TITLE: Temporaries and const member functions

PROBLEM: Dimitri Tischenko (dimitri@dutiws.tudelft.nl)

class A 
{
public:
	A()	{ i=0; }
virtual A&	foc() const	{ return *this; }
//   18:error  : declaration of object of abstract class A
//   18:warning: const initializer: temporary used to initialize reference
//   18:warning: reference to temporary returned (return value is not lvalue
private:
	int i;
};	


RESPONSE: Robert C. Martin (rmartin@rational.com)

The ARM section 9.3.1 says:  "The type of 'this' in a member function
of class X is X *const unless the member function is declared 'const'
or 'volatile'; in those cases, the type of 'this' is const X* const and
volatile X* const, respectively."

In your case, within the function 'foc' the type of 'this' is const A*
const.  But foc returns A& which is not const.  Thus the compiler has
to create a temporary volatile version of A.  However A is abstract so
the compiler can't instantiate it.  

You can get rid of the compiler errors by changing the declaration of
foc to 'virtual const A& foc() const {return *this;}'

If you look closely you will realized that the original form of foc
would have been a nasty bug, even if A had not been abstract.  foc
declares itself to be const, promising that it will not change its
object.  'return *this;' forces the compiler to create a temporary
since it can't pass a const object (this) out through a volatile
interface (the return value).  So the function returns a reference to
a temporary.  But the temporary was created *inside* foc and will be
destroyed when foc exits.  Thus foc returns a reference to an object
which no longer exists.    

[ Ouch! Be sure you understand this. It would be time-consuming to
  find this one. -adc ]