STL Functors Functions vs Function Objects STL Functors

STL Functors: Functions vs. Function Objects • • STL Functors support function call syntax Algorithms invoke functions and operators – E. g. , calling operator< during a sort algorithm – Those expressions define type requirements – In many cases, can plug in alternative behaviors • The STL allows diverse functor types – A function pointer (from Lippman, La. Joie, Moo) bool (*PF) (const string &, const string &); // function pointer bool pf (const string &, const string &); // function – Instance of a class/struct providing operator() – Either one can be invoked (called) • But may have to deal with signature (and return type) CSE 332: C++ STL functors

STL Functors Extend STL Algorithms • • Make the algorithms/containers even more general Can be used parameterize policy – – • E. g. , the order produced by a sorting algorithm E. g. , the order maintained by an associative container Each functor does a single, specific operation – Often implemented as small functions or classes/structs • • E. g. , a struct with one public member function, operator() Function objects may also have member variables – – Arguments not stored may be supplied at point of call Member variables can parameterize the operation E. g. , the value k for a functor that adds k to another value E. g. , arguments for an invocation on a remote object CSE 332: C++ STL functors

Function Object Use in an Algorithm struct GT_magnitude : public binary_function<double, bool> { bool operator() (double x, double y) { return fabs(y) < fabs(x); } }; struct LT_magnitude : public binary_function<double, bool> { bool operator() (double x, double y) { return fabs(x) < fabs(y); } }; CSE 332: C++ STL functors int main (int, char **) { vector<double> u, v; for (double d = 0. 0; d < 10. 1; d += 1. 0){ u. push_back (d); v. push_back (d); } sort (u. begin(), u. end(), GT_magnitude()); sort (v. begin(), v. end(), LT_magnitude()); ostream_iterator<double> o (cout, “ ”); copy (u. begin(), u. end(), o); copy (v. begin(), v. end(), o); return 0; }

Function Use in an Algorithm int main (int, char *[]) { heap object vector<Employee. Ptr> v; v. push_back(new Employee("Claire", 23451)); v. push_back(new Employee("Bob", 12345)); v. push_back(new Employee("Alice", 54321)); cout << "v: " ; struct Employee { copy (v. begin(), v. end(), Employee (const char * n, int i) : name_(n), ostream_iterator<Employee. Ptr>(cout)); id_(i) {} cout << endl; string name_; int id_; // "v: Claire 23451 Bob 12345 Alice 54321 " }; #include <iostream> #include <vector> #include <string> #include <iterator> #include <algorithm> using namespace std; sort (v. begin(), v. end(), id_compare); cout << "v: " ; copy (v. begin(), v. end(), function name ok here ostream_iterator<Employee. Ptr>(cout)); cout << endl; // "v: Bob 12345 Claire 23451 Alice 54321 " typedef Employee * Employee. Ptr; ostream& operator<< (ostream & os, const Employee. Ptr & e) { os << e->name_ << " " << e->id_ << " "; return os; } // clean up: pointers "own" the heap objects for (vector<Employee. Ptr>: : iterator i = v. begin(); i != v. end(); ++i) { delete *i; } return 0; // function for comparing Employee. Ptrs bool id_compare (const Employee. Ptr & e, const Employee. Ptr & f) { return e->id_< f->id_|| (e->id_ == f->id_ && e->name_ < f->name_); } } CSE 332: C++ STL functors

