shared_ptr and use of undefined type

[nobody]

class A;

class B
{
public:
    B();
// ~B();
private:
    boost::shared_ptr<A> a_p; 
};

B b;    // fails to compile 

The above line fails to compile with the message "use 
of undefined type 'A'".

It compiles successfully if B has an explicite 
destructor. i.e. the line "// ~B();" is uncommented.

I am using MSVC 6.