<!DOCTYPE html>
<html lang="en"><head>
  <meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0288R7: any_invocable</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
 *   - table.data for general data tables
 *     -> 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
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .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
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     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.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

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

	p.copyright,
	p.copyright small { font-size: small }

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

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

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

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

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

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

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

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

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


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

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

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

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

	sup {
		vertical-align: super;
		font-size: 80%
	}

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

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

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

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

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

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** 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".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 </head><body class="h-entry toc-sidebar"><p id="toc-nav"><a id="toc-jump" href="#toc"><span aria-hidden="true">↑</span> <span>Jump to Table of Contents</span></a><a id="toc-toggle" href="#toc"><span aria-hidden="true">←</span> <span>Collapse Sidebar</span></a></p>
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0288R7<br>any_invocable</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-07-31">2020-07-31</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     </dt><dd><a class="u-url" href="https://htmlpreview.github.io/?https://github.com/mattcalabrese/proposal-unique_function/master/0288-current-draft.html">https://htmlpreview.github.io/?https://github.com/mattcalabrese/proposal-unique_function/master/0288-current-draft.html</a>
     </dd><dt>Authors:
     </dt><dd>
      </dd><dd class="editor p-author h-card vcard"><span class="p-name fn">Matt Calabrese &lt;metaprogrammingtheworld@gmail.com&gt;</span>
     </dd><dd>
      </dd><dd class="editor p-author h-card vcard"><span class="p-name fn">Ryan McDougall &lt;mcdougall.ryan@gmail.com&gt;</span>
     </dd><dt>Audience:
     </dt><dd>LWG
     </dd><dt>Project:
     </dt><dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dd></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>
    This paper proposes a conservative, move-only equivalent of 
    <tt>std::function</tt>
    .
   </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="#History"><span class="secno">1</span> <span class="content">Brief History</span></a>
    </li><li><a href="#Overview"><span class="secno">2</span> <span class="content">Overview</span></a>
    </li><li><a href="#Specification"><span class="secno">3</span> <span class="content">Specification</span></a>
    </li><li>
     <a href="#SD2018"><span class="secno">4</span> <span class="content">Polls from LEWG San Diego Review (2018)</span></a>
     <ol class="toc">
      <li><a href="#support-func-func-const-func"><span class="secno">4.1</span> <span class="content">Support func(), func() const, func() &amp;&amp;</span></a>
      </li><li><a href="#support-func--only"><span class="secno">4.2</span> <span class="content">Support func() &amp;&amp; only</span></a>
      </li><li><a href="#remove-target-target_type"><span class="secno">4.3</span> <span class="content">Remove target/target_type</span></a>
      </li><li><a href="#require-more-stuff-noexcept-const"><span class="secno">4.4</span> <span class="content">Require more stuff (noexcept, const&amp;&amp;, ...)</span></a>
      </li><li><a href="#name-options"><span class="secno">4.5</span> <span class="content">Name Options</span></a>
     </li></ol>
    </li><li>
     <a href="#K2019"><span class="secno">5</span> <span class="content">Polls from LEWG Kona Review (2019)</span></a>
     <ol class="toc">
      <li><a href="#we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"><span class="secno">5.1</span> <span class="content">We want to spend time on this now in order to include it in C++20</span></a>
      </li><li><a href="#add-support-for-func-const-and-func"><span class="secno">5.2</span> <span class="content">Add support for func() const&amp; and func()&amp;</span></a>
      </li><li><a href="#add-support-for-func-noexcept-x-all-of-the-above"><span class="secno">5.3</span> <span class="content">Add support for func() noexcept (x all of the above)</span></a>
