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  <title>P1877R0: Saving Private Ranges: Recovering Lost Information from Comparison and Predicate Algorithms</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
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 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
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 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
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 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
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 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
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 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
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 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

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/** Copyright *****************************************************************/

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/** Back to Top / ToC Toggle **************************************************/

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		/* See also Overflow section at the bottom */

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/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

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/** Subheadings ***************************************************************/

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/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

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	 display: inline-block;
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/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
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	a > i { /* Instance of term */
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	dt dfn code, code.idl {
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	dfn var {
		font-style: normal;
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/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

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/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

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	pre {
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		margin-bottom: 1em;
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/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

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		text-decoration: none;
		border-bottom: 1px solid #707070;
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	/* Backout above styling for W3C logo */
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		text-decoration: none;
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/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

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	/* For autogen numbers, add
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	*/

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/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

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/** Blockquotes ***************************************************************/

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/** Open issue ****************************************************************/

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/** Non-normative Note ********************************************************/

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/** Assertion Box *************************************************************/
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/** Advisement Box ************************************************************/
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/** Spec Obsoletion Notice ****************************************************/
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	.annoying-warning:not(details),
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}

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/** Entity Definition Boxes ***************************************************/

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		margin: 1.2em 0;
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/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

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/** Property/Descriptor Definition Tables *************************************/

