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CMSC 202

CMSC 202. Lesson 25 Miscellaneous Topics. Warmup. User iterators to print each item in a vector: ____________________________________________________________ ____________________________________________________________ for ( ______________________________________________________ )

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CMSC 202

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  1. CMSC 202 Lesson 25 Miscellaneous Topics

  2. Warmup • User iterators to print each item in a vector: ____________________________________________________________ ____________________________________________________________ for ( ______________________________________________________ ) cout << ________________ << endl; vector<int> integers; // assume we initialize this… vector<int>::iterator iter; iter = integers.begin(); iter != integers.end(); ++iter *iter

  3. Announcments • 2 more days of class (after today) • Thursday • Overload the dereferencing operator • Bit-wise operators (important for 341) • Tuesday • Review for final (cumulative, focus on 2nd half) • Homework: • Go over review questions • Go over last semester’s final • Bring a question that you want answered

  4. Inline Functions • Problem • Calling a 1-line function is inefficient • Solution • Inline functions • Compiler replaces function call with function body • Drawbacks? • Mix definition and implementation • Make executables bigger (ack!)

  5. Writing Inline Functions • 2 ways • Put the function body in the class definition class A { public: int GetData() { return data; } private: int data; }; • Put keyword ‘inline’ before the signature line inline void foobar() { /* some code */ }

  6. Friend Classes • Problem: • Class A relies heavily on Class b • Tightly coupled – lots of interconnectivity • Both classes required to represent 1 data structure • Inefficient to use methods to access data • Solution: • Declare Class A as a friend of Class B • State that Class A has access to private data of Class B • Drawbacks: • “Break” Encapsulation and Information Hiding

  7. template < class T > class List<T>; // a "forward declaration" template< class T > class Node { private: Node( ); Node* m_next; int m_data; friend class List<T>; }; template < class T > class List { public: List( ); // other public stuff private: Node<T> *m_header; }; Friend Classes

  8. Nested Classes • Same Problem – tightly coupled classes • Solution: • Nested Classes • Class defined INSIDE of another class • If private? • Entirely hidden from everyone else • If public? • Accessible THROUGH the outer-class

  9. Nested Classes template< class T > class List { public: List( ) { m_header = NULL; } // other public stuff private: template < class T1 > class Node { private: Node( ); Node<T1>* m_next; int m_data; }; Node<T> *m_header; }; Node is “nested” inside of List class If it were public, the Node’s classname (and type!) would be: List<T>::Node<T> The node is scoped INSIDE the List class…

  10. Namespaces • Problem: • Your class-names, function-names, or variable-names clash with existing names • Example: • You want your own ‘cout’ object • You want to define your own ‘vector’ class • Solution: • Namespaces • Groups of classes, functions, or variables • Allow you to specify exactly which version • Kind of like overloading…

  11. Namespaces • Assume ‘Fred’ is a namespace… • There are different ways to use Fred… • Using everything from a namespace • using namespace Fred; • Use only ‘f’ from Fred • using Fred::f; • Qualify each use of something from Fred • Fred::f() • Instead of just f() • Example: • using namespace std; • using std::string; • std::string myString = “Hello World!”; • std::cout << myString << std::endl;

  12. Creating a Namespace • Really simple… namespace <name> { /* functions, classes, or variables */ } • Example: namespace CMSC202 { /* functions, classes, or variables */ }

  13. Constants and Pointers • Problem: • What happens with the following? const int age = 42; int *pAge = &age; *pAge = 57; • Solution: • Pointers to Constants const int age = 42; int age2 = 37; const int *pAge = &age; // OK! *pAge = 57; // compiler error! pAge = & age2; // OK!