      </li><li><a href="#include-the-option-for-ctad"><span class="secno">5.4</span> <span class="content">Include the option for CTAD</span></a>
      </li><li><a href="#name-callable-vs-any-invocable"><span class="secno">5.5</span> <span class="content">Name: callable vs any invocable</span></a>
      </li><li><a href="#any_invocable-vs-invocable"><span class="secno">5.6</span> <span class="content">any_invocable vs invocable</span></a>
      </li><li><a href="#header-choice"><span class="secno">5.7</span> <span class="content">Header choice</span></a>
      </li><li><a href="#can-get-stdfunction-from-any_invocable"><span class="secno">5.8</span> <span class="content">Can get std::function from &lt;any_invocable&gt;</span></a>
      </li><li><a href="#can-get-stdfunction-from-invocable"><span class="secno">5.9</span> <span class="content">Can get std::function from &lt;invocable&gt;</span></a>
      </li><li><a href="#remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"><span class="secno">5.10</span> <span class="content">Remove the null-check in the call operator and throwing of bad_function_call</span></a>
      </li><li><a href="#remove-the-null-check-in-constructors-that-are-not-nullptr_t"><span class="secno">5.11</span> <span class="content">Remove the null-check in constructors that are not nullptr_t</span></a>
      </li><li><a href="#perfect-forwarding-for-converting-constructor-instead-of-by-value"><span class="secno">5.12</span> <span class="content">Perfect forwarding for converting constructor instead of by-value</span></a>
      </li><li><a href="#forward-to-lwg-for-c20"><span class="secno">5.13</span> <span class="content">Forward to LWG for C++20</span></a>
     </li></ol>
    </li><li><a href="#Impl"><span class="secno">6</span> <span class="content">Implementation Experience</span></a>
    </li><li><a href="#Ack"><span class="secno">7</span> <span class="content">Acknowledgments</span></a>
    </li><li><a href="#References"><span class="secno">8</span> <span class="content">References</span></a>
   </li></ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="History"><span class="secno">1. </span><span class="content">Brief History</span><a class="self-link" href="#History"></a></h2>
   <p>This paper started as a proposal by David Krauss, N4543[1], from 2015 and there has
been an open issue in the LEWG bugzilla requesting such a facility since 2014[2].</p>
   <p>
    Since then, the paper has gone through 4 revisions and has been considered in
small groups in LEWG multiple times. Gradual feedback has led to the conservative
proposal seen here. The most-recent draft prior to this was a late-paper written
and presented by Ryan McDougall in LEWGI in San Diego[3]. It included multiple
references to implementations of move-only functions and made a strong case for
the importance of a move-only form of 
    <tt>std::function</tt>
    .
   </p>
   <p>Feebdack given was encouragement for targetting C++20.</p>
   <p>An updated version of that paper was presented on Saturday at the end of the
San Diego meeting. Poll results from that presentation are presented after the
overview in this document.</p>
   <p>The revision was presented in Kona, receiving additional polls and feedback, and
was to be forwarded to LWG pending updates reflecting additional poll results. Those
changes have been applied to the wording in this paper. Polls from the LEWG Kona
review are also provided in this document.</p>
   <h2 class="heading settled" data-level="2" id="Overview"><span class="secno">2. </span><span class="content">Overview</span><a class="self-link" href="#Overview"></a></h2>
   <p>
    This conservative 
    <tt>any_invocable</tt>
     is intended to be the same as 
    <tt>std::function</tt>
    ,
with the exceptions of the following:
   </p>
   <ol>
    <li data-md="">
     <p>It is move-only.</p>
    </li><li data-md="">
     <p>
      It does not have the const-correctness bug of 
      <tt>std::function</tt>
       detailed in n4348.[4]
     </p>
    </li><li data-md="">
     <p>It provides support for cv/ref/noexcept qualified function types.</p>
    </li><li data-md="">
     <p>
      It does not have the 
      <tt>target_type</tt>
       and 
      <tt>target</tt>
       accessors (direction requested by users and implementors).
     </p>
    </li><li data-md="">
     <p>Invocation has strong preconditions.</p>
   </li></ol>
   <h2 class="heading settled" data-level="3" id="Specification"><span class="secno">3. </span><span class="content">Specification</span><a class="self-link" href="#Specification"></a></h2>
   <p>The following is relative to N4820.[5]</p>
   <p>
    Add 
    <tt>&lt;any_invocable&gt;</tt>
     to [tab:headers.cpp] — C++ library headers
   </p>
   <p>Add the following entry to [version.syn#2]:</p>
   <tt>#define __cpp_lib_any_invocable   xxxxxxL // also in &lt;any_invocable&gt;</tt>
   <p>
    Make it so that 
    <tt>&lt;functional&gt;</tt>
     is guaranteed to include 
    <tt>&lt;any_invocable&gt;</tt>
     by adding the following at the start of the synopsis of [functional.syn]:
   </p>
   <blockquote>
<pre>        <b>20.14.1  Header &lt;functional&gt; synopsis                         [functional.syn]</b>

