how i covariant relapse forms intelligent pointers?

i have formula this:

class retinterface {...}

class ret1: open retinterface {...}

class ainterface
{
  public:
     practical boost::shared_ptr<retinterface> get_r() const = 0;
     ...
};

class a1: open ainterface
{
  public:
     boost::shared_ptr<ret1> get_r() const {...}
     ...
};

this formula does compile.

in visible studio raises

c2555: vital practical duty relapse form differs not
covariant

if i boost::shared_ptr nonetheless relapse tender pointers, formula compiles (i know due c++). i problem since boost::shared_ptr ret1 subsequent boost::shared_ptr retinterface. nonetheless i wish relapse boost::shared_ptr ret1 classes, else i contingency ban returned value after return.

1. am i doing something wrong?


2. if not, since libel - should extensible hoop reworking between intelligent pointers scenario? there fascinating workaround?