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Update html
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Peter Dimov
@ -2,22 +2,21 @@
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<head>
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<meta name="generator" content="Asciidoctor 2.0.9">
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<meta name="author" content="Peter Dimov">
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<title>Boost.Variant2: A never valueless variant type</title>
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@ -70,7 +69,7 @@ select{width:100%}
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@ -80,7 +79,7 @@ h2{font-size:1.6875em}
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@ -142,9 +144,9 @@ p a>code:hover{color:rgba(0,0,0,.9)}
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@ -153,8 +155,8 @@ p a>code:hover{color:rgba(0,0,0,.9)}
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@ -165,13 +167,13 @@ p a>code:hover{color:rgba(0,0,0,.9)}
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.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:#999}
|
||||
.listingblock:hover code[data-lang]::before{display:block}
|
||||
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|
||||
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
|
||||
table.pyhltable{border-collapse:separate;border:0;margin-bottom:0;background:none}
|
||||
table.pyhltable td{vertical-align:top;padding-top:0;padding-bottom:0;line-height:1.45}
|
||||
table.pyhltable td.code{padding-left:.75em;padding-right:0}
|
||||
pre.pygments .lineno,table.pyhltable td:not(.code){color:#999;padding-left:0;padding-right:.5em;border-right:1px solid #ddddd8}
|
||||
pre.pygments .lineno{display:inline-block;margin-right:.25em}
|
||||
table.pyhltable .linenodiv{background:none!important;padding-right:0!important}
|
||||
.prettyprint{background:#f7f7f8}
|
||||
pre.prettyprint .linenums{line-height:1.45;margin-left:2em}
|
||||
pre.prettyprint li{background:none;list-style-type:inherit;padding-left:0}
|
||||
pre.prettyprint li code[data-lang]::before{opacity:1}
|
||||
pre.prettyprint li:not(:first-child) code[data-lang]::before{display:none}
|
||||
table.linenotable{border-collapse:separate;border:0;margin-bottom:0;background:none}
|
||||
table.linenotable td[class]{color:inherit;vertical-align:top;padding:0;line-height:inherit;white-space:normal}
|
||||
table.linenotable td.code{padding-left:.75em}
|
||||
table.linenotable td.linenos{border-right:1px solid currentColor;opacity:.35;padding-right:.5em}
|
||||
pre.pygments .lineno{border-right:1px solid currentColor;opacity:.35;display:inline-block;margin-right:.75em}
|
||||
pre.pygments .lineno::before{content:"";margin-right:-.125em}
|
||||
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
|
||||
.quoteblock>.title{margin-left:-1.5em;margin-bottom:.75em}
|
||||
.quoteblock blockquote,.quoteblock blockquote p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
|
||||
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
|
||||
.quoteblock blockquote{margin:0;padding:0;border:0}
|
||||
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
|
||||
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
|
||||
.quoteblock .attribution{margin-top:.5em;margin-right:.5ex;text-align:right}
|
||||
.quoteblock .quoteblock{margin-left:0;margin-right:0;padding:.5em 0;border-left:3px solid rgba(0,0,0,.6)}
|
||||
.quoteblock .quoteblock blockquote{padding:0 0 0 .75em}
|
||||
.quoteblock .quoteblock blockquote::before{display:none}
|
||||
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
|
||||
.verseblock{margin:0 1em 1.25em}
|
||||
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
|
||||
.verseblock pre strong{font-weight:400}
|
||||
@ -251,13 +254,17 @@ table.pyhltable .linenodiv{background:none!important;padding-right:0!important}
|
||||
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
|
||||
.quoteblock .attribution br,.verseblock .attribution br{display:none}
|
||||
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
|
||||
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
|
||||
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
