Topic: Enhanced fold expressions
Author: Roman Orlov <mr.gordon.freman@gmail.com>
Date: Wed, 8 Mar 2017 11:11:26 -0800 (PST)
Raw View
------=_Part_369_1573466444.1489000286871
Content-Type: multipart/alternative;
boundary="----=_Part_370_1023903247.1489000286872"
------=_Part_370_1023903247.1489000286872
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
At this moment fold expressions have 4 forms
1. ( pack op ... )
2. ( ... op pack )
3. ( pack op ... op init )
4. ( init op ... op pack )
=20
where 'op' is one of the following 32 binary operators:
+ - * / % =CB=86 & | << >> +=3D -=3D *=3D /=3D %=3D =CB=86=3D &=3D |=3D <<=
=3D >>=3D =3D =3D=3D !=3D < > <=3D >=3D &&=20
|| , .* ->*
It is insufficient for writing powerful fold expressions. So it is=20
impossible to find min/max element in compile-time sequence of integers.
template<typename T, T... Args>
constexpr T min(std::integer_sequence<T, Args...>)
{
return (... ??? Args); // what operator should be here?
}
template<typename T, T... Args>
constexpr T max(std::integer_sequence<T, Args...>)
{
return (... ??? Args); // what operator should be here?
}
To solve that we need binary function *f(x,y)* returning either *x* or *y*.=
=20
And there is no standard binary operator for doing this.
But we already have these functions in algorithm library=20
template< class T >=20
constexpr const T& min( const T& a, const T& b );
template< class T >=20
constexpr const T& max( const T& a, const T& b );
I suggest to extend the meaning of binary operator in fold expression. *Ope=
rator=20
is a function*, so any *constexpr binary function* could be used in fold=20
expression.
I've invented a little hack to proof the idea
#include <iostream>
#include <algorithm>
// it's a user defined type convertible to int
enum dummy_int: int {};=20
// now we can define operators for it
// it's a constexpr wrapper around min function
constexpr dummy_int const& operator << (dummy_int const& lhs, dummy_int=20
const& rhs)
{
return std::min(lhs, rhs);
}
// it's a constexpr wrapper around max function
constexpr dummy_int const& operator >> (dummy_int const& lhs, dummy_int=20
const& rhs)
{
return std::max(lhs, rhs);
}
template<typename T, T... Args>
constexpr T min(std::integer_sequence<T, Args...>)
{
return (... << dummy_int(Args)); // explicitly convert to dummy_int
}
template<typename T, T... Args>
constexpr T max(std::integer_sequence<T, Args...>)
{
return (... >> dummy_int(Args)); // explicitly convert to dummy_int
}
int main()
{
typedef std::integer_sequence<int, 3, 4, 1, 2> ints;
// to be sure in compile-time evaluation use the result as template=20
parameter
std::cout << std::integral_constant<int, min(ints())>::value << std::
endl;
std::cout << std::integral_constant<int, max(ints())>::value << std::
endl;
return 0;
}
The code would be simpler if I can write it like this
template<typename T, T... Args>
constexpr T min(std::integer_sequence<T, Args...>)
{
return (... std::min Args);
}
template<typename T, T... Args>
constexpr T max(std::integer_sequence<T, Args...>)
{
return (... std::max Args);=20
}
--=20
You received this message because you are subscribed to the Google Groups "=
ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e=
mail to std-proposals+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.com/a/isocpp=
..org/d/msgid/std-proposals/c2b847b0-dd26-4d79-aad7-8c808008c0a4%40isocpp.or=
g.
------=_Part_370_1023903247.1489000286872
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
<div dir=3D"ltr"><div>At this moment fold expressions have 4 forms<br></div=
><div><ol><li>( pack op ... )<br></li><li>( ... op pack )<br></li><li>( pac=
k op ... op init )<br></li><li>( init op ... op pack )<br></li></ol></div><=
div>where 'op' is one of the following 32 binary operators:</div><d=
iv><br></div><div><div class=3D"prettyprint" style=3D"background-color: rgb=
(250, 250, 250); border-color: rgb(187, 187, 187); border-style: solid; bor=
der-width: 1px; word-wrap: break-word;"><code class=3D"prettyprint"><div cl=
ass=3D"subprettyprint"><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: #660;" class=3D"styled-by-prettify">*</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #080;" =
class=3D"styled-by-prettify">/ % =CB=86 & | << >> +=3D -=3D=
*=3D /</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=3D=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">%=3D</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">=CB=86=3D</span><span style=3D"col=
or: #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"> </span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">|=3D</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify"><<=3D</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">>=
;>=3D</span><span 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"> </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">=3D=3D</span><span 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"> </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-prett=
ify">></span><span style=3D"color: #000;" class=3D"styled-by-prettify"> =
</span><span style=3D"color: #660;" class=3D"styled-by-prettify"><=3D</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span=
style=3D"color: #660;" class=3D"styled-by-prettify">>=3D</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">&&</span><span style=3D"co=
lor: #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"> </span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">.*<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">->*</span></div><=
/code></div></div><div><br></div><div>It is insufficient for writing powerf=
ul fold expressions. So it is impossible to find min/max element in compile=
-time sequence of integers.</div><div><br></div><div class=3D"prettyprint" =
style=3D"background-color: rgb(250, 250, 250); border-color: rgb(187, 187, =
187); border-style: solid; border-width: 1px; word-wrap: break-word;"><code=
class=3D"prettyprint"><div class=3D"subprettyprint"><span style=3D"color: =
#008;" class=3D"styled-by-prettify">template</span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify"><</span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">typename</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"> T</span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> T</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>...</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </spa=
n><span style=3D"color: #606;" class=3D"styled-by-prettify">Args</span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">></span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"c=
olor: #008;" class=3D"styled-by-prettify">constexpr</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"> T min</span><span style=3D"color:=
#660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">std</span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">::</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify">integer_sequence</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify"><</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify">T</span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">,</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </sp=
an><span style=3D"color: #606;" class=3D"styled-by-prettify">Args</span><sp=
an 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: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">(...</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">???</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"> </span><span style=3D"color: #606;" class=3D"styled-by-prettif=
y">Args</span><span style=3D"color: #660;" class=3D"styled-by-prettify">);<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><sp=
an style=3D"color: #800;" class=3D"styled-by-prettify">// what operator sho=
uld be here?</span><span style=3D"color: #000;" class=3D"styled-by-prettify=
"><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify">}</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"><br><br></spa=
n><span style=3D"color: #008;" class=3D"styled-by-prettify">template</span>=
<span style=