code:

class Base 
{
public:
  Base( int initX = 0 ); 
  Base( const Base& b );
  Base& operator=( const Base& b );
private:
  int x;
};

class Derived : public Base 
{
public:
  Derived( int initValue );
  Derived& operator=( const Derived& d );
private:
  int y;
};

Base::Base( int initX )
: x( initX ) 
{}; 

Base::Base( const Base& b )
{
  x = b.x;
};

Base& Base::operator=( const Base& b )
{
  if( this == &b ) {
    return *this;
  }
  x = b.x;
  return *this;
}

//=============================================

Derived::Derived( int initValue )
:Base( initValue ), y( initValue )
{}

Derived& Derived::operator=( const Derived& d )
{
  if( this == &d ) {
    return *this;
  }
  static_cast<Base&>(*this) = d;

  y = d.y;
  return *this;
}
 

code:

  static_cast<Base&>(*this) = d;
 

code:

  static_cast<Base>(*this) = d;