If you’re curious like me, you probably ventured at least once in the scary and mind-bending world of functional programming, came back and told yourself: “It would be nice if I could do this or that in c++”. FP languages have been present for decades but only recently, we’ve been starting to see the adoption of some of their techniques in classical imperative languages… like higher order functions, closures/lambdas functions, currying and lazy evaluation. For example, Javascript supports closures since version 1.7 and C# from 3.0.

Seeing how useful these techniques are, it’s normal to want them in our favorite programming language. We’re already doing a bit a FP without knowing thanks to the standard library algorithms. Lots of them takes functors/predicates as arguments so they mimics fairly well the behavior of higher order functions. Besides that, C++ has no built-in support for other idioms like lambda functions or closures but we can achieve similar effects due to the lazy nature of templates and a technique known as “expression templates”. More on that technique on a future post…

To demonstrate my point, let’s take a small program that takes a string as input and return the most frequent character. In the old classical C++ way, it could be implemented as follow:

#include <iostream> #include <locale> #include <map> #include <string> namespace { char most_frequent_letter(const std::string &str) { typedef std::map<char, unsigned int> char_counts_t; char_counts_t char_counts; for(std::string::const_iterator itr = str.begin(); itr != str.end(); ++itr) if(std::isalpha(*itr, std::locale())) ++char_counts[*itr]; for(char_counts_t::const_iterator itr = char_counts.begin(); itr != char_counts.end(); ++itr) std::cout << itr->first << " => " << itr->second << std::endl; if(!char_counts.empty()) { char_counts_t::const_iterator highest_count = char_counts.begin(); for(char_counts_t::const_iterator itr = ++char_counts.begin(); itr != char_counts.end(); ++itr) if(itr->second > highest_count->second) highest_count = itr; return highest_count->first; } return ' '; } } int main(int argc, char *argv[]) { if(argc > 1) { std::string some_string = argv[1]; std::cout << "The string is: " << some_string << "

" << std::endl; std::cout << "The most frequent letter is: " << most_frequent_letter(some_string) << std::endl; } else std::cout << "Usage: " << argv[0] << " <string>" << std::endl; } [/sourcecode] So far so good, it works and does the job. We're putting the characters in a map using the character as the key and the count as the value. Then, we print the content of the map and finally iterate through it to find the character with the highest value. The problems with this code is that we're reinventing parts already in the standard library and that code lacks expressiveness. Let's see how the code could look like if we used the standard algorithms. [sourcecode language='cpp'] namespace { template <typename map_t> struct map_filler { typedef void result_type; map_filler(map_t &map): map_(map) { } template <typename T> result_type operator()(const T &t) const { if(std::isalpha(t, std::locale())) ++map_[t]; } private: map_t &map_; }; struct pair_printer { typedef void result_type; template <typename pair_t> result_type operator()(const pair_t &pair) const { std::cout << pair.first << " => " << pair.second << std::endl; } }; struct pair_value_comparer { typedef bool result_type; template <typename pair_t> result_type operator()(const pair_t &a, const pair_t &b) { return a.second < b.second; } }; char most_frequent_letter(const std::string &str) { typedef std::map<char, unsigned int> char_counts_t; char_counts_t char_counts; std::for_each(str.begin(), str.end(), map_filler<char_counts_t>(char_counts)); std::for_each(char_counts.begin(), char_counts.end(), pair_printer()); char_counts_t::const_iterator result = std::max_element( char_counts.begin(), char_counts.end(), pair_value_comparer()); return (result != char_counts.end()) ? result->first : ' '; } }

