Understanding C++ LINQ and Ranges Pipeline Implementation

This article introduces C++ LINQ implementation, which is similar to C# LINQ and serves as a foundational mechanism for C++20 ranges library and the upcoming execution library.

The article begins with a ranges example demonstrating filter and transform operations using the pipe operator:

auto const ints = {0, 1, 2, 3, 4, 5};

auto even_func = [](int i) { return i % 2 == 0; };

auto square_func = [](int i) { return i * i; };

auto tmpv = ints | std::views::filter(even_func) | std::views::transform(square_func);

The article explains that this pipe syntax is essentially syntactic sugar equivalent to:

std::views::transform(std::views::filter(ints, even_func), square_func)

The core design involves a DSL with three components: (1) DSL definition using BNF: Ranges Pipeline ::= Data Source {'|' Range Adapter}, (2) Compiler (Pipeline operation) - the | operator processing, and (3) Execute - the iterator operations like std::ranges::begin(), std::ranges::end().

Key benefits include lazy evaluation (no data access during compilation), compile-time type safety, and efficient code generation. The article details the implementation of key concepts and classes:

namespace _Pipe {

template <class _Left, class _Right>

concept _Can_pipe = requires(_Left&& __l, _Right&& __r) {

static_cast<_Right&&>(__r)(static_cast<_Left&&>(__l));

};

}

This concept ensures that pipeable objects can be called as r(l). The _Can_compose concept checks constructibility for storing objects in _Pipeline.

The _Base<class _Derived> class provides pipe operator overloads for chaining range adapters, while _Pipeline<class _Left, class _Right> handles composition of two adapters:

template <class _Left, class _Right>

struct _Pipeline : _Base<_Pipeline<_Left, _Right>> {

_Left __l;

_Right __r;

template <class _Ty>

constexpr auto operator()(_Ty&& _Val)

requires requires { __r(__l(static_cast<_Ty&&>(_Val))); }

{ return __r(__l(std::forward<_Ty>(_Val))); }

};

The article also covers C++17's custom template argument deduction rules that allow simpler syntax like _Pipeline(a, b) instead of _Pipeline<a, b>().