C++ lambda powered matrix

lambda_powered matrix.h

#pragma once
// (c) 2019-2020 by dbj@dbj.org
// License: CC BY SA 4.0

#include <iomanip> // setw
#include <sstream> 
#include <array>
#include <cassert>
#include <string>
#include <vector>

namespace dbj::mtx {
	
	enum class version { major = 5, minor = 0, patch = 0 };

	template< typename T, size_t len_ >
	inline constexpr auto arr_stack() { return std::array<T, len_>{ {}}; }

	template< typename T, size_t len_ >
	inline std::vector<T> arr_heap() { return std::vector<T>(len_); }

	template< typename T, size_t rows, size_t cols, typename F >
	inline constexpr auto mx(F source_)
	{
		return[arry = source_()]
		(size_t row_, size_t col_) /*constexpr*/ mutable->T&
		{
			assert(row_ <= rows);
			assert(col_ <= cols);
			return 	arry[row_ * rows + col_];
		};
	} // mx

#undef dbj_mx_make
#undef dbj_mx_make_heap
#undef dbj_mx_make_stack

#define dbj_mx_make(T, R, C, K) dbj::mtx::mx<T, R, C>(dbj::mtx::K<T, R * C>)
#define dbj_mx_make_heap(T,R,C) dbj_mx_make(T, R, C, arr_heap)
#define dbj_mx_make_stack(T,R,C) dbj_mx_make(T, R, C, arr_stack)

	///
	/// Testing
	///
#ifdef DBJ_MTX_TESTING 

	inline auto changer = []( auto matrix_, size_t row_, size_t col_, auto value_) {
		matrix_(row_, col_) = value_;
		return matrix_;
	};

	// print to buffer
	inline auto
		printer = [](auto matrix_, size_t width_, size_t height_) 
		-> std::string {
		using namespace std;

		std::ostringstream rez_;

		for (size_t row_ = 0; row_ < width_; row_++) {
			rez_ << endl << "{";
			for (size_t col_ = 0; col_ < height_; col_++) {
				rez_ << setw(3) << matrix_(row_, col_) << " ";
			}
			rez_ << "}";
		}
		rez_ << endl;
		return rez_.str();		
	};

	///
	/// log callback signature:
	/// void (*)( std::string_view  )
	///
	template<typename log_callback >
	inline void test_dbj_matrix_creation( log_callback log )
	{
		using namespace dbj::mtx;
		using namespace std;

		constexpr size_t width_ = 3; // 0,1,2
		constexpr size_t height_ = 3; // 0,1,2

		constexpr size_t last_col_ = width_ - 1; // 0,1,2
		constexpr size_t last_row_ = height_ - 1; // 0,1,2

		// test move in, move out and the rest
		auto reporter = [&](auto matrix_, const char* prompt_) {
			string report = prompt_ +
				printer(changer(matrix_, last_col_, last_row_, 42), width_, height_);
			log(report);
			return matrix_;
		};
					   
		auto mxs = reporter(dbj_mx_make_stack(int, width_, height_),
			"\n matrix_on_stack \n" );

		auto mxh = reporter(dbj_mx_make_heap(int, width_, height_),
			"\n matrix_on_heap \n");
	}
#endif // DBJ_MTX_TESTING 

} // dbj::mtx

5.0.0 is "dramatic" change. Had to use std::vector for heap based matrix after all. std::unique_ptr disables ability to copy and std::shared_ptr is abomination.