Better Software - June 2008 - (Page 15)

Code Craft template class Composer { private: typedef typename iterator_traits ::value_type Fun; typedef typename Fun::result_type T; typedef reverse_iterator RevIter; RevIter beg, end; static T apply(T sofar, Fun f) { return f(sofar); } public: Composer(Iter b, Iter e) : beg(RevIter(e)), end(RevIter(b)) {} T operator()(T x) { return accumulate(beg, end, x, apply); // Function Applicator } }; Listing 3 main in listing 5, two unnamed functions are created and stored in funs. The variable c holds the delegate returned by the call to compose. So T delegate(T) compose(T)(T function(T)[] funs) { T doit(T n) { T result = n; foreach_reverse (f; funs) result = f(result); return result; } return &doit; } Listing 4 what is a delegate in D? It is the very mechanism that allows funs to persist and be used by the instance of doit returned by a call to compose. A delegate is a pair that contains a pointer to a function (which could be a class method) and the calling context of the function. That calling context could be an activation (stack frame) of an enclosing function, as compose is for doit, or it could be an object or class used as context for a method. So that its calling context—the activation of the call to compose—is preserved for doit to do its work, doit must be returned as a delegate. That context is preserved even after com- pose has terminated. Such a persistent calling context is called a closure. The closure for compose is moved from the runtime stack to the garbage-collected heap so it persists as long as the variable c in listing 5 does. There is no need to create a class to solve this problem, and the nested-function approach is simpler anyway. Whither Function Objects? C++ popularized the use of function objects—sometimes called functors—with the introduction of STL. A function object is nothing more than an instance of a class that overloads the function-call operator. To illustrate, listing 6 creates a C++ function object, gtn, which determines whether its argument is greater than a previously stored value. Similar code can be written in D using the opCall special function, but the nested-function version in listing 7 is simpler. Once again we have an outer function that returns a nested function that has access to the outer calling context. It appears that nested functions and closures can replace function objects template class gtn { T n; public: gtn(T x) : n(x) {} bool operator()(T m) { return m > n; } }; int main() { gtn g5(5); cout<< g5(1) <<endl; cout<< g5(6) <<endl; } Listing 6 pointer to the function doit. The function doit itself uses the array funs, which is defined in compose’s parameter list. It is common to say that funs is in the “calling environment” of doit. So what happens when a pointer to doit is returned from a call to compose? In order for funs to persist for use by subsequent calls to doit, it has to somehow be saved and connected to doit. Consider how compose is used in listing 5. The function keyword has two related uses in D: to declare a function pointer and to define an anonymous function literal (like “lambda” expressions in functional programming languages). The first usage occurs in the definition of compose in the first line of listing 4 and also in the first line inside main in listing 5. In each case, funs is declared a dynamic array of single-valued function pointers. In the second and third lines of import std.stdio; void main() { int function(int)[] funs; funs ~= function int(int x){return x*x;}; funs ~= function int(int x){return x+1;}; auto c = compose(funs); writeln(c(3)); // 16 } Listing 5 // false // true www.StickyMinds.com JUNE 2008 BETTER SOFTWARE 15 http://www.StickyMinds.com

Table of Contents Feed for the Digital Edition of Better Software - June 2008

Better Software - June 2008 Contents Mark Your Calendar Contributors Technically Speaking eLightenment Code Craft Test Connection Management Chronicles Agile Model-Driven Development The Myth of Risk Management Stop the Insanity! Product Announcements 10 Things You Might Not Know About … The Last Word Ad Index

Better Software - June 2008

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