         <ins>#include &lt;any_invocable&gt; // see [inv.syn]
</ins>
          namespace std {
</pre>
   </blockquote>
   <p>Insert the following section at the end of <b>Function Objects</b> [function.objects], where SECTION is a placeholder for the root of the section numbering:</p>
   <blockquote>
<pre>        <b>SECTION Storage for any callable                                         [inv]</b>

     1  This subclause describes components that C++ programs may use to perform
        operations on callable objects of a given signature type.


        <b>SECTION.1 Header &lt;any_invocable&gt; synopsis                            [inv.syn]</b>

     1  The header <tt>&lt;any_invocable&gt;</tt> provides partial specializations of <tt>any_invocable</tt> for
        each combination of the possible replacements of the placeholders <tt><i>cv</i></tt>, <tt><i>ref</i></tt>, and
        <tt><i>noex</i></tt> where:

 (1.1)   — <tt><i>cv</i></tt> is either <tt>const</tt> or empty.

 (1.2)   — <tt><i>ref</i></tt> is either <tt>&amp;</tt>, <tt>&amp;&amp;</tt>, or empty.

 (1.3)   — <tt><i>noex</i></tt> is either <tt>true</tt> or <tt>false</tt>.

     2  For each of the possible combinations of the placeholders mentioned above,
        there is a placeholder <tt><i>inv-quals</i></tt> defined as follows:

 (2.1)   — If <tt><i>ref</i></tt> is empty, let <tt><i>inv-quals</i></tt> be <tt><i>cv</i>&amp;</tt>

 (2.2)   — Otherwise, let <tt><i>inv-quals</i></tt> be <tt><i>cv ref</i></tt>.

<tt>          namespace std {
            template&lt;class... S&gt; class any_invocable; // not defined

            template&lt;class R, class... ArgTypes&gt;
              class any_invocable&lt;R(ArgTypes...) <i>cv</i> <i>ref</i> noexcept(<i>noex</i>)&gt;;
          }</tt>


        <b>SECTION.2 Class template any_invocable                              [inv.class]</b>

<tt>          namespace std {
            template&lt;class... S&gt; class any_invocable; // not defined

            template&lt;class R, class... ArgTypes&gt;
            class any_invocable&lt;R(ArgTypes...) <i>cv</i> <i>ref</i> noexcept(<i>noex</i>)&gt; {
            public:
              using result_type = R;

              // SECTION.3, construct/move/destroy
              any_invocable() noexcept;
              any_invocable(nullptr_t) noexcept;
              any_invocable(any_invocable&amp;&amp;) noexcept;
              template&lt;class F&gt; any_invocable(F&amp;&amp;);

              template&lt;class T, class... Args&gt;
                explicit any_invocable(in_place_type_t&lt;T&gt;, Args&amp;&amp;...);
              template&lt;class T, class U, class... Args&gt;
                explicit any_invocable(in_place_type_t&lt;T&gt;, initializer_list&lt;U&gt;, Args&amp;&amp;...);
    
              any_invocable&amp; operator=(any_invocable&amp;&amp;);
              any_invocable&amp; operator=(nullptr_t) noexcept;
              template&lt;class F&gt; any_invocable&amp; operator=(F&amp;&amp;);

              ~any_invocable();

              // SECTION.4, any_invocable invocation
              explicit operator bool() const noexcept;

              R operator()(ArgTypes...) <i>cv</i> <i>ref</i> noexcept(<i>noex</i>);

              // SECTION.5, any_invocable utility
              void swap(any_invocable&amp;) noexcept;

              friend void swap(any_invocable&amp;, any_invocable&amp;) noexcept;

              friend bool operator==(const any_invocable&amp;, nullptr_t) noexcept;

            private:
              template&lt;class VT&gt;
                static constexpr bool is-callable-from = see below; // exposition-only
            };
          }</tt>

     1  The <tt>any_invocable</tt> class template provides polymorphic wrappers that generalize
        the notion of a callable object [func.def]. These wrappers can store, move, and
        call arbitrary callable objects, given a call signature, allowing functions to
        be first-class objects.