|
||||
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
|
||||
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
|
||||
.quoteblock.abstract blockquote,.quoteblock.abstract blockquote p{word-spacing:0;line-height:1.6}
|
||||
.quoteblock.abstract blockquote::before,.quoteblock.abstract p::before{display:none}
|
||||
.quoteblock.excerpt,.quoteblock .quoteblock{margin:0 0 1.25em;padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
|
||||
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
|
||||
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;text-align:left;margin-right:0}
|
||||
table.tableblock{max-width:100%;border-collapse:separate}
|
||||
p.tableblock:last-child{margin-bottom:0}
|
||||
td.tableblock>.content{margin-bottom:-1.25em}
|
||||
td.tableblock>.content>:last-child{margin-bottom:-1.25em}
|
||||
td.tableblock>.content>:last-child.sidebarblock{margin-bottom:0}
|
||||
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
|
||||
table.grid-all>thead>tr>.tableblock,table.grid-all>tbody>tr>.tableblock{border-width:0 1px 1px 0}
|
||||
table.grid-all>tfoot>tr>.tableblock{border-width:1px 1px 0 0}
|
||||
@ -269,8 +276,7 @@ table.grid-all>tbody>tr:last-child>.tableblock,table.grid-all>thead:last-child>t
|
||||
table.frame-all{border-width:1px}
|
||||
table.frame-sides{border-width:0 1px}
|
||||
table.frame-topbot,table.frame-ends{border-width:1px 0}
|
||||
table.stripes-all tr,table.stripes-odd tr:nth-of-type(odd){background:#f8f8f7}
|
||||
table.stripes-none tr,table.stripes-odd tr:nth-of-type(even){background:none}
|
||||
table.stripes-all tr,table.stripes-odd tr:nth-of-type(odd),table.stripes-even tr:nth-of-type(even),table.stripes-hover tr:hover{background:#f8f8f7}
|
||||
th.halign-left,td.halign-left{text-align:left}
|
||||
th.halign-right,td.halign-right{text-align:right}
|
||||
th.halign-center,td.halign-center{text-align:center}
|
||||
@ -282,7 +288,6 @@ tbody tr th{display:table-cell;line-height:1.6;background:#f7f8f7}
|
||||
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
|
||||
p.tableblock>code:only-child{background:none;padding:0}
|
||||
p.tableblock{font-size:1em}
|
||||
td>div.verse{white-space:pre}
|
||||
ol{margin-left:1.75em}
|
||||
ul li ol{margin-left:1.5em}
|
||||
dl dd{margin-left:1.125em}
|
||||
@ -313,8 +318,8 @@ td.hdlist1{font-weight:bold;padding-bottom:1.25em}
|
||||
.colist td:not([class]):first-child img{max-width:none}
|
||||
.colist td:not([class]):last-child{padding:.25em 0}
|
||||
.thumb,.th{line-height:0;display:inline-block;border:solid 4px #fff;-webkit-box-shadow:0 0 0 1px #ddd;box-shadow:0 0 0 1px #ddd}
|
||||
.imageblock.left,.imageblock[style*="float: left"]{margin:.25em .625em 1.25em 0}
|
||||
.imageblock.right,.imageblock[style*="float: right"]{margin:.25em 0 1.25em .625em}
|
||||
.imageblock.left{margin:.25em .625em 1.25em 0}
|
||||
.imageblock.right{margin:.25em 0 1.25em .625em}
|
||||
.imageblock>.title{margin-bottom:0}
|
||||
.imageblock.thumb,.imageblock.th{border-width:6px}
|
||||
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
|
||||
@ -341,37 +346,37 @@ div.unbreakable{page-break-inside:avoid}
|
||||
.overline{text-decoration:overline}
|
||||
.line-through{text-decoration:line-through}
|
||||
.aqua{color:#00bfbf}
|
||||
.aqua-background{background-color:#00fafa}
|
||||
.aqua-background{background:#00fafa}
|
||||
.black{color:#000}
|
||||
.black-background{background-color:#000}
|
||||
.black-background{background:#000}
|
||||
.blue{color:#0000bf}
|
||||
.blue-background{background-color:#0000fa}
|
||||
.blue-background{background:#0000fa}
|
||||
.fuchsia{color:#bf00bf}
|
||||
.fuchsia-background{background-color:#fa00fa}
|
||||
.fuchsia-background{background:#fa00fa}
|
||||
.gray{color:#606060}
|
||||
.gray-background{background-color:#7d7d7d}
|
||||
.gray-background{background:#7d7d7d}
|
||||
.green{color:#006000}
|
||||
.green-background{background-color:#007d00}
|
||||
.green-background{background:#007d00}
|
||||
.lime{color:#00bf00}
|
||||
.lime-background{background-color:#00fa00}
|
||||
.lime-background{background:#00fa00}
|
||||
.maroon{color:#600000}
|
||||
.maroon-background{background-color:#7d0000}
|
||||
.maroon-background{background:#7d0000}