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/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1877R0<br>Saving Private Ranges: Recovering Lost Information from Comparison and Predicate Algorithms</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-10-07">2019-10-07</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://thephd.github.io/vendor/future_cxx/papers/d1803.html">https://thephd.github.io/vendor/future_cxx/papers/d1803.html</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Target:
     <dd>C++20
     <dt>Latest:
     <dd><a href="https://thephd.github.io/vendor/future_cxx/papers/d1803.html">https://thephd.github.io/vendor/future_cxx/papers/d1803.html</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Currently, many of the new algorithms in std::ranges algorithms copy their `bool`-returning predecessors by returning a single `bool` value. And while this makes perfect sense, developers who build algorithms on top of the standard ones often have to replicate the information that these algorithms already perform. This proposal adjusts the return types of std::ranges algorithms to return more information and be less lossy, preventing people from redoing work already performed by the called algorithm and without having to replicate the internal metaprogramming and state information that implementations already come across as a natural consequence of the algorithm.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#changelog-r0"><span class="secno">1.1</span> <span class="content">Revision 0 - October 7th, 2019</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
     <ol class="toc">
      <li><a href="#design-predicate"><span class="secno">3.1</span> <span class="content">Single-Boolean Predicate Returns</span></a>
      <li><a href="#design-comparison"><span class="secno">3.2</span> <span class="content">Single-Boolean Comparison Returns</span></a>
      <li><a href="#design-questions"><span class="secno">3.3</span> <span class="content">Questions</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changelog-r0"><span class="secno">1.1. </span><span class="content">Revision 0 - October 7th, 2019</span><a class="self-link" href="#changelog-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial release.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>When building wrapping ranges and iterators, it is useful for individuals working with these wrapped iterators to provide algorithms synonymous to the C++ Standard. For example, if someone is writing a <code class="highlight"><c- n>bit_iterator</c-></code> or a <code class="highlight"><c- n>code_point_iterator</c-></code> which wraps an underlying iterator and performs operations on the underlying iterator, one can optimize for the case where the wrapped iterators are potentially of the same <code class="highlight"><c- n>iterator_category</c-></code> (<code class="highlight"><c- n>iterator_concept</c-></code>) and have the same <code class="highlight"><c- n>value_type</c-></code>s. In an <a href="https://github.com/ThePhD/itsy_bitsy/blob/9556008313029b3deaabd67fabdcb3f5b84e8a3d/include/itsy/detail/algorithm.hpp#L166">example from real-world code</a>:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>_It0</c-><c- p>,</c-> <c- k>typename</c-> <c- n>_It1</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c->
<c- n>bit_equal</c-><c- p>(</c-><c- n>bit_iterator</c-><c- o>&lt;</c-><c- n>_It0</c-><c- o>></c-> <c- n>__first0</c-><c- p>,</c-> <c- n>bit_iterator</c-><c- o>&lt;</c-><c- n>_It0</c-><c- o>></c-> <c- n>__last0</c-><c- p>,</c-> 
	<c- n>bit_iterator</c-><c- o>&lt;</c-><c- n>_It1</c-><c- o>></c-> <c- n>__first1</c-><c- p>)</c-> 
<c- p>{</c->
	<c- k>using</c-> <c- n>__iterator0</c->        <c- o>=</c-> <c- n>__bit_iterator</c-><c- o>&lt;</c-><c- n>_It0</c-><c- o>></c-><c- p>;</c->
	<c- k>using</c-> <c- n>__iterator1</c->        <c- o>=</c-> <c- n>__bit_iterator</c-><c- o>&lt;</c-><c- n>_It1</c-><c- o>></c-><c- p>;</c->
	<c- k>using</c-> <c- n>__difference_type0</c-> <c- o>=</c-> <c- k>typename</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>iterator_traits</c-><c- o>&lt;</c-><c- n>__iterator0</c-><c- o>>::</c-><c- n>difference_type</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__iterator_category0</c-> <c- o>=</c-> <c- k>typename</c-> <c- n>__iterator0</c-><c- o>::</c-><c- n>iterator_category</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__iterator_category1</c-> <c- o>=</c-> <c- k>typename</c-> <c- n>__iterator1</c-><c- o>::</c-><c- n>iterator_category</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__base_iterator0</c->     <c- o>=</c-> <c- k>typename</c-> <c- n>__iterator0</c-><c- o>::</c-><c- n>iterator_type</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__base_iterator1</c->     <c- o>=</c-> <c- k>typename</c-> <c- n>__iterator1</c-><c- o>::</c-><c- n>iterator_type</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__base_value_type0</c-> <c- o>=</c-> <c- k>typename</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>iterator_traits</c-><c- o>&lt;</c-><c- n>__base_iterator0</c-><c- o>>::</c-><c- n>value_type</c-><c- p>;</c->
	<c- k>using</c-> <c- n>__base_value_type1</c-> <c- o>=</c-> <c- k>typename</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>iterator_traits</c-><c- o>&lt;</c-><c- n>__base_iterator1</c-><c- o>>::</c-><c- n>value_type</c-><c- p>;</c->
	<c- k>if</c-> <c- nf>constexpr</c-> <c- p>(</c-><c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>is_unsigned_v</c-><c- o>&lt;</c-><c- n>__base_value_type0</c-><c- o>></c-> <c- o>&amp;&amp;</c->
				<c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>is_unsigned_v</c-><c- o>&lt;</c-><c- n>__base_value_type1</c-><c- o>></c-> <c- o>&amp;&amp;</c->
				<c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>__base_value_type0</c-><c- p>,</c-> <c- n>__base_value_type1</c-><c- o>></c-><c- p>)</c->
