boost::shared_ptr<foo> f = get_foo();
     if(f == NULL)
     {
          /* .. */
     }
     return 0;
}

Any help would be great.