CSE 332: C++ 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, LaJoie, 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, double, bool> { bool operator() (double x, double y) { return fabs(y) < fabs(x); } }; struct LT_magnitude : public binary_function<double, double, bool> { bool operator() (double x, double y) { return fabs(x) < fabs(y); } }; int main (int, char **) { vector 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 o (cout, “ ”); copy (u.begin(), u.end(), o); copy (v.begin(), v.end(), o); return 0; }

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

CSE 332: C++ STL functors Function Object Use in a Container #include using namespace std; 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 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 EmployeeIdComp { bool operator() (const Employee & e, const Employee & f) { return e.id_ < f.id_ || (e.id_ == f.id_ && e.name_ < f.name_); } }; int main (int, char *[]) { vector 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 (cout)); // "v: Claire Bob Alice " set s; s.insert(v.begin(), v.end()); cout << "s: " ; copy (s.begin(), s.end(), ostream_iterator (cout)); // "s: Alice Bob Claire “ set t; t.insert(v.begin(), v.end()); cout << "t: " ; copy (t.begin(), t.end(), ostream_iterator (cout)); // "t: Bob Claire Alice “ return 0; } function object needed temporary object

CSE 332: C++ STL functors STL Before and After C++11 Parts were deprecated in C++11, but are still available in Visual C –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 binder1st –E.g., bind second argument using binder2nd –See for more details New versions introduced in C++11, are also available in Visual C –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 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 D1 : public B {void print() {cout<<"I'm a D1"<< endl;}}; struct D2 : public B {void print() {cout<<"I'm a D2"<< endl;}}; int main(int, char **) { D1 d1; D2 d2; vector v; vector w; v.push_back(&d1); v.push_back(&d2); v.push_back(&d2); v.push_back(&d1); w.push_back(d2); for_each(v.begin(), v.end(), mem_fun(& B::print)); cout << endl << endl; for_each(w.begin(), w.end(), mem_fun_ref(& B::print)); return 0; } different wrappers needed

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 D1 : public B {void print() {cout<<"I'm a D1"<< endl;}}; struct D2 : public B {void print() {cout<<"I'm a D2"<< endl;}}; int main(int, char **) { D1 d1; D2 d2; vector v; vector w; v.push_back(&d1); v.push_back(&d2); v.push_back(&d2); v.push_back(&d1); w.push_back(d2); for_each(v.begin(), v.end(), mem_fn(& B::print)); cout << endl << endl; for_each(w.begin(), w.end(), mem_fn(& B::print)); return 0; } same wrapper for both

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 Predicate –Adaptable Binary Predicate –Strict Weak Ordering Specialized Concepts –Random Number Generator –Hash Function

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

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 (0-ary) rand() function vector 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 operator()(double x) const { return sin(x); } }; sine func; C++11 Example using function function 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 operator()(double x, double y) const { return pow(x,y); } }; C++11 Example using function function 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 F1::result_type Adaptable Unary Function F2::argument_type F2::result_type Adaptable Binary Function F3::first_argument_type F3::second_argument_type F3::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 (f)

CSE 332: C++ STL functors Adaptable Function Object Example Each value printed out will be 3.0 larger than the corresponding element in v1 int main(int, char **) { const int vector_size = 10; vector v1(vector_size); generate(v1.begin(), v1.end(), rand); // random values transform(v1.begin(), v1.end(), ostream_iterator (cout, " "), bind1st(plus (), 3.0)); transform(v1.begin(), v1.end(), ostream_iterator (cout, " "), std::bind(plus (), std::placeholders::_1, 3.0)); return 0; } C++11 function adapter Adaptable function objects Pre-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 typedef s 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 typedef s for the 2 arguments, return types Strict Weak Ordering –Refinement of Binary Predicate (for comparison operations) –Similar semantics to operator< but with type constraints