Function Object Use in a Container #include <set> #include <string> #include <iterator> #include <algorithm> using namespace std; int main (int, char *[]) { temporary object vector<Employee> v; v. push_back(Employee("Claire", 23451)); v. push_back(Employee("Bob", 12345)); v. push_back(Employee("Alice", 54321)); cout << "v: " ; copy (v. begin(), v. end(), ostream_iterator<Employee>(cout)); // "v: Claire 23451 Bob 12345 Alice 54321 " struct Employee { Employee (const char * n, int i) : name_(n), id_(i) {} string name_; int id_; }; ostream& operator<< (ostream & os, const Employee & e) { os << e. name_ << " " << e. id_ << “ "; return os; } set<Employee> s; s. insert(v. begin(), v. end()); cout << "s: " ; copy (s. begin(), s. end(), ostream_iterator<Employee>(cout)); // "s: Alice 54321 Bob 12345 Claire 23451 “ // set needs this (orders by name then id) bool operator< (const Employee & e, const Employee & f) { return e. name_ < f. name_ || (e. name_ == f. name_ && e. id_ < f. id_); } // orders by id then name) struct Employee. Id. Comp { bool operator() (const Employee & e, const Employee & f) { return e. id_ < f. id_ || (e. id_ == f. id_ && e. name_ < f. name_); } }; CSE 332: C++ STL functors function object needed set<Employee, Employee. Id. Comp> t; t. insert(v. begin(), v. end()); cout << "t: " ; copy (t. begin(), t. end(), ostream_iterator<Employee>(cout)); // "t: Bob 12345 Claire 23451 Alice 54321 “ return 0; }

STL <functional> Before and After C++11 • Parts were deprecated in C++11, but are still available in Visual C++ 2010 – E. g. , inherit from unary_function and binary_function to decorate a callable object with associated types reflecting its function call signature – E. g. , make a functor for a member function call through a pointer using mem_fun – E. g. , make a functor for a member function call through a reference using mem_fun_ref – E. g. , bind first argument using binder 1 st<Binary. Fun> – E. g. , bind second argument using binder 2 nd<Binary. Fun> – See http: //www. sgi. com/tech/stl/table_of_contents. html for more details • New versions introduced in C++11, are also available in Visual C++ 2010 – – – E. g. , bind any argument using bind and _1 _2 _3 etc. E. g. , make a functor for a member function call through pointer or reference using mem_fn E. g. , wrap a callable object with a function call signature using function New versions provide similar capabilities, but are more consistent and general than before Availability may depend on compiler version, older code may involve previous versions See http: //en. cppreference. com/w/cpp/utility/functional for more details CSE 332: C++ STL functors

Pre-C++11 Member Function Adaptor Example // Based on the SGI C++ STL page examples struct B { virtual void print() = 0; }; struct D 1 : public B {void print() {cout<<"I'm a D 1"<< endl; }}; struct D 2 : public B {void print() {cout<<"I'm a D 2"<< endl; }}; int main(int, char **) { D 1 d 1; D 2 d 2; vector<B*> v; vector<D 2> w; v. push_back(&d 1); v. push_back(&d 2); v. push_back(&d 1); w. push_back(d 2); for_each(v. begin(), v. end(), mem_fun(& B: : print)); cout << endl; different wrappers needed for_each(w. begin(), w. end(), mem_fun_ref(& B: : print)); return 0; } CSE 332: C++ STL functors

C++11 Member Function Adaptor Example // Previous example modified to use mem_fn in both cases struct B { virtual void print() = 0; }; struct D 1 : public B {void print() {cout<<"I'm a D 1"<< endl; }}; struct D 2 : public B {void print() {cout<<"I'm a D 2"<< endl; }}; int main(int, char **) { D 1 d 1; D 2 d 2; vector<B*> v; vector<D 2> w; v. push_back(&d 1); v. push_back(&d 2); v. push_back(&d 1); w. push_back(d 2); for_each(v. begin(), v. end(), mem_fn(& B: : print)); cout << endl; same wrapper for both for_each(w. begin(), w. end(), mem_fn(& B: : print)); return 0; } CSE 332: C++ STL functors

STL Functor Concepts • Basic Functor Concepts – Generator – Unary Function – Binary Function • Adaptable Function Objects (turn functions into function objects) – Adaptable Generator – Adaptable Unary Function – Adaptable Binary Function • Predicates (return a boolean result) – – – Predicate Binary Predicate Adaptable Binary Predicate Strict Weak Ordering • Specialized Concepts – Random Number Generator – Hash Function CSE 332: C++ STL functors

