view::generate

README.md

@@ -40,9 +40,8 @@ Should be something like:
 
 	for (int i{}; i < outerLimit; i++)
 	{
-		auto v = std::views::iota(0, innerLimit)
-			| oel::view::transform([i](auto j) { return i * j; });
-		arr.append(v);
+		auto fn = [i, j = 0]() mutable { return i * j++; };
+		arr.append(oel::view::generate(fn, innerLimit));
 	}
 
 Another good way, using `resize_for_overwrite`:

unit_test/view_gtest.cpp

@@ -244,6 +244,25 @@ TEST(viewTest, viewTransformAsOutput)
 	EXPECT_EQ(-2, test[1].second);
 }
 
+struct Ints
+{
+	int i;
+
+	int operator()()  { return i++; }
+};
+
+TEST(viewTest, viewGenerate)
+{
+	auto d = view::generate(Ints{1}, 2) | oel::to_dynarray();
+
+	ASSERT_EQ(2U, d.size());
+	EXPECT_EQ(1, d[0]);
+	EXPECT_EQ(2, d[1]);
+
+	d.assign(oel::view::generate(Ints{}, 0));
+	EXPECT_TRUE(d.empty());
+}
+
 TEST(viewTest, viewMoveEndDifferentType)
 {
 	auto nonEmpty = [i = -1](int j) { return i + j; };
@@ -265,6 +284,9 @@ TEST(viewTest, viewMoveMutableEmptyAndSize)
 	EXPECT_EQ(1U, v.size());
 }
 
+using IntGenIter = oel::iterator_t<decltype( view::generate(Ints{}, 0) )>;
+static_assert(std::input_or_output_iterator<IntGenIter>);
+
 TEST(viewTest, chainWithStd)
 {
 	auto f = [](int i) { return -i; };

view/generate.h

@@ -0,0 +1,51 @@
+#pragma once
+
+// Copyright 2021 Ole Erik Peistorpet
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+
+#include "counted.h"
+#include "../auxi/assignable.h"
+
+/** @file
+*/
+
+namespace oel
+{
+
+template< typename Generator >
+class _generateIterator
+{
+	typename _detail::AssignableWrap<Generator>::Type _g;
+
+public:
+	using iterator_category = std::input_iterator_tag;
+	using reference         = decltype( std::declval<Generator &>()() );
+	using pointer           = void;
+	using value_type        = std::remove_cv_t< std::remove_reference_t<reference> >;
+	using difference_type   = ptrdiff_t;
+
+	constexpr explicit _generateIterator(Generator g)   : _g{std::move(g)} {}
+
+	constexpr reference operator*()
+		{
+			return static_cast<Generator &>(_g)();
+		}
+
+	constexpr _generateIterator & operator++()        OEL_ALWAYS_INLINE { return *this; }
+	constexpr void                operator++(int) &   OEL_ALWAYS_INLINE {}
+};
+
+
+namespace view
+{
+//! Returns a view that generates `count` elements by calling the given generator function
+/**
+* Like `generate_n` in the Range-v3 library, but this is only for use within OE-Lib. */
+inline constexpr auto generate =
+	[](auto generator, ptrdiff_t count)  { return counted(_generateIterator{std::move(generator)}, count); };
+}
+
+} // oel

views.h

@@ -12,6 +12,7 @@
 */
 
 #include "view/counted.h"
+#include "view/generate.h"
 #include "view/move.h"
 #include "view/owning.h"
 #include "view/subrange.h"