	<c- p>{</c->
		<c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>__is_iterator_category_or_better_v</c-><c- o>&lt;::</c-><c- n>std</c-><c- o>::</c-><c- n>forward_iterator_tag</c-><c- p>,</c->
				<c- n>__iterator_category0</c-><c- o>></c->
			<c- o>&amp;&amp;</c-> <c- n>__is_iterator_category_or_better_v</c-><c- o>&lt;::</c-><c- n>std</c-><c- o>::</c-><c- n>forward_iterator_tag</c-><c- p>,</c->
				<c- n>__iterator_category1</c-><c- o>></c-><c- p>)</c->
			<c- p>{</c->
				<c- c1>// get .base() and use internal algorithms...</c->
			<c- p>}</c->
	<c- p>}</c->
	<c- c1>// use baseline input algorithm...</c->
<c- p>}</c->
</pre>
   <p>In the innermost branch after checking iterator categories, we would like to use <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal</c-></code> on the <code class="highlight"><c- p>.</c-><c- n>base</c-><c- p>()</c-></code> iterators, to compare whole words at a time or entire sequences at a time rather than just compare 1 bit at a time. The problem with using <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal</c-></code> here is that it only returns a <code class="highlight"><c- b>bool</c-></code> value: if there is any additional "work" left over after <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal</c-></code> is done comparing the fully populated underlying iterators, we now have to manually re-increment all the way to the end.</p>
   <p>This problem is present with a large number of algorithms in the standard. From <code class="highlight"><c- n>copy</c-></code>/<code class="highlight"><c- n>copy_n</c-></code> to <code class="highlight"><c- n>equal</c-></code>, many algorithms advance the iterator or perform useful computation on the iterators that is then discarded, leaving higher levels to re-do that work and incur a performance penalty (violating the idea that it could not be done by hand better with optimizations turned on).</p>
   <p>This performance penalty was fixed for certain <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> algorithms. For example, <code class="highlight"><c- n>copy</c-></code>, where a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>copy</c-></code> returns a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>copy_result</c-><c- o>&lt;</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>OutputIterator</c-><c- o>></c-></code>, allows someone to retrieve the underlying incremented <code class="highlight"><c- n>Iterator</c-></code> type.</p>
   <p>There are three ways around the problem:</p>
   <ol>
    <li data-md>
     <p>re-implement what the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> algorithms do now, <a href="https://github.com/gcc-mirror/gcc/blob/41d6b10e96a1de98e90a7c0378437c3255814b16/libstdc%2B%2B-v3/include/ext/algorithm#L79">which is what libstdc++ has done</a> for a handful of algorithms before <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> came along. This works only for an individual library’s implementation of that algorithm;</p>
    <li data-md>
     <p>re-implement any of the time-complexity checks and then use a "lower level" algorithm to perform the rest of the work. For example, this would include duplicating logic from <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>equal</c-></code> to check sizes in the case of random access. After doing explicit <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>distance</c-></code> checks upon getting <code class="highlight"><c- n>random_access_iterator</c-></code>s or better to meet the complexity requirements of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal</c-></code>, a developer would then dispatch to its "lower level" algorithmic form by calling <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>mismatch</c-></code>, which does return iterator information;</p>
    <li data-md>
     <p>or, return iterator information from the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> version of the algorithms that currently lose this information by returning only a <code class="highlight"><c- b>bool</c-></code>.</p>
   </ol>
   <p>This paper proposes Option 3, which is enhancing only the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> versions of these algorithms to return additional iterator information, in the same way <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>copy</c-></code> was enhanced over its non-<code class="highlight"><c- n>ranges</c-></code> counterpart. This is a backwards-compatible change since it does not touch the original algorithms.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <p>The design here is fairly straightforward: we go through all algorithms returning <code class="highlight"><c- b>bool</c-></code> in the standard library’s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> namespace and change it to have a return type similar to the below structure.  This proposal uses the same machinery that other range algorithms like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>mismatch</c-></code> and friends to produce an <code class="highlight"><c- n>X_result</c-></code> type. Most algorithms only need a <code class="highlight"><c- n>predicate_result</c-></code> type, while others require a little extra information.</p>
   <h3 class="heading settled" data-level="3.1" id="design-predicate"><span class="secno">3.1. </span><span class="content">Single-Boolean Predicate Returns</span><a class="self-link" href="#design-predicate"></a></h3>
   <p>The following result structure...</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>ranges</c-> <c- p>{</c->
	<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>I1</c-><c- o>></c->
	<c- k>struct</c-> <c- n>predicate_result</c-> <c- p>{</c->
		<c- p>[[</c-><c- n>no_unique_address</c-><c- p>]]</c-> <c- n>I1</c-> <c- n>in</c-><c- p>;</c->
		<c- b>bool</c-> <c- n>value</c-><c- p>;</c->
		