Hmm… Okay… Let’s see, our “most_frequent_letter” function is now using the standard library algorithms. It does make the function clearer and way more expressive but at the cost of around 40 lines of “support code” whose are in our case, functors. Even when thinking in terms of reusability, the chance of needing that same support code in the future is small, if not inexistant. What would we do in that case in a FP language? Use a small lambda functions/closure instead. For that example, I’m going to use boost::phoenix 2.0, an efficient FP library part of boost which is in my opinion the best general, multi-purpose C++ library and a must-have for any serious C++ programmer. Let’s see what phoenix can do:

namespace { namespace phx = boost::phoenix; using namespace phx::arg_names; using namespace phx::local_names; using phx::at_c; char most_frequent_letter(const std::string &str) { typedef std::map<char, unsigned int> char_counts_t; char_counts_t char_counts; std::for_each(str.begin(), str.end(), phx::if_(phx::bind(std::isalpha<char>, _1, phx::construct<std::locale>())) [ phx::let(_a = phx::ref(char_counts)[_1]) [ ++_a ] ]); std::for_each(char_counts.begin(), char_counts.end(), std::cout << at_c<0>(_1) << " => " << at_c<1>(_1) << std::endl); char_counts_t::const_iterator result = std::max_element( char_counts.begin(), char_counts.end(), at_c<1>(_1) < at_c<1>(_2)); return (result != char_counts.end()) ? result->first : ' '; } }

I made a few using statements to make the code easier to understand. Let’s take a look the for_each statement, the 2 first arguments are the usual .begin() and .end() but then you see that strange if_ as the third argument. if_, like every phoenix statement, returns a functor object created at compile time via template composition (Expression templates). So with this library, you can create inline functors on the fly without the “support code” bloat. You can use your own functors as long as they’re lazy which means they don’t do anything before the operator () is called on them. Fortunately, the lib also provides wrapper for “normal” functions.

Now for that code, nothing much to say for the if_ statement, it’s just a lazy version of the classic if keyword. phx::bind is one of the included wrappers, it creates a lazy version of a function passed as the first argument binded with the arguments passed as additional parameters. _1 and _2 are placeholders, they’re the actual parameters passed by the algorithm to the functor and phx::construct returns a new object of the type passed as the template parameter. Knowing that, we can now understand that “phx::bind(std::isalpha, _1, phx::construct())” returns a lazy version of std::isalpha with the current argument from std::for_each binded as the first argument to std::isalpha and an object of type std::locale binded as the second. phx::let’s only purpose is to create scoped local variables. phx::ref returns a reference to the object passed as the parameter. phx::at_c is simple, on a std::pair phx::at_c returns .first and phx::at_c .second.

For more information, consult the boost::phoenix documentation.

With that new tool, we can now more easily than ever use C++ to imitate some FP idioms:

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

int main(int argc, char *argv[])

{

namespace phx = boost::phoenix;

using namespace phx::arg_names;

using namespace phx::local_names;

std::vector input;

input.push_back(1);

input.push_back(2);

input.push_back(3);

input.push_back(4);

input.push_back(5);

//map( Make a new sequence with all the elements multiplied by 2 )

std::transform(input.begin(),

input.end(),

std::ostream_iterator (std::cout, “, “),

_1 * 2);

std::cout << std::endl; //filter( Make a new sequence containing all the odd numbers ) std::remove_copy_if(input.begin(), input.end(), std::ostream_iterator (std::cout, “, “),

!(_1 % 2));

std::cout << std::endl; //fold/reduce (Builds up and returns a value based on the sequence) //I use std::string here because it makes it easier to show what is //going on exactly. std::vector words;

words.push_back(“H”);

words.push_back(“e”);

words.push_back(“l”);

words.push_back(“l”);

words.push_back(“o”);

//foldl

std::string result = std::accumulate(words.begin(),

words.end(),

static_cast (“”),

_1 + _2);

std::cout << result << std::endl; //foldr result = std::accumulate(words.rbegin(), words.rend(), static_cast (“”),

_1 + _2);

std::cout << result << std::endl; } [/sourcecode] In a near future, we'll probably see more FP techniques being applied to imperative languages because they can make the code cleaner and more expressive without penalties. In the case of C++, like we just saw, they can leverage existing standard library algorithms and make them more convenient.