c++ - Need work around for limitation: abstract class cannot be used for return types -

i have c++ class implementation wish hide using pimpl pointer. of work done, , everything's ok except operator '+=' returns object of same class. operator function required in class. problem when made abstract class, no member functions can return object of class, '+=' isn't allowed.

i asking advice on dealing limitation. i'd prefer way doesn't require changing existing code uses class. if way doesn't exist, else can done?

here's simplified version of class wish make abstract:

class aclass  { public:     aclass( int x0, int y0);     ~aclass(void);     void operator=(const aclass &a2);     aclass operator+(const aclass &a2);     void operator+=(const aclass &a2); protected;     int x;     int y; };  aclass aclass::operator+(const aclass &a2) { aclass aout(a2); aout += a2;  return aout; } aclass aclass::operator+=(const aclass &a2) { this->x += a2.x; this->y += a2.y; }    // 'operator+' necessary perform following operation on aclass objects. i.e. = b + c; // 'operator+=' necessary perform following operation on aclass objects. i.e. += b; 

except operator '+=' returns object of same class

operator+= (as other assignment operators) need return aclass& instead of aclass.

see e.g. scott meyers "effective c++" item 10:

have assignment operators return reference *this

this way avoid problem of returning class value, , make code closer c++ guidelines.

p.s. operator+= not seem point. instance, operator+ has same issue, , should not return reference. may consider post literal answer question, not solution problem.