		<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>II1</c-><c- o>></c->
			<c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I1</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>II1</c-><c- o>></c->
			<c- k>operator</c-> <c- n>predicate_result</c-><c- o>&lt;</c-><c- n>II1</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- o>&amp;</c-> <c- p>{</c->
				<c- k>return</c-> <c- p>{</c-><c- n>in</c-><c- p>,</c-> <c- n>value</c-><c- p>};</c->
			<c- p>}</c->

		<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>II1</c-><c- o>></c->
			<c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>I1</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c->
			<c- k>operator</c-> <c- n>predicate_result</c-><c- o>&lt;</c-><c- n>II1</c-><c- o>></c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- p>{</c->
				<c- k>return</c-> <c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>in</c-><c- p>),</c-> <c- n>value</c-><c- p>};</c->
			<c- p>}</c->

		<c- k>explicit</c-> <c- k>operator</c-> <c- b>bool</c-> <c- p>()</c-> <c- k>const</c-> <c- p>{</c->
			<c- k>return</c-> <c- n>value</c-><c- p>;</c->
		<c- p>}</c->
	<c- p>};</c->
<c- p>}</c->
</pre>
   <p>... works for the following algorithm return types.</p>
   <ul>
    <li data-md>
     <p>In Non-modifying Algorithms <a href="http://eel.is/c++draft/alg.nonmodifying">[alg.nonmodifying</a>]:</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>all_of</c-></code>;</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>any_of</c-></code>;</p>
    <li data-md>
     <p>and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>none_of</c-></code>.</p>
    <li data-md>
     <p>In Sorting and related operations <a href="eel.is/c++draft/alg.sorting">[alg.sorting</a>]:</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_sorted</c-></code>;</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_partitioned</c-></code>;</p>
    <li data-md>
     <p>and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_heap</c-></code>.</p>
   </ul>
   <p>It is notable that <code class="highlight"><c- n>is_sorted</c-></code>, <code class="highlight"><c- n>is_partitioned</c-></code>, and <code class="highlight"><c- n>is_heap</c-></code> all have versions of themselves that return an iterator and also do not have additional complexity requirements specified in the standard over their <code class="highlight"><c- n>is_</c-><c- p>{}</c-><c- n>_until</c-></code> versions. Therefore, it may be prudent to just leave changes to these algorithms off entirely rather than also return both a convenience <code class="highlight"><c- b>bool</c-></code> value and the iterator.</p>
   <p>Likewise, <code class="highlight"><c- n>any_of</c-></code>, <code class="highlight"><c- n>all_of</c-></code>, and <code class="highlight"><c- n>none_of</c-></code> can be seen as wrappers around the <code class="highlight"><c- n>find_if</c-><c- p>(</c-><c- n>_not</c-><c- p>)</c-></code> algorithms. None of them impose additional complexity requirements, nor do standard libraries today do anything particularly special in the general implementation of these wrappers either. While the information could be returned, simply using a lower-level facility would be suitable in the cases here.</p>
   <h3 class="heading settled" data-level="3.2" id="design-comparison"><span class="secno">3.2. </span><span class="content">Single-Boolean Comparison Returns</span><a class="self-link" href="#design-comparison"></a></h3>
   <p>Similarly, the following <code class="highlight"><c- n>comparison_result</c-></code> structure...</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>ranges</c-> <c- p>{</c->
	<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>I1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>I2</c-><c- o>></c->
	<c- k>struct</c-> <c- n>comparison_result</c-> <c- p>{</c->
		<c- p>[[</c-><c- n>no_unique_address</c-><c- p>]]</c-> <c- n>I1</c-> <c- n>in1</c-><c- p>;</c->
		<c- p>[[</c-><c- n>no_unique_address</c-><c- p>]]</c-> <c- n>I2</c-> <c- n>in2</c-><c- p>;</c->
		<c- b>bool</c-> <c- n>value</c-><c- p>;</c->
		
		<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>II1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>II2</c-><c- o>></c->
			<c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I1</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>II1</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I2</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>II2</c-><c- o>></c->
			<c- k>operator</c-> <c- n>comparison_result</c-><c- o>&lt;</c-><c- n>II1</c-><c- p>,</c-> <c- n>II2</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- o>&amp;</c-> <c- p>{</c->
				<c- k>return</c-> <c- p>{</c-><c- n>in1</c-><c- p>,</c-> <c- n>in2</c-><c- p>,</c-> <c- n>value</c-><c- p>};</c->
			<c- p>}</c->

