cpplinq
如今在C++中LINQ已经有了众多实现，其中实现度最高，更新最及时，，文档最完整的当推LINQ for C++ (cpplinq)。
以下代码使用C++14再现了cpplinq的主要实现技巧。
#include <iostream>#include <vector>template<typename TIterator>struct from_range{typedef typename std::iterator_traits<TIterator>::value_type value_type;TIterator current, upcoming, end;from_range(TIterator begin, TIterator end): current(begin), upcoming(begin), end(end) {}template<typename TRangeBuilder>decltype(auto) operator>>(TRangeBuilder builder) const { return builder.build(*this); }decltype(auto) front() const { return *current; }bool next() { return upcoming == end ? false : (current = upcoming++, true); }};template<typename TRange, typename TPredicate>struct where_range{typedef typename TRange::value_type value_type;TRange range;TPredicate predicate;where_range(TRange range, TPredicate predicate): range(range), predicate(predicate) {}template<typename TRangeBuilder>decltype(auto) operator>>(TRangeBuilder builder) const { return builder.build(*this); }decltype(auto) front() const { return range.front(); }bool next(){while(range.next())if(predicate(range.front()))return true;return false;}};template<typename TPredicate>struct where_builder{TPredicate predicate;where_builder(TPredicate predicate) : predicate(predicate) {}template<typename TRange>auto build(TRange range){return where_range<TRange, TPredicate>(range, predicate);}};struct to_vector_builder{template<typename TRange>auto build(TRange range){std::vector<TRange::value_type> result;while(range.next())result.push_back(range.front());return result;}};template<typename TContainer>auto from(TContainer const& container){return from_range<decltype(std::begin(container))>(std::begin(container), std::end(container));}auto to_vector() { return to_vector_builder(); }template<typename TPredicate>auto where(TPredicate predicate) { return where_builder<TPredicate>(predicate); }int main(){int a[] = {1, 2, 3, 4};auto v = from(a) >> where([](int n){return n % 2 == 0;}) >> to_vector();for(int i : v) std::cout << i << std::endl;}/*24*/
代码分析以上代码总共实现了cpplinq中的三个操作符from，where以及to_vector。
from
首先让我们来看from操作符的实现
艰苦能磨练人的意志。
