AnOrQuery merges the set of line numbers returned by its two operandsits result is the union of the results for its two operands:

// returns union of its operands' result sets set<TextQuery::line_no> OrQuery::eval(const TextQuery& file) const { // virtual calls through theQuery handle to get result sets for the operands set<line_no> right = rhseval(file), ret_lines = lhseval(file); // destination to hold results // inserts the lines from right that aren't already inret_lines ret_linesinsert(rightbegin(), rightend()); return ret_lines; }

Theeval function starts by calling eval on each of its Query operands Those calls call Query::eval, which in turn makes a virtual call to eval on the underlying Query_base object Each of these calls yields a set of line numbers in which its operand appears We then call insert on ret_lines, passing a pair of iterators denoting the set returned from evaluating the right-hand operand Becauseret_lines is a set , this call adds the elements from right that are not also in left into ret_lines After the call to insert, ret_lines contains each line number that was in either of the left or right sets We complete the function by returning ret_lines

C++ Primer, Fourth Edition TheAndQuery version of eval uses one of the library algorithms that performs setlike operations By Stanley B Lippman,Jos e Library These algorithms are described in the Lajoie, Appendix, in Section A28 (p 821): Barbara E Moo Publisher:Addison Wesley // returns intersection of its operands' result sets Professional set<TextQuery::line_no> Pub Date: February 14, 2005 AndQuery::eval(const TextQuery& file) const Print ISBN: 0-201-72148-1 { Pages: 912 // virtual calls through theQuery handle to get result sets for the operands

left = lhseval(file), right = rhseval(file); set<line_no> ret_lines; // destination to hold results // writes intersection of two ranges to a destination iterator This popular tutorialdestination iterator in this call adds elements to updated, reorganized, and rewritten to help // introduction to standard C++ has been completely ret programmers learn the language faster and use it in a more modern, effective wayJust as C++ has evolved since the last set_intersection(leftbegin(), leftend(), edition, so has the authors' approach rightbegin(), rightend(),standard library from the beginning, giving to teaching it They now introduce the C++ readers the means to write useful programs without first having to master every language detail Highlighting today's best inserter(ret_lines, ret_linesbegin())); practices, they return to write programs that are safe, can be built quickly, and yet offer outstanding performance show how ret_lines; Examples that take advantage of the library, and explain the features of C++, also show how to make the best use of the }

language As in its previous editions, the book's authoritative discussion of fundamental C++ concepts and techniques makes it a valuable resource even for more experienced programmersProgram Faster and More Effectively with This Rewritten Classic Restructured for quicker learning, using the C++ standard library Updated to teach the most current programming styles and program design techniques Filled with new learning aids that emphasize important points, warn about common This version of eval uses the set_intersection algorithm to find the lines in common to both pitfalls, suggest good programming practices, and provide general usage tips Complete with exercises that reinforce skills queries: That algorithm takes five iterators: The first four denote two input ranges, and the last learned Authoritative and comprehensive in its coverageThe source code for the book's extended examples is available on denotes a destination The algorithm writes each element that is in both of the two input ranges the Web at the address below

into the destination The destination in this call is an insert iterator (Section 1131, p 406) which inserts new elements into ret_lines