|
||||
.navy{color:#000060}
|
||||
.navy-background{background-color:#00007d}
|
||||
.navy-background{background:#00007d}
|
||||
.olive{color:#606000}
|
||||
.olive-background{background-color:#7d7d00}
|
||||
.olive-background{background:#7d7d00}
|
||||
.purple{color:#600060}
|
||||
.purple-background{background-color:#7d007d}
|
||||
.purple-background{background:#7d007d}
|
||||
.red{color:#bf0000}
|
||||
.red-background{background-color:#fa0000}
|
||||
.red-background{background:#fa0000}
|
||||
.silver{color:#909090}
|
||||
.silver-background{background-color:#bcbcbc}
|
||||
.silver-background{background:#bcbcbc}
|
||||
.teal{color:#006060}
|
||||
.teal-background{background-color:#007d7d}
|
||||
.teal-background{background:#007d7d}
|
||||
.white{color:#bfbfbf}
|
||||
.white-background{background-color:#fafafa}
|
||||
.white-background{background:#fafafa}
|
||||
.yellow{color:#bfbf00}
|
||||
.yellow-background{background-color:#fafa00}
|
||||
.yellow-background{background:#fafa00}
|
||||
span.icon>.fa{cursor:default}
|
||||
a span.icon>.fa{cursor:inherit}
|
||||
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
|
||||
@ -380,7 +385,7 @@ a span.icon>.fa{cursor:inherit}
|
||||
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
|
||||
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
|
||||
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
|
||||
.conum[data-value]{display:inline-block;color:#fff!important;background-color:rgba(0,0,0,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
|
||||
.conum[data-value]{display:inline-block;color:#fff!important;background:rgba(0,0,0,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
|
||||
.conum[data-value] *{color:#fff!important}
|
||||
.conum[data-value]+b{display:none}
|
||||
.conum[data-value]::after{content:attr(data-value)}
|
||||
@ -393,7 +398,7 @@ p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
|
||||
p,blockquote,dt,td.content,span.alt{font-size:1.0625rem}
|
||||
p{margin-bottom:1.25rem}
|
||||
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
|
||||
.exampleblock>.content{background-color:#fffef7;border-color:#e0e0dc;-webkit-box-shadow:0 1px 4px #e0e0dc;box-shadow:0 1px 4px #e0e0dc}
|
||||
.exampleblock>.content{background:#fffef7;border-color:#e0e0dc;-webkit-box-shadow:0 1px 4px #e0e0dc;box-shadow:0 1px 4px #e0e0dc}
|
||||
.print-only{display:none!important}
|
||||
@page{margin:1.25cm .75cm}
|
||||
@media print{*{-webkit-box-shadow:none!important;box-shadow:none!important;text-shadow:none!important}
|
||||
@ -408,7 +413,7 @@ svg{max-width:100%}
|
||||
p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
|
||||
h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
|
||||
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
|
||||
#toc{border-bottom:1px solid #ddddd8!important;padding-bottom:0!important}
|
||||
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
|
||||
body.book #header{text-align:center}
|
||||
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
|
||||
body.book #header .details{border:0!important;display:block;padding:0!important}
|
||||
@ -479,55 +484,353 @@ body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-b
|
||||
<h2 id="overview">Overview</h2>
|
||||
<div class="sectionbody">
|
||||
<div class="paragraph">
|
||||
<p>This library implements a type-safe discriminated union (variant) type,
|
||||
<code>variant<T…​></code>, that almost conforms to the C++17 Standard’s
|
||||
<a href="http://en.cppreference.com/w/cpp/utility/variant"><code>std::variant<T…​></code></a>. The
|
||||
main differences between the two are:</p>
|
||||
</div>
|
||||
<div class="ulist">
|
||||
<ul>
|
||||
<li>
|
||||
<p><code>variant<T…​></code> does not have a valueless-by-exception state;</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>A converting constructor from, e.g. <code>variant<int, float></code> to
|
||||
<code>variant<float, double, int></code> is provided as an extension;</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>The reverse operation, going from <code>variant<float, double, int></code> to
|
||||
<code>variant<int, float></code> is provided as the member function <code>subset<U…​></code>.
