Topic: Add overloaded relational operators for std::reference_wrapper
Author: Lingxi Li <lilingxi.cs@gmail.com>
Date: Fri, 9 Oct 2015 01:20:47 -0700 (PDT)
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It turns out that with the current design, std::reference_wrapper does not
work in certain cases where it is expected to work. The issue can be
illustrated by the following example:
#include <functional>
#include <string>
int main() {
int x = 0, y = 0;
//std::string x, y;
auto ref_x = std::ref(x);
auto ref_y = std::ref(y);
ref_x < ref_y;
}
The code compiles fine when x and y are of type int, but does not when they
are of type std::string, despite the fact that
std::reference_wrapper<std::string> is implicitly convertible to
std::string& for which an overloaded operator< is defined. Such is
inconsistent behavior. The issue turns out to be caused by the template
nature of the overloaded operator< for std::basic_string, which prevents
implicit conversion from happening. Specifically, it is said in N4296
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf> 14.8.1
[temp.arg.explicit] para. 6 that
Implicit conversions (Clause 4) will be performed on a function argument to
> convert it to the type of the corresponding function parameter if the
> parameter type contains no template-parameters that participate in template
> argument deduction.
One resolution to this issue is discussed here
<https://groups.google.com/a/isocpp.org/d/topic/std-proposals/1pBkN5ZvLhg/discussion>.
But it is way too aggressive, requiring a lot of modifications. So, a new
resolution is proposed here that solves the most important cases and
requires just a small modification to the standard. Specifically, it is
proposed to add overloaded relational operators for std::reference_wrapper
as follows:
template<class T> bool operator==(const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() == y.get(); }
template<class T> bool operator!=(const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() != y.get(); }
template<class T> bool operator< (const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() < y.get(); }
template<class T> bool operator<=(const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() <= y.get(); }
template<class T> bool operator> (const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() > y.get(); }
template<class T> bool operator>=(const reference_wrapper<T>& x, const
reference_wrapper<T>& y) { return x.get() >= y.get(); }
This modification should improve consistency in the standard library.
--
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<div dir=3D"ltr"><div>It turns out that with the current design,<span style=
=3D"background-color: rgb(238, 238, 238);"><font face=3D"courier new, monos=
pace"> std::reference_wrapper</font></span> does not work in certain cases =
where it is expected to work. The issue can be illustrated by the following=
example:<br><br></div><div class=3D"prettyprint" style=3D"border: 1px soli=
d rgb(187, 187, 187); word-wrap: break-word; background-color: rgb(250, 250=
, 250);"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span st=
yle=3D"color: #800;" class=3D"styled-by-prettify">#include</span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"colo=
r: #080;" class=3D"styled-by-prettify"><functional></span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"co=
lor: #800;" class=3D"styled-by-prettify">#include</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #080;"=
class=3D"styled-by-prettify"><string></span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #008=
;" class=3D"styled-by-prettify">int</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> main</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettif=
y">{</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=
=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">int=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> x </span>=
<span style=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"c=
olor: #066;" class=3D"styled-by-prettify">0</span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> y </span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">=3D</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"> </span><span style=3D"color: #066;" class=3D"styled-by-prettify"=
>0</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 </span=
><span style=3D"color: #800;" class=3D"styled-by-prettify">//std::string x,=
y;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=
=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">auto</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify"> ref_x </spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span=
style=3D"color: #000;" class=3D"styled-by-prettify"> std</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"colo=
r: #008;" class=3D"styled-by-prettify">ref</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify">x</span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">);</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"><br>=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-pr=
ettify">auto</span><span style=3D"color: #000;" class=3D"styled-by-prettify=
"> ref_y </span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
=3D</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> std</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span><spa=
n style=3D"color: #008;" class=3D"styled-by-prettify">ref</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify">y</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">);</span><span style=3D"color: #000;" class=3D=
"styled-by-prettify"><br>=C2=A0 ref_x </span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify"><</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> ref_y</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">;</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"> =C2=A0<br></span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">}</span></div></code></div><div><br>The code compiles fine when=
<span style=3D"background-color: rgb(238, 238, 238);"><font face=3D"courie=
r new, monospace">x</font></span> and <font face=3D"courier new, monospace"=
color=3D"#000000" style=3D"background-color: rgb(238, 238, 238);">y</font>=
are of type <span style=3D"background-color: rgb(238, 238, 238);"><font fa=
ce=3D"courier new, monospace">int</font></span>, but does not when they are=
of type <font face=3D"courier new, monospace" style=3D"background-color: r=
gb(238, 238, 238);">std::string</font><font style=3D"background-color: rgb(=
255, 255, 255);" face=3D"arial, sans-serif">,</font> despite the fact that =
<span style=3D"background-color: rgb(238, 238, 238);"><font face=3D"courier=
new, monospace">std::reference_wrapper<std::string></font></span> is=
implicitly convertible to <span style=3D"background-color: rgb(238, 238, 2=
38);"><font face=3D"courier new, monospace">std::string&</font></span> =
for which an overloaded <font face=3D"courier new, monospace" style=3D"back=
ground-color: rgb(238, 238, 238);">operator<</font> is defined. Such is =
inconsistent behavior. The issue turns out to be caused by the template nat=
ure of the overloaded <span style=3D"background-color: rgb(238, 238, 238);"=
><font face=3D"courier new, monospace">operator<</font></span> for <span=
style=3D"background-color: rgb(238, 238, 238);"><font face=3D"courier new,=
monospace">std::basic_string</font></span>, which prevents implicit conver=
sion from happening. Specifically, it is said in=C2=A0<a href=3D"http://www=
..open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf" rel=3D"nofollow" t=
arget=3D"_blank" style=3D"cursor: pointer; font-family: Arial, sans-serif; =
font-size: 13.3333px;">N4296</a><span style=3D"font-family: Arial, sans-ser=
if; font-size: 13.3333px;">=C2=A014.8.1 [temp.arg.explicit] para. 6 that<br=
></span><br><blockquote class=3D"gmail_quote" style=3D"margin: 0px 0px 0px =
0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); borde=
r-left-style: solid; padding-left: 1ex;">Implicit conversions (Clause 4) wi=
ll be performed on a function argument to convert it to the type of the cor=
responding function parameter if the parameter type contains no template-pa=
rameters that participate in template argument deduction.</blockquote><br>O=
ne resolution to this issue is discussed <a href=3D"https://groups.google.c=
om/a/isocpp.org/d/topic/std-proposals/1pBkN5ZvLhg/discussion">here</a>. But=
it is way too aggressive, requiring a lot of modifications. So, a new reso=
lution is proposed here that solves the most important cases and requires j=
ust a small modification to the standard. Specifically, it is proposed to a=
dd overloaded relational operators for <font face=3D"courier new, monospace=
