call QueryResult's default copy constructor.

virtuals:

interface functions: call the corresponding Query_base operatopns

virtuals:

interface functions: call the corresponding Query_base operatopns

call QueryResult's default copy constructor.

^^^--fixed to override the inherited version

for ex17.5 return a vector with an entry for each store that sold the given book

for ex17.6 return a vector with an entry for each store that sold the given book.

* @brief destroy the elements and deallocate the space previously allocated. */

return the underlying pointer

return the underlying pointer

Get underlying pointer