15.37.txt

Exercise 15.37: What changes would your classes need if the derived classes had
members of type shared_ptr<Query_base> rather than of type Query?

By Faisal Saadatmand

1) The constructor AndQuery, OrQuery and NotQuery parameters' types must be
changed from Query to shared_ptr<Query_base>

2) The parameter(s) and the return type of the overloaded operators, with the
exception of <<, must be change from type Query to shared_ptr<Query_base>

3) All the calls to rep and eval must use the -> access member operator
instead of the "." Member access operator.

4) As a result of 3, the virtual function rep in Query_base must be public,
otherwise it will not be accessible through dereferencing the
shared_ptr<Query_base> within the derived classes. Alternatively, we could have
the derived classes befriend Query_base (but that is awkward syntax). Note:
private or protected access specifier will not work, because dereferencing the
shared_ptr involves derived-to-base conversion, which has its own subtle
restriction on accessibility. (p 613-614

5) The same would apply to eval in Query_base.

6) Query class must define a conversion operator (with a public access
specifier) from Query to shared_ptr<Query_base>

7) Query's constructor that takes a shared_ptr (and the conversion operator)
must be public in order to be accessible to users. This is so for the same
reason mention in 4.

For an implementation, see 15.37a.cpp.