Overview of STL Function Objects

1. Overview of STL Function Objects Function Object (also known as a Functor) STL Function Objects support function call syntax E.g., a class that declares and defines operator() E.g., by being a function pointer GoF Command pattern may be applied to classes and structs (user-defined types) Encapsulate a function call as an object Some or all of these can be member variables Target object reference, function to invoke, arguments Generic programming allows diverse types A function pointer (example from Lippman, LaJoie, Moo) bool (*pf) (const string &, const & string) // function pointer bool *pf (const string &, const & string) // function named pf An instance of a class providing operator()

2. Function Objects Extend STL Algorithms Make the algorithms even more general Parameterize policy E.g., the order produced by a sorting algorithm Each functor does a single, specific operation Often implemented as small classes or structs Often has only one public member function, operator() Functors 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 +k functor E.g., arguments for an invocation on a remote object

3. Examples of Function Object Usage 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); } };

4. Function Object Concepts Basic Function Object Concepts Generator Unary Function Binary Function Adaptable Function Objects (turn functions into functors) 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

5. Function Object Concept Hierarchy

6. 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 Here, all Basic Function Object concepts Generator, Unary Function, Binary Function And the concepts that specialize them

7. Generator Concept Refines Assignable Abstracts 0-ary functions (take 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

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

9. Unary Function Also a refinement of Assignable Valid Expression Function call signature with one argument f(x) Semantics May ignore or use single argument Similar return, const semantics to generator Example - Unary Sine Functor struct sine : public unary_function<double,double> { double operator()(double x) const { return sin(x); } };

10. Binary Function Also a refinement of Assignable Valid Expression Function call signature with two arguments f(x,y) Semantics May use or ignore either or both of its arguments Similar const and return semantics to Unary Function Example - Exponentiation Functor struct exponentiate : public binary_function<double,double,double> { double operator()(double x, double y) const { return pow(x,y); } };

11. 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 Functions like Result(*f)(Arg) are not models of these concepts Helper adapters make Adaptable Function Objects from these functions For example ptr_fun(f) is a model of Adaptable Unary Function

12. Adaptable Function Object Example Each value in v2 will be 3.0 larger than the corresponding element in v1 const int N = 1000; vector<double> v1(N); vector<double> v2(N); // random values generate(v1.begin(), v1.end(), rand); transform(v1.begin(), v1.end(), v2.begin(), bind1st(plus<double>(), 3.0));

13. 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

14. Random Number Generator Refinement of Unary Function Unfortunately name is similar to Generator (0-ary) Valid Expression Function call f(N), where N is a positive integer Semantics Returns some value of type Result Different invocations may return different values Normal pseudo-random distribution If function f is called many times with the same argument N then Each value in range [0,N) will appear ~ equal number of times

15. Hash Function Refinement of Unary Function Return type must be size_t Valid Expression Function call f(x) Semantics Map from argument to a size_t result Equivalent arguments must yield same result Used by Hashed Associative Containers

16. Function Object Adaptors Adaptors transform one interface to another I.e., they implement the Adapter pattern Function Object Adaptors Perform function composition and binding Allows fewer ad hoc function objects Allows you to build functions as graphs (especially chains and trees) of other functions

17. Composition and Binding with Function Objects Function Composition f and g are both Adaptable Unary Functions g return type is convertible to f argument type (f ? g)(x) which is f(g(x)) can be written using unary_compose<f,g> or compose1(f,g) Function Binding Adaptable Binary Function to Adaptable Unary Function Bind first argument using binder1st<BinaryFun> Where f is an object of type binder1st<F> f(x) ? F(c,x) where c is constant Bind second argument using binder2nd<BinaryFun> Where f is an object of type binder2nd<F> f(x) ? F(x,c) where c is constant

18. vector<double> angles; // ... push in some radian values ... vector<double> sines; const double pi = 3.14159265358979323846; transform(angles.begin(), angles.end(), sines.begin(), compose1(negate<double>(), compose1(ptr_fun(sin), bind2nd(multiplies<double>(), pi / 180.0))));

19. Carry out operations in some specified order template <class R, class List> class Chain { Fun2& operator()(const Fun1<R, List>& fun1, const Fun2<R, List>& fun2) { fun1(); return fun2(); } };

20. Member Function Adaptors Allows use of member functions as Function Objects Similar adaptors available for non-member functions Take an X* or X& argument and call one of the X object�s member functions through that argument If the member function is virtual, call is polymorphic Three variations Argument type X* vs. X& Member Function takes zero vs. one argument Encapsulates non-const vs. const member function

21. Member Function Adaptor Example 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 **) { vector<B*> v; v.push_back(new D1); v.push_back(new D2); v.push_back(new D2); v.push_back(new D1); for_each(v.begin(), v.end(), mem_fun(& B::print)); return 0; }

22. Concluding Remarks Passing parameters to Functors Can do by value or by reference Same kinds of aliasing issues, etc. as with any other object Watch performance with many small function objects Watch out especially for creation, destruction overhead May want to inline functors constructors, destructors Put function objects on stack instead of heap Function objects are a powerful, general mechanism Reduces programming complexity, increases reuse Several new uses of generic programming Could go farther with parameterization than just algorithms Use functors to make smarter iterators and containers