  14. Const Pointers • Problem: • Want to define a pointer that cannot be changed • i.e. it cannot point to a different object! • Solution • Const Pointers int width = 56; int length = 42; int *const pLength = & length; // initialized, unchangeable *pLength = 86; // ok - length is not const pLength = &height; // error - pLength is const

  15. Const Pointer, Const Object • Problem: • Can we make an unmovable pointer that points to an unchangeable object? • Solution: • Const Pointer to a Const Object…(ack!) int size = 43; // non-const int const int weight = 89; // const int const int *const pWeight = &weight; // const pointer to a const int *pWeight = 88; // compiler error pWeight = &size; // compiler error cout << *pWeight << endl; // ok - just //dereferencing

  16. Consts and Pointers… • 4 different ways: int *pInt; const int *pInt; int *const pInt; const int *const pInt; What do each of these mean??? Hint: read from the “inside” to the “outside… Pointer to integer Pointer to constant integer Constant pointer to integer Constant pointer to constant integer

  17. STL Algorithms • STL provides many algorithms for use with container classes • #include <algorithm> • Some are: • for_each() – performs a function on each item • Pass by reference if you want to change the item • transform() – performs for_each, but stores result in another container • fill() – fills every item with a supplied value • replace() – replaces a subset of the items with value • sort() – uses Quicksort to sort items based on some comparison function

  18. Examples • Assume appropriate containers exist… • Square, print, and GreaterThan are user-defined functions (i.e. you must write them!) for_each( myList.begin(), myList.end(), square ); for_each( myList.begin(), myList.end(), print ); transform( v1.begin(), v1.end(), v2.begin(), square); fill( vString.begin() + 1, vString.begin() + 3, "tommy"); replace( vString.begin(), vString.end(), string("steve"), string("bill")); sort( v1.begin(), v1.end()); // default, uses operator< sort( v1.begin(), v1.end(), GreaterThan);

  19. Function Objects • Problem: • Can we dynamically build functions at runtime? • Solution: • Function Objects • Classes (objects) that behave like functions • Overloading the operator( ) • I told you it was possible!

  20. Function Objects • Why? • Functions that have more properties than just the operator() • Can store a state • Separate copies, each with own state • Can be initialized in constructor

  21. class Add { public: // const Add (int value) : m_value( value ) { }; void operator( ) (int& n ) const { n += m_value; } private: int m_value; // value to add }; void print(const int& i) { cout << i << endl; } int main ( ) { vector<int> iVector; // insert some data for (int i = 1; i < 10; i++) iVector.push_back( int(i) ); // print elements // 1 2 3 4 5 6 7 8 9 // create a function adds 42 to parameter Add add42( 42 ); // add 42 to each element of the set for_each( iVector.begin(), iVector.end(), add42); // print the elements // 43 44 45 46 47 48 49 50 51 for_each( iVector.begin(), iVector.end(), print); }; Function Objects Code in lecture notes used ‘set’ instead of ‘vector’. Why won’t that work?

  22. class RNG // Random Number Generator { public: RNG (unsigned int seed = 1) : m_lastValue( seed ), m_seed( seed ) { srand(seed); } unsigned int operator( ) () { // modify last Value for new value m_lastValue += rand() % m_seed; return m_lastValue; } private: unsigned int m_lastValue; unsigned int m_seed; }; int main ( ) { RNG rng ( 42 ); for (int i = 0; i < 10; i++) cout << rng() << endl; } Function Objects Key understanding: Create functions on the fly! Often used in Artifical Intelligence applications!

  23. Practice • Which of the following is legal? • Assume that illegal statements are skipped… BeachBall a(7.0); BeachBall b(6.0); const BeachBall c(5.0); const BeachBall* p = &a; BeachBall* const q = &b; p->SetRadius(1.0); // 1 q->SetRadius(2.0); // 2 p = &c; // 3 q = &c; // 4 p->SetRadius(1.0); // 5 q->SetRadius(2.0); // 6

  24. Challenge • Use a Function Object to create a function that computes the polynomial of its parameter • n2, n3, n4, … • Exponent is parameter in constructor • Use for_each to compute the square and cube of each float in a vector

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