FP编程实践：在C++0x中模拟F#的List.map和List.choose库函数

 

F#代码

let list1 = [1; 2; 3]let newList = List.map (fun x -> x + 1) list1printfn "%A" newList//[2; 3; 4]let listWords = [ "and"; "Rome"; "Bob"; "apple"; "zebra" ]let isCapitalized (string1:string) = string1.Chars(0) >= 'A' && string1.Chars(0) <= 'Z'let results = List.choose (fun elem -> if isCapitalized elem then Some(elem) else None) listWordsprintfn "%A" results//["Rome"; "Bob"]

C++代码

#include <iostream>#include <list>#include <string>#include <algorithm>#include <boost/assign.hpp>#include <boost/optional/optional.hpp>#include <boost/spirit/home/karma.hpp>using namespace std;using namespace boost::assign;using boost::optional;namespace fsharp_list{template<typename Fn, typename T>auto map(Fn mapping, list<T> const& list1) -> list<decltype(mapping(declval<T>()))>{list<decltype(mapping(declval<T>()))> list_result(list1.size());transform(list1.cbegin(), list1.cend(), list_result.begin(), mapping);return list_result;}template<typename Fn, typename T>auto choose(Fn chooser, list<T> const& list1) -> list<typename remove_reference<decltype(*chooser(declval<T>()))>::type>{list<typename remove_reference<decltype(*chooser(declval<T>()))>::type> list_result;for_each(list1.cbegin(), list1.cend(),[&](T const& elem){if(auto v = chooser(elem))list_result.push_back(*v);});return list_result;}}namespace std{namespace karma = boost::spirit::karma;template<typename T>ostream& operator<<(ostream& os, list<T> const& list1){return os << karma::format('[' << -(karma::auto_ % "; ") << ']', list1);}}int main(){list<int> list1 = list_of(1)(2)(3);auto newList = fsharp_list::map([](int x){return x + 1;}, list1);cout << newList << endl;//[2; 3; 4]list<string> listWords = list_of<string>("and")("Rome")("Bob")("apple")("zebra");auto isCapitalized = [](string const& str){return str[0] >= 'A' && str[0] <= 'Z';};auto results = fsharp_list::choose([&](string const& elem){return isCapitalized(elem) ? optional<string>(elem) : optional<string>();}, listWords);cout << results << endl;//["Rome"; "Bob"]}