" style=3D"background-color: rgb(238, 238, 238);">std::reference_wrapper</f=
ont> as follows:<br><br><div class=3D"prettyprint" style=3D"border: 1px sol=
id rgb(187, 187, 187); word-wrap: break-word; background-color: rgb(250, 25=
0, 250);"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span s=
tyle=3D"color: #008;" class=3D"styled-by-prettify">template</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify"><</span><span style=3D"=
color: #008;" class=3D"styled-by-prettify">class</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"color: #660;"=
class=3D"styled-by-prettify">></span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">bool</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">=
operator</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
=3D=3D(</span><span style=3D"color: #008;" class=3D"styled-by-prettify">con=
st</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> referen=
ce_wrapper</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
<</span><span style=3D"color: #000;" class=3D"styled-by-prettify">T</spa=
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><span style=3D"color: #000;" class=3D"styled-by-prettify"> x</span><span s=
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class=3D"styled-by-prettify"><br></span><span style=3D"color: #008;" class=
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> reference_wrapper</span><span style=3D"color: #660;" class=3D"styled-by-p=
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y-prettify">.</span><span style=3D"color: #008;" class=3D"styled-by-prettif=
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rettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
(</span><span style=3D"color: #008;" class=3D"styled-by-prettify">const</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> reference_wra=
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class=3D"styled-by-prettify">const</span><span style=3D"color: #000;" class=
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led-by-prettify">>&</span><span style=3D"color: #000;" class=3D"styl=
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</span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span><s=
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=3D"color: #660;" class=3D"styled-by-prettify">();</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">}</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">template</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify"><</span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">class</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">>=
;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><=
span style=3D"color: #008;" class=3D"styled-by-prettify">bool</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"c=
olor: #008;" class=3D"styled-by-prettify">operator</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify"><=3D(</span><span style=3D"color=
: #008;" class=3D"styled-by-prettify">const</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> reference_wrapper</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify"><</span><span style=3D"color: #=
000;" class=3D"styled-by-prettify">T</span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">>&</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> x</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">c=
onst</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> refer=
ence_wrapper</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
"><</span><span style=3D"color: #000;" class=3D"styled-by-prettify">T</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">>&</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> y</span><span=
style=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"sty=
led-by-prettify">return</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> x</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">.</span><span style=3D"color: #008;" class=3D"styled-by-prettify">get<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">()</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=
=3D"color: #660;" class=3D"styled-by-prettify"><=3D</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> y</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">.</span><span style=3D"color: #008;" cl=
ass=3D"styled-by-prettify">get</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">();</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br><=
/span><span style=3D"color: #008;" class=3D"styled-by-prettify">template</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify"><</span><s=
pan style=3D"color: #008;" class=3D"styled-by-prettify">class</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"=
color: #660;" class=3D"styled-by-prettify">></span><span style=3D"color:=
#000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">bool</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">operator</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">></span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">(<=
/span><span style=3D"color: #008;" class=3D"styled-by-prettify">const</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"> reference_wrapp=
er</span><span style=3D"color: #660;" class=3D"styled-by-prettify"><</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify">T</span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">>&</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> x</span><span style=3D"=
color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" clas=
s=3D"styled-by-prettify">const</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> reference_wrapper</span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify"><</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify">T</span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">>&</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> y</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">)</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> x</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">.</span><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">get</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">></span><span style=3D"color: #000;" class=3D"styled-by-prettify"> y</=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">.</span><spa=
n style=3D"color: #008;" class=3D"styled-by-prettify">get</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">();</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">}</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">template</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify"><</span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">class</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">>=
;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><=
span style=3D"color: #008;" class=3D"styled-by-prettify">bool</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"c=
olor: #008;" class=3D"styled-by-prettify">operator</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify">>=3D(</span><span style=3D"color=
: #008;" class=3D"styled-by-prettify">const</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> reference_wrapper</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify"><</span><span style=3D"color: #=
000;" class=3D"styled-by-prettify">T</span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">>&</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> x</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">c=
onst</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> refer=
ence_wrapper</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
"><</span><span style=3D"color: #000;" class=3D"styled-by-prettify">T</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">>&</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> y</span><span=
style=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"sty=
led-by-prettify">return</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> x</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">.</span><span style=3D"color: #008;" class=3D"styled-by-prettify">get<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">()</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">>=3D</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> y</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">.</span><span style=3D"color: #008;" cl=
ass=3D"styled-by-prettify">get</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">();</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">}</span></div></code></div><div><br></div>This modification should impr=
ove consistency in the standard library.<br><br></div></div>
<p></p>
-- <br />
<br />
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.