     2  Implementations are encouraged to avoid the use of dynamically allocated memory
        for a small contained value.

        [ Note: Such small-object optimization can only be applied to types <tt>T</tt> for
        which <tt>is_nothrow_move_constructible_v&lt;T&gt;</tt> is <tt>true</tt>. -- end note ]


        <b>SECTION.3  Constructors and destructor                              [inv.con]</b>

        <tt>template&lt;class VT&gt;
              static constexpr bool is-callable-from = see below; // exposition-only</tt>

     1  If <tt><i>noex</i></tt> is true, <tt>is-callable-from&lt;VT&gt;</tt> is equal to
          <tt>is_nothrow_invocable_r_v&lt;R, VT <i>cv ref</i>, Args...&gt; &amp;&amp;
            is_nothrow_invocable_r_v&lt;R, VT <i>inv-quals</i>, Args...&gt;</tt>.
        Otherwise, <tt>is-callable-from&lt;VT&gt;</tt> is equal to
          <tt>is_invocable_r_v&lt;R, VT <i>cv ref</i>, Args...&gt; &amp;&amp;
            is_invocable_r_v&lt;R, VT <i>inv-quals</i>, Args...&gt;</tt>.

        <tt>any_invocable() noexcept;</tt>
        <tt>any_invocable(nullptr_t) noexcept;</tt>

     2        <em>Postconditions:</em> <tt>*this</tt> has no target object.
 
        <tt>any_invocable(any_invocable&amp;&amp; f) noexcept;</tt>
 
     3        <em>Postconditions:</em> The target object of <tt>*this</tt> is the target object <tt>f</tt> had before
              construction, and <tt>f</tt> is in a valid state with an unspecified value.

        <tt>template&lt;class F&gt; any_invocable(F&amp;&amp; f);</tt>

     4        Let <tt>VT</tt> be <tt>decay_t&lt;F&gt;</tt>.

     5        <em>Constraints:</em>

 (5.1)            — <tt>remove_cvref_t&lt;F&gt;</tt> is not the same type as <tt>any_invocable</tt>, and

 (5.2)            — <tt>remove_cvref_t&lt;F&gt;</tt> is not a specialization of <tt>in_place_type_t</tt>, and

 (5.3)            — <tt>is_constructible_v&lt;VT, F&gt;</tt> is <tt>true</tt>, and

 (5.4)            — <tt>is-callable-from&lt;VT&gt;</tt> is <tt>true</tt>.

     6        <em>Preconditions:</em> <tt>VT</tt> meets the <i>Cpp17Destructible</i> requirements, and if
              <tt>is_move_constructible_v&lt;VT&gt;</tt> is <tt>true</tt>, <tt>VT</tt> meets the <i>Cpp17MoveConstructible</i>
              requirements.

     7        <em>Postconditions:</em> <tt>*this</tt> has a target object unless any of the following hold:

 (7.1)            — <tt>f</tt> is a null function pointer value, or

 (7.2)            — <tt>f</tt> is a null member pointer value, or

 (7.3)            — <tt>remove_cvref_t&lt;F&gt;</tt> is a specialization of the <tt>any_invocable</tt> class template,
                     and <tt>f</tt> has no target object.

     8        Otherwise, <tt>*this</tt> targets an object of type <tt>VT</tt>
              direct-non-list-initialized with <tt>std::forward&lt;F&gt;(f)</tt>.

     9        <em>Throws:</em> Any exception thrown by the initialization of the target object.
              May throw <tt>bad_alloc</tt> unless <tt>VT</tt> is a function pointer or a specialization of
              <tt>reference_wrapper</tt>.

