C++ Lecture 9: Custom Comparison Operators, STL - Programming

1 Lecture #9 Custom Comparison Operators Templates The Standard Template Library (STL) STL Iterators STL Algorithms (sort, etc.) On-your-own Study: – Inline Functions, Template Exercise, More STL Algorithms 2...

1 Lecture #9 Custom Comparison Operators Templates The Standard Template Library (STL) STL Iterators STL Algorithms (sort, etc.) On-your-own Study: – Inline Functions, Template Exercise, More STL Algorithms 2 WRITES 50 DIFFERENT SORT FUNCTIONS Implement polymorphism in C++. DATA WAS ALREADY SORTED  Generic Programming What’s the big picture? Generic programming is when you write a function or class in a manner so that it can process many different types of data. Like a generic sorting function that can sort an array holding AN Y type of value Or a generic linked list class that can hold AN Y type of variable int main() { Create a linked list holding list list_of_integers; any type of data in ONE list list_of_strings; LINE! Uses: list_of_integers.push_back(42); Used in virtually every list_of_strings.push_back(“It’s LIT!”); C++ program to speed } up development – search engines, video games, Once you define such a generic function or class, you can quickly reuse it etc. to solve many different problems. 4 Part 1: Allowing Generic Comparisons Consider the following main function that compares various objects to each other… int main() Notice that the way we compare two dogs (by { weight) is int i1 = 3, i2 = 5; different than the way we if (i1 > i2) compare two circles (by radius). cout b) > b.radius()) cout spot) > (fido.weight() cout =(const Dog &a, NOTE: if (a = b.getWeight()) return true; from your class! return false; // otherwise } 6 Custom Comparison Operators bool operator>=(const Dog &a, const Dog &b) And here’s how they work! { 5 3 if (a.getWeight() >= b.getWeight()) return true; else return false; } If you forget the const If we define an operator like == does C++ classkeyword… Dog Simply using the operator in Oh, andautomatically by the way…define since arelated and b operators are const, Answer: { yourNo! Youcauses code must define each C++ to call our function public: operator explicitly! like !=can only call const functions in for us? your comparison int getWeight() function! const Dog! Thinking time! { So you’d better make int main() return m_weight; getWeight() const too! { } fido weight Dog... fido(5), spot(3); 5 if (fido >= spot) private: You’ll see this kind of cryptic cout int b. getBite()) For instance, this function will place dogs return true if A belongs before n.push_back("alex"); return true; // Dog a has a nastier bite! with a bigger bite before dogs with a smaller B if (a.getBite() < b.getBite()) bite… // sorts returnthe whole false; vectorb has a nastier bite! // Dog sort ( n.begin( ), n.end(The ) ); sort() function uses the return false if A belongs after return a.getBark() > b.getBark(); B. } // sorts just the firstpassed-in 2 items offunction n to figure out int main() how sort ( n.begin( ), n.begin() + 2to);order the items! { And break tiesfunction’s by the You then pass this Dog arr int arr == {5,2,1,-7}; {...}; address asloudest bark… a parameter to sort()! // sorts the first 4 array items } sort sort( arr, ( arr,arr+4, arr+4&customCompare); ); } 65 Part 6: Compound STL Data Structures Let’s say you want to maintain a #include list of courses for each UCLA #include crsMap student. class Course “carey” c1 c2 { public: “david” c1 c3 How could you do it … with the STL? }; int main() Well, how about creating a map between { a map< , > crsmap; student’s stringname and their Course c1(“cs”,”32”), list of courses? list c2(“math”,”3b”), c3(“english”,”1”); In many cases, you’ll want to combine crsmap[“carey”].push_back(c1); multiple STL containers to represent crsmap[“carey”].push_back(c2); more complex associations like this! crsmap[“david”].push_back(c1); crsmap[“david”].push_back(c3); 66 STL Challenges Design a compound STL data structure that allows us to associate people (Person objects) and each person’s unique group of friends (also Person objects). Remember: If you’re mapping your own class class Person toThinking time! something else you’ll need to { define the operator< for it. public: Person(string name); string getName();... Answer: private:... map facebook ; }; and: Also, if youbool have a set containing operator