STL Functor Concept Hierarchy is-ref Assignable ined-b Generator Adaptable Unary Function Binary Function Unary Function Random Number Generator Predicate Adaptable Predicate y Hash Function Adaptable Binary Function Binary Predicate Adaptable Binary Predicate Strict Weak Ordering Basic Function Object Predicate Adaptable Function Object Specialized CSE 332: C++ STL functors

Assignable Concept • Does not refine any other STL concept • Valid Expressions – Copy Constructor X(x); X x(y); X x = y; – Assignment x = y; • Models of Assignable – – Almost every non-const C++ built-in type … … and function pointers … … but not functions (cannot construct or assign them) Here, all Basic Function Object concepts • Generator, Unary Function, Binary Function • And the concepts that specialize them CSE 332: C++ STL functors

Generator Concept • Refines Assignable • Abstracts pointers to 0 -ary functions (no arguments) • Valid Expressions – Function call signature with no arguments f() • Semantics – Returns some value of type Result – Different invocations may return different values • Or, can represent a constant as a 0 -ary functor – Invocation may change the function object’s internal state • So, operator() need not be a const member function CSE 332: C++ STL functors

Generator Example • Goal: fill a vector with random numbers • Generic generate algorithm – Fills in a range given in its 1 st and 2 nd arguments – applies Generator Concept to its 3 rd argument • Here, the functor is simply a function pointer – To the <cmath> (0 -ary) rand() function vector<int> v(100); generate(v. begin(), v. end(), rand); CSE 332: C++ STL functors

Unary Function Concept • Also a refinement of Assignable • Valid Expression – Function call signature with one argument f(x) – May ignore or use single argument – Similar return, const semantics to generator • Pre-C++11 Example using unary_function struct sine : public unary_function<double, double> { double operator()(double x) const { return sin(x); } }; sine func; • C++11 Example using function<double (double)> func(sin); CSE 332: C++ STL functors

Binary Function Concept • Also a refinement of Assignable • Valid Expression – Function call signature with two arguments f(x, y) – May use or ignore either or both of its arguments – Similar const and return semantics to Unary Function • Pre-C++11 Example using binary_function struct exponentiate : public binary_function<double, double> { double operator()(double x, double y) const { return pow(x, y); } }; • C++11 Example using function<double (double, double)> func(pow); CSE 332: C++ STL functors

Adaptable Function Objects • Allow functors to be used with Function Object Adaptors • Associated types – of argument(s), and especially return value • How to access these associated types ? – Define Adaptable Function Object Concepts • Adaptable Generator F 1: : result_type • Adaptable Unary Function F 2: : argument_type F 2: : result_type • Adaptable Binary Function F 3: : first_argument_type F 3: : second_argument_type F 3: : result_type • Models – Function pointers like Result(*f)(Arg) do not model these concepts – Helper adapters make Adaptable Function Objects from these functions • Pre-C++11 ptr_fun(f) or C++11 function<Result (Arg)>(f) CSE 332: C++ STL functors

Adaptable Function Object Example • Each value printed out will be 3. 0 larger than the corresponding element in v 1 int main(int, char **) { const int vector_size = 10; vector<double> v 1(vector_size); generate(v 1. begin(), v 1. end(), rand); // random values transform(v 1. begin(), v 1. end(), ostream_iterator<double>(cout, " "), bind 1 st(plus<double>(), 3. 0)); Pre-C++11 function adapter transform(v 1. begin(), v 1. end(), ostream_iterator<double>(cout, " "), std: : bind(plus<double>(), Adaptable function objects std: : placeholders: : _1, 3. 0)); return 0; } C++11 function adapter CSE 332: C++ STL functors

Predicate Concepts • Predicate – Refinement of Unary Function – Return type must be convertible to bool • Adaptable Predicate – Refinement of Predicate, Adaptable Unary Function – Adds typedefs for argument, return types • Binary Predicate – Refinement of Binary Function – Return type again must be convertible to bool • Adaptable Binary Predicate – Refinement of Binary Predicate, Adaptable Binary Function – Adds typedefs for the 2 arguments, return types • Strict Weak Ordering – Refinement of Binary Predicate (for comparison operations) – Similar semantics to operator< but with type constraints CSE 332: C++ STL functors