        <tt>template&lt;class T, class... Args&gt;
            explicit any_invocable(in_place_type_t&lt;T&gt;, Args&amp;&amp;... args);</tt>

    10        Let <tt>VT</tt> be <tt>decay_t&lt;T&gt;</tt>.

    11        <em>Constraints:</em>

(11.1)            — <tt>is_constructible_v&lt;VT, Args...&gt;</tt> is <tt>true</tt>, and

(11.2)            — <tt>is-callable-from&lt;VT&gt;</tt> is <tt>true</tt>.

    12        <em>Mandates:</em> <tt>VT</tt> is the same type as <tt>T</tt>.

    13        <em>Preconditions:</em> <tt>VT</tt> meets the <i>Cpp17Destructible</i> requirements, and if
              <tt>is_move_constructible_v&lt;VT&gt;</tt> is <tt>true</tt>, <tt>VT</tt> meets the <i>Cpp17MoveConstructible</i>
              requirements.

    14        <em>Postconditions:</em> <tt>*this</tt> targets an object of type <tt>VT</tt>
              direct-non-list-initialized with <tt>std::forward&lt;Args&gt;(args)...</tt>.

    15        <em>Throws:</em> Any exception thrown by the initialization of the target object.
              May throw <tt>bad_alloc</tt> unless <tt>VT</tt> is a function pointer or a specialization of
              <tt>reference_wrapper</tt>.

        <tt>template&lt;class T, class U, class... Args&gt;
            explicit any_invocable(in_place_type_t&lt;T&gt;, initializer_list&lt;U&gt; ilist, Args&amp;&amp;... args);</tt>

    16        Let <tt>VT</tt> be <tt>decay_t&lt;T&gt;</tt>.

    17        <em>Constraints:</em>

(17.1)            — <tt>is_constructible_v&lt;VT, initializer_list&lt;U&gt;&amp;, ArgTypes...&gt;</tt> is <tt>true</tt>, and

(17.2)            — <tt>is-callable-from&lt;VT&gt;</tt> is <tt>true</tt>.

    18        <em>Mandates:</em> <tt>VT</tt> is the same type as <tt>T</tt>.

    19        <em>Preconditions:</em> <tt>VT</tt> meets the <i>Cpp17Destructible</i> requirements, and if
              <tt>is_move_constructible_v&lt;VT&gt;</tt> is <tt>true</tt>, <tt>VT</tt> meets the <i>Cpp17MoveConstructible</i>
              requirements.

    20        <em>Postconditions:</em> <tt>*this</tt> targets an object of type <tt>VT</tt>
              direct-non-list-initialized with <tt>ilist, std::forward&lt;ArgTypes&gt;(args)...</tt>.

    21        <em>Throws:</em> Any exception thrown by the initialization of the target object.
              May throw <tt>bad_alloc</tt> unless <tt>VT</tt> is a function pointer or a specialization of
              <tt>reference_wrapper</tt>.

        <tt>any_invocable&amp; operator=(any_invocable&amp;&amp; f);</tt>

    22        <em>Effects:</em> Equivalent to: <tt>any_invocable(std::move(f)).swap(*this);</tt>

    23        <em>Returns:</em> <tt>*this</tt>.

        <tt>any_invocable&amp; operator=(nullptr_t) noexcept;</tt>

    24        <em>Effects:</em> Destroys the target object of <tt>*this</tt>, if any.

    25        <em>Returns:</em> <tt>*this</tt>.

        <tt>template&lt;class F&gt; any_invocable&amp; operator=(F&amp;&amp; f);</tt>

    26        <em>Effects:</em> Equivalent to: <tt>any_invocable(std::forward&lt;F&gt;(f)).swap(*this);</tt>

    27        <em>Returns:</em> <tt>*this</tt>.


        <tt>~any_invocable()</tt>;

    28        <em>Effects:</em> Destroys the target object of <tt>*this</tt>, if any.

         <b>SECTION.4  Invocation                                              [inv.inv]</b>

        <tt>explicit operator bool() const noexcept;</tt>

     1        <em>Returns:</em> <tt>true</tt> if <tt>*this</tt> has a target object, otherwise <tt>false</tt>.