|
||||
(This operation can throw if the current state of the variant cannot be
|
||||
represented.)</p>
|
||||
</li>
|
||||
<li>
|
||||
<p><code>variant<T…​></code> is not trivial when all contained types are trivial, as
|
||||
mandated by C++17.</p>
|
||||
</li>
|
||||
</ul>
|
||||
<p>This library implements a type-safe discriminated/tagged union type,
|
||||
<code>variant<T…​></code>, that is API-compatible with the C++17 Standard’s
|
||||
<a href="http://en.cppreference.com/w/cpp/utility/variant"><code>std::variant<T…​></code></a>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>To avoid the valueless-by-exception state, this implementation falls
|
||||
back to using double storage unless</p>
|
||||
</div>
|
||||
<div class="ulist">
|
||||
<ul>
|
||||
<li>
|
||||
<p>one of the alternatives is the type <code>monostate</code>,</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>one of the alternatives has a nonthrowing default constructor, or</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>all the contained types are nothrow move constructible.</p>
|
||||
</li>
|
||||
</ul>
|
||||
<p>A <code>variant<T1, T2, …​, Tn></code> variable can hold a value of any of the
|
||||
types <code>T1</code>, <code>T2</code>, …​, <code>Tn</code>. For example,
|
||||
<code>variant<int64_t, double, std::string></code> can hold an <code>int64_t</code> value, a
|
||||
<code>double</code> value, or a <code>string</code> value.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>If the first two bullets don’t hold, but the third does, the variant uses
|
||||
single storage, but <code>emplace</code> constructs a temporary and moves it into place
|
||||
if the construction of the object can throw. In case this is undesirable, one
|
||||
can force <code>emplace</code> into always constructing in-place by adding <code>monostate</code> as
|
||||
one of the alternatives.</p>
|
||||
<p>Such a type is sometimes called a "tagged union", because it’s roughly
|
||||
equivalent to</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>struct V
|
||||
{
|
||||
enum tag { tag_int64_t, tag_double, tag_string };
|
||||
|
||||
tag tag_;
|
||||
|
||||
union
|
||||
{
|
||||
int64_t i_;
|
||||
double d_;
|
||||
std::string s_;
|
||||
};
|
||||
};</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Variants can be used to represent dynamically-typed values. A configuration
|
||||
file of the form</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>server.host=test.example.com
|
||||
server.port=9174
|
||||
cache.max_load=0.7</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>can be represented as <code>std::map<std::string, variant<int64_t, double,
|
||||
std::string>></code>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Variants can also represent polymorphism. To take a classic example, a
|
||||
polymorphic collection of shapes:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>#define _USE_MATH_DEFINES
|
||||
#include <iostream>
|
||||
#include <vector>
|
||||
#include <memory>
|
||||
#include <cmath>
|
||||
|
||||
class Shape
|
||||
{
|
||||
public:
|
||||
|
||||
virtual ~Shape() = default;
|
||||
virtual double area() const = 0;
|
||||
};
|
||||
|
||||
class Rectangle: public Shape
|
||||
{
|
||||
private:
|
||||
|
||||
double width_, height_;
|
||||
|
||||
public:
|
||||
|
||||
Rectangle( double width, double height ):
|
||||
width_( width ), height_( height ) {}
|
||||
|
||||
virtual double area() const { return width_ * height_; }
|
||||
};
|
||||
|
||||
class Circle: public Shape
|
||||
{
|
||||
private:
|
||||
|
||||
double radius_;
|
||||
|
||||
public:
|
||||
|
||||
explicit Circle( double radius ): radius_( radius ) {}
|
||||
virtual double area() const { return M_PI * radius_ * radius_; }
|
||||
};
|
||||
|
||||
double total_area( std::vector<std::unique_ptr<Shape>> const & v )
|
||||
{
|
||||
double s = 0.0;
|
||||
|
||||
for( auto const& p: v )
|
||||
{
|
||||
s += p->area();
|
||||
}
|
||||
|
||||
return s;
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
std::vector<std::unique_ptr<Shape>> v;
|
||||
|
||||
v.push_back( std::unique_ptr<Shape>( new Circle( 1.0 ) ) );