		<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>II1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>II2</c-><c- o>></c->
			<c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>I1</c-><c- p>,</c-> <c- n>II1</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>I2</c-><c- p>,</c-> <c- n>II2</c-><c- o>></c->
			<c- k>operator</c-> <c- n>predicate_result</c-><c- o>&lt;</c-><c- n>II1</c-><c- p>,</c-> <c- n>II2</c-><c- o>></c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- p>{</c->
				<c- k>return</c-> <c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>in1</c-><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>in2</c-><c- p>),</c-> <c- n>value</c-><c- p>};</c->
			<c- p>}</c->

		<c- k>explicit</c-> <c- k>operator</c-> <c- b>bool</c-> <c- p>()</c-> <c- k>const</c-> <c- p>{</c->
			<c- k>return</c-> <c- n>value</c-><c- p>;</c->
		<c- p>}</c->
	<c- p>};</c->
<c- p>}</c->
</pre>
   <p>... works for algorithms which take 2 ranges.</p>
   <ul>
    <li data-md>
     <p>In Non-modifying Algorithms <a href="http://eel.is/c++draft/alg.nonmodifying">[alg.nonmodifying</a>]:</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>equal</c-></code>;</p>
    <li data-md>
     <p>and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_permutation</c-></code>.</p>
    <li data-md>
     <p>In Sorting and related operations <a href="eel.is/c++draft/alg.sorting">[alg.sorting</a>]:</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>binary_search</c-></code>;</p>
    <li data-md>
     <p>and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>lexicographic_compare</c-></code>.</p>
   </ul>
   <p>Algorithms such as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>includes</c-></code> are not included in this fix up because that algorithm works with input ranges, and the iterators returned from it by the time the algorithm completes can essentially be empty husks that contain no valuable information. The goal would be to have a well-specified return value for the iterators on algorithm success, where logically that information implies the whole range was examined.</p>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>lexicographic_compare</c-></code> returns whether or not the range is in lexicographic order: an enhancement to that would be to return the incremented-to-the-end <code class="highlight"><c- n>first1</c-></code> and <code class="highlight"><c- n>first2</c-></code> iterators in the case where the algorithm returns <code class="highlight">true</code> for the result. Otherwise, the value of <code class="highlight"><c- n>in1</c-></code> and <code class="highlight"><c- n>in2</c-></code> are unspecified. Similarly for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>equal</c-></code>: the incremented iterators should be returned from <code class="highlight"><c- n>equal</c-></code> in <code class="highlight"><c- n>in1</c-></code> and <code class="highlight"><c- n>in2</c-></code> if the algorithm returns true: otherwise, the value of the returned iterators is unspecified.</p>
   <h3 class="heading settled" data-level="3.3" id="design-questions"><span class="secno">3.3. </span><span class="content">Questions</span><a class="self-link" href="#design-questions"></a></h3>
   <p>Question 1: These algorithms take in a range. We recognize that the result type is likely more ergonomic and efficient in terms of the amount of times an iterator has to be moved to store the result, and so recommended return types that return iterators like the other modified <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> algorithms. Still,</p>
   <blockquote>
    <p>is it worthwhile to have the return results return a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- p>...</c-><c- o>></c-></code> type, rather than just a <code class="highlight"><c- n>result</c-></code> type with the iterators?</p>
   </blockquote>
   <p>Question 2: Some of these algorithms are more or less trivial wrappers around their counterparts with no additional complexity requirements or guarantees obtain by metaprogramming checks. These are <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_sorted</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_partitioned</c-></code>, and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>is_heap</c-></code>.</p>
   <blockquote>
    <p>Is returning the iterator worth it when there are <code class="highlight"><c- n>X_until</c-></code> versions of all these algorithms which do return the iterator and can be defined as convenience wrappers?</p>
   </blockquote>
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