        <tt>R operator()(ArgTypes... args) <i>cv</i> <i>ref</i> noexcept(<i>noex</i>);</tt>

     2        <em>Preconditions:</em> <tt>*this</tt> has a target object.

     3        <em>Effects:</em> Equivalent to:
              <tt>return INVOKE&lt;R&gt;(static_cast&lt;F <i>inv-quals</i>&gt;(f), std::forward&lt;ArgTypes&gt;(args)...);</tt>
              where <tt>f</tt> is the target object of <tt>*this</tt> and <tt>f</tt> is lvalue of type <tt>F</tt>.

         <b>SECTION.5  Utility                                                 [inv.util]</b>

        <tt>void swap(any_invocable&amp; other) noexcept;</tt>

     1        <em>Effects:</em> Exchanges the targets of <tt>*this</tt> and <tt>other</tt>.

        <tt>friend void swap(any_invocable&amp; f1, any_invocable&amp; f2) noexcept;</tt>

     2        <em>Effects</em>: Equivalent to: <tt>f1.swap(f2)</tt>.

        <tt>friend bool operator==(const any_invocable&amp; f, nullptr_t) noexcept;</tt>

     3        <em>Returns:</em> <tt>true</tt> if <tt>f</tt> has no target object.

</pre>
   </blockquote>
   <h2 class="heading settled" data-level="4" id="SD2018"><span class="secno">4. </span><span class="content">Polls from LEWG San Diego Review (2018)</span><a class="self-link" href="#SD2018"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="support-func-func-const-func"><span class="secno">4.1. </span><span class="content">Support func(), func() const, func() &amp;&amp;</span><a class="self-link" href="#support-func-func-const-func"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">6</c->  <c- mi="">6</c-> <c- mi="">2</c-> <c- mi="">1</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="4.2" id="support-func--only"><span class="secno">4.2. </span><span class="content">Support func() &amp;&amp; only</span><a class="self-link" href="#support-func--only"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">2</c->  <c- mi="">2</c-> <c- mi="">7</c-> <c- mi="">1</c-> <c- mi="">1</c->
</pre>
   <h3 class="heading settled" data-level="4.3" id="remove-target-target_type"><span class="secno">4.3. </span><span class="content">Remove target/target_type</span><a class="self-link" href="#remove-target-target_type"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">12</c-> <c- mi="">5</c-> <c- mi="">0</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="4.4" id="require-more-stuff-noexcept-const"><span class="secno">4.4. </span><span class="content">Require more stuff (noexcept, const&amp;&amp;, ...)</span><a class="self-link" href="#require-more-stuff-noexcept-const"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">0</c->  <c- mi="">1</c-> <c- mi="">8</c-> <c- mi="">6</c-> <c- mi="">0</c->
</pre>
   <p>Note that the final poll (require more stuff) was not due to members being
against the design, but because we could easily add those facilities in a later
standard without any breakage.</p>
   <h3 class="heading settled" data-level="4.5" id="name-options"><span class="secno">4.5. </span><span class="content">Name Options</span><a class="self-link" href="#name-options"></a></h3>
   <p>
    There was one final poll, which brought us to the name 
    <tt>any_invocable</tt>
    .
   </p>
<pre class="highlight"><c- mi="">3</c->  <c- n="">unique_function</c->
<c- mi="">3</c->  <c- n="">move_function</c->
<c- mi="">2</c->  <c- n="">move_only_function</c->
<c- mi="">7</c->  <c- n="">movable_function</c->
<c- mi="">8</c->  <c- n="">mfunction</c->
<c- mi="">10</c-> <c- n="">any_invocable</c->
<c- mi="">8</c->  <c- n="">mofun</c->
<c- mi="">8</c->  <c- n="">mofunc</c->
<c- mi="">0</c->  <c- n="">callback</c->
<c- mi="">4</c->  <c- n="">mvfunction</c->
<c- mi="">2</c->  <c- n="">func</c->
<c- mi="">0</c->  <c- n="">callable</c->
<c- mi="">2</c->  <c- n="">any_function</c->
</pre>