|
||||
v.push_back( std::unique_ptr<Shape>( new Rectangle( 2.0, 3.0 ) ) );
|
||||
|
||||
std::cout << "Total area: " << total_area( v ) << std::endl;
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>can instead be represented as a collection of <code>variant<Rectangle, Circle></code>
|
||||
values. This requires the possible <code>Shape</code> types be known in advance, as is
|
||||
often the case. In return, we no longer need virtual functions, or to allocate
|
||||
the values on the heap with <code>new Rectangle</code> and <code>new Circle</code>:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>#define _USE_MATH_DEFINES
|
||||
#include <iostream>
|
||||
#include <vector>
|
||||
#include <cmath>
|
||||
|
||||
#include <boost/variant2/variant.hpp>
|
||||
using namespace boost::variant2;
|
||||
|
||||
struct Rectangle
|
||||
{
|
||||
double width_, height_;
|
||||
double area() const { return width_ * height_; }
|
||||
};
|
||||
|
||||
struct Circle
|
||||
{
|
||||
double radius_;
|
||||
double area() const { return M_PI * radius_ * radius_; }
|
||||
};
|
||||
|
||||
double total_area( std::vector<variant<Rectangle, Circle>> const & v )
|
||||
{
|
||||
double s = 0.0;
|
||||
|
||||
for( auto const& x: v )
|
||||
{
|
||||
s += visit( []( auto const& y ){ return y.area(); }, x );
|
||||
}
|
||||
|
||||
return s;
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
std::vector<variant<Rectangle, Circle>> v;
|
||||
|
||||
v.push_back( Circle{ 1.0 } );
|
||||
v.push_back( Rectangle{ 2.0, 3.0 } );
|
||||
|
||||
std::cout << "Total area: " << total_area( v ) << std::endl;
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>If we look at the</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code> v.push_back( Circle{ 1.0 } );</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>line, we can deduce that <code>variant<Rectangle, Circle></code> can be (implicitly)
|
||||
constructed from <code>Circle</code> (and <code>Rectangle</code>), and indeed it can. It can also
|
||||
be assigned a <code>Circle</code> or a <code>Rectangle</code>:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>variant<Rectangle, Circle> v = Circle{ 1.0 }; // v holds Circle
|
||||
v = Rectangle{ 2.0, 3.0 }; // v now holds Rectangle</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>If we try to construct <code>variant<int, float></code> from something that is neither
|
||||
<code>int</code> nor <code>float</code>, say, <code>(short)1</code>, the behavior is "as if" the <code>variant</code> has
|
||||
declared two constructors,</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>variant::variant(int x);
|
||||
variant::variant(float x);</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>and the standard overload resolution rules are used to pick the one that will
|
||||
be used. So <code>variant<int, float>((short)1)</code> will hold an <code>int</code>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Putting values into a <code>variant</code> is easy, but taking them out is necessarily a
|
||||
bit more convoluted. It’s not possible for <code>variant<int, float></code> to define a
|
||||
member function <code>get() const</code>, because such a function will need its return
|
||||
type fixed at compile time, and whether the correct return type is <code>int</code> or
|
||||
<code>float</code> will only become known at run time.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>There are a few ways around that. First, there is the accessor member function</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>std::size_t variant::index() const noexcept;</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>that returns the zero-based index of the current type. For <code>variant<int,
|
||||
float></code>, it will return <code>0</code> for <code>int</code> and <code>1</code> for <code>float</code>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Once we have the index, we can use the free function <code>get<N></code> to obtain the
|
||||
value. Since we’re passing the type index to <code>get</code>, it knows what to return.