   <h2 class="heading settled" data-level="5" id="K2019"><span class="secno">5. </span><span class="content">Polls from LEWG Kona Review (2019)</span><a class="self-link" href="#K2019"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"><span class="secno">5.1. </span><span class="content">We want to spend time on this now in order to include it in C++20</span><a class="self-link" href="#we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">8</c->  <c- mi="">8</c-> <c- mi="">2</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="5.2" id="add-support-for-func-const-and-func"><span class="secno">5.2. </span><span class="content">Add support for func() const&amp; and func()&amp;</span><a class="self-link" href="#add-support-for-func-const-and-func"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">0</c->  <c- mi="">8</c-> <c- mi="">7</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="5.3" id="add-support-for-func-noexcept-x-all-of-the-above"><span class="secno">5.3. </span><span class="content">Add support for func() noexcept (x all of the above)</span><a class="self-link" href="#add-support-for-func-noexcept-x-all-of-the-above"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c->  <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">2</c->  <c- mi="">12</c-> <c- mi="">2</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="5.4" id="include-the-option-for-ctad"><span class="secno">5.4. </span><span class="content">Include the option for CTAD</span><a class="self-link" href="#include-the-option-for-ctad"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">0</c->  <c- mi="">1</c-> <c- mi="">5</c-> <c- mi="">9</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="5.5" id="name-callable-vs-any-invocable"><span class="secno">5.5. </span><span class="content">Name: callable vs any invocable</span><a class="self-link" href="#name-callable-vs-any-invocable"></a></h3>
<pre class="highlight"><c- n="">SC</c-> <c- n="">C</c-> <c- n="">N</c-> <c- n="">AI</c-> <c- n="">SAI</c->
<c- mi="">3</c->  <c- mi="">2</c-> <c- mi="">3</c-> <c- mi="">5</c->  <c- mi="">6</c->
</pre>
   <h3 class="heading settled" data-level="5.6" id="any_invocable-vs-invocable"><span class="secno">5.6. </span><span class="content">any_invocable vs invocable</span><a class="self-link" href="#any_invocable-vs-invocable"></a></h3>
<pre class="highlight"><c- n="">SAI</c-> <c- n="">AI</c-> <c- n="">N</c-> <c- n="">I</c-> <c- n="">SI</c->
<c- mi="">3</c->   <c- mi="">7</c->  <c- mi="">2</c-> <c- mi="">5</c-> <c- mi="">1</c->
</pre>
   <h3 class="heading settled" data-level="5.7" id="header-choice"><span class="secno">5.7. </span><span class="content">Header choice</span><a class="self-link" href="#header-choice"></a></h3>
<pre class="highlight"><c- mi="">7</c-> <c- o="">&lt;</c-><c- n="">functional</c-><c- o="">&gt;</c->
<c- mi="">11</c-> <c- o="">&lt;</c-><c- n="">any_invocable</c-><c- o="">&gt;</c->
<c- mi="">11</c-> <c- o="">&lt;</c-><c- n="">invocable</c-><c- o="">&gt;</c->
<c- mi="">3</c-> <c- o="">&lt;</c-><c- p="">()</c-><c- o="">&gt;</c->
</pre>
   <h3 class="heading settled" data-level="5.8" id="can-get-stdfunction-from-any_invocable"><span class="secno">5.8. </span><span class="content">Can get std::function from &lt;any_invocable&gt;</span><a class="self-link" href="#can-get-stdfunction-from-any_invocable"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">0</c->  <c- mi="">1</c-> <c- mi="">4</c-> <c- mi="">4</c-> <c- mi="">7</c->
</pre>
   <h3 class="heading settled" data-level="5.9" id="can-get-stdfunction-from-invocable"><span class="secno">5.9. </span><span class="content">Can get std::function from &lt;invocable&gt;</span><a class="self-link" href="#can-get-stdfunction-from-invocable"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">1</c->  <c- mi="">3</c-> <c- mi="">6</c-> <c- mi="">3</c-> <c- mi="">2</c->
</pre>
   <p>Decide on &lt;any_invocable&gt;. Unanimous for &lt;functional&gt; to pull it in,
even if in its own header.</p>