|
||||
<code>get<0>(v)</code> will return <code>int</code>, and <code>get<1>(v)</code> will return <code>float</code>:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>void f( variant<int, float> const& v )
|
||||
{
|
||||
switch( v.index() )
|
||||
{
|
||||
case 0:
|
||||
|
||||
// use get<0>(v)
|
||||
break;
|
||||
|
||||
case 1:
|
||||
|
||||
// use get<1>(v)
|
||||
break;
|
||||
|
||||
default:
|
||||
|
||||
assert(false); // never happens
|
||||
}
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>If we call <code>get<0>(v)</code>, and <code>v.index()</code> is not currently <code>0</code>, an exception
|
||||
(of type <code>bad_variant_access</code>) will be thrown.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>An alternative approach is to use <code>get<int>(v)</code> or <code>get<float>(v)</code>. This
|
||||
works similarly.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Another alternative that avoids the possibility of <code>bad_variant_access</code> is
|
||||
to use <code>get_if</code>. Instead of a reference to the contained value, it returns
|
||||
a pointer to it, returning <code>nullptr</code> to indicate type mismatch. <code>get_if</code>
|
||||
takes a pointer to the <code>variant</code>, so in our example we’ll use something along
|
||||
the following lines:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>void f( variant<int, float> const& v )
|
||||
{
|
||||
if( int const * p = get_if<int>(&v) )
|
||||
{
|
||||
// use *p
|
||||
}
|
||||
else if( float const * p = get_if<float>(&v) )
|
||||
{
|
||||
// use *p
|
||||
}
|
||||
else
|
||||
{
|
||||
assert(false); // never happens
|
||||
}
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Last but not least, there’s <code>visit</code>. <code>visit(f, v)</code> calls the a function object
|
||||
<code>f</code> with the value contained in the <code>variant</code> <code>v</code> and returns the result. When
|
||||
<code>v</code> is <code>variant<int, float></code>, it will call <code>f</code> with either an <code>int</code> or a
|
||||
<code>float</code>. The function object must be prepared to accept both.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>In practice, this can be achieved by having the function take a type that can
|
||||
be passed either <code>int</code> or <code>float</code>, such as <code>double</code>:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>double f( double x ) { return x; }
|
||||
|
||||
double g( variant<int, float> const& v )
|
||||
{
|
||||
return visit( f, v );
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>By using a function object with an overloaded <code>operator()</code>:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>struct F
|
||||
{
|
||||
void operator()(int x) const { /* use x */ }
|
||||
void operator()(float x) const { /* use x */ }
|
||||
};
|
||||
|
||||
void g( variant<int, float> const& v )
|
||||
{
|
||||
visit( F(), v );
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Or by using a polymorphic lambda, as we did in our <code>Circle</code>/<code>Rectangle</code>
|
||||
example:</p>
|
||||
</div>
|
||||
<div class="listingblock">
|
||||
<div class="content">
|
||||
<pre class="highlight"><code>void g( variant<int, float> const& v )
|
||||
{
|
||||
visit( [&]( auto const& x ){ std::cout << x << std::endl; }, v );
|
||||
}</code></pre>
|
||||
</div>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p><code>visit</code> can also take more than one <code>variant</code>. <code>visit(f, v1, v2)</code> calls
|
||||
<code>f(x1, x2)</code>, where <code>x1</code> is the value contained in <code>v1</code> and <code>x2</code> is the value
|
||||
in <code>v2</code>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>The default constructor of <code>variant</code> value-initializes the first type in
|
||||
the list. <code>variant<int, float>{}</code> holds <code>0</code> (of type <code>int</code>), and
|
||||
<code>variant<float, int>{}</code> holds <code>0.0f</code>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>This is usually the desired behavior. However, in cases such as
|
||||
<code>variant<std::mutex, std::recursive_mutex></code>, one might legitimately wish to
|
||||
avoid constructing a <code>std::mutex</code> by default. A provided type, <code>monostate</code>,
|
||||
can be used as the first type in those scenarios. <code>variant<monostate,
|
||||
std::mutex, std::recursive_mutex></code> will default-construct a <code>monostate</code>,
|
||||
which is basically a no-op, as <code>monostate</code> is effectively an empty <code>struct</code>.</p>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
@ -2189,12 +2492,13 @@ the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Versi
|
||||
</div>
|
||||
<div id="footer">
|
||||
<div id="footer-text">
|
||||
Last updated 2019-02-22 19:21:28 GTBST
|
||||
Last updated 2019-05-11 17:49:15 +0300
|
||||
</div>
|
||||
</div>
|
||||
<style>
|
||||
|
||||
*:not(pre)>code { background: none; color: #600000; }
|
||||
:not(pre):not([class^=L])>code { background: none; color: #600000; }
|
||||
|
||||
</style>
|
||||
</body>
|
||||
|
||||
Reference in New Issue
Block a user