   <h3 class="heading settled" data-level="5.10" id="remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"><span class="secno">5.10. </span><span class="content">Remove the null-check in the call operator and throwing of bad_function_call</span><a class="self-link" href="#remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">8</c->  <c- mi="">2</c-> <c- mi="">1</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h3 class="heading settled" data-level="5.11" id="remove-the-null-check-in-constructors-that-are-not-nullptr_t"><span class="secno">5.11. </span><span class="content">Remove the null-check in constructors that are not nullptr_t</span><a class="self-link" href="#remove-the-null-check-in-constructors-that-are-not-nullptr_t"></a></h3>
<pre class="highlight"><c- n="">std</c-><c- o="">::</c-><c- n="">any_callable</c-><voidu00028voidu00029> <c- n="">ac</c-> <c- o="">=</c-> <c- n="">my_ptr_object</c-><c- p="">;</c->
<c- k="">if</c-><c- p="">(</c-><c- n="">ac</c-><c- p="">)</c->  <c- p="">{</c-> <c- d="">/* true even if my_ptr is nullptr */</c-> <c- p="">}</c->
</voidu00028voidu00029></pre>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">0</c->  <c- mi="">2</c-> <c- mi="">2</c-> <c- mi="">4</c-> <c- mi="">3</c->
</pre>
   <h3 class="heading settled" data-level="5.12" id="perfect-forwarding-for-converting-constructor-instead-of-by-value"><span class="secno">5.12. </span><span class="content">Perfect forwarding for converting constructor instead of by-value</span><a class="self-link" href="#perfect-forwarding-for-converting-constructor-instead-of-by-value"></a></h3>
   <p>Unanimous</p>
   <h3 class="heading settled" data-level="5.13" id="forward-to-lwg-for-c20"><span class="secno">5.13. </span><span class="content">Forward to LWG for C++20</span><a class="self-link" href="#forward-to-lwg-for-c20"></a></h3>
<pre class="highlight"><c- n="">SF</c-> <c- n="">F</c-> <c- n="">N</c-> <c- n="">A</c-> <c- n="">SA</c->
<c- mi="">8</c->  <c- mi="">5</c-> <c- mi="">0</c-> <c- mi="">0</c-> <c- mi="">0</c->
</pre>
   <h2 class="heading settled" data-level="6" id="Impl"><span class="secno">6. </span><span class="content">Implementation Experience</span><a class="self-link" href="#Impl"></a></h2>
   <p>
    There are many implementations of a move-only 
    <tt>std::function</tt>
     with a
design that is similar to this. What is presented is a conservative subset of
those implementations. The changes suggested in LEWG, though minimal, have not
been used in a large codebase.
   </p>
   <p>Previous revisions of this paper have included publicly accessible move-only
function implementations, notably including implementations in HPX, Folly, and
LLVM.</p>
   <h2 class="heading settled" data-level="7" id="Ack"><span class="secno">7. </span><span class="content">Acknowledgments</span><a class="self-link" href="#Ack"></a></h2>
   <p>Thanks to Tomasz Kamiński, Tim Song, and Nevin Liber for suggestions on wording simplifications.</p>
   <h2 class="heading settled" data-level="8" id="References"><span class="secno">8. </span><span class="content">References</span><a class="self-link" href="#References"></a></h2>
   <p>[1]: David Krauss: N4543 "A polymorphic wrapper for all Callable objects" <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4543.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4543.pdf</a></p>
   <p>[2]: Geoffrey Romer: "Bug 34 - Need type-erased wrappers for move-only callable objects" <a href="https://issues.isocpp.org/show_bug.cgi?id=34">https://issues.isocpp.org/show_bug.cgi?id=34</a></p>
   <p>[3]: Ryan McDougall: P0288R2 "The Need for std::unique_function" <a href="https://wg21.link/p0288r2">https://wg21.link/p0288r2</a></p>
   <p>[4]: Geoffrey Romer: N4348 "Making std::function safe for concurrency" <a href="https://api.csswg.org/bikeshed/www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4348.html">www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4348.html</a></p>
   <p>[5]: Richard Smith: N4820 "Working Draft, Standard for Programming Language C++" <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4820.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4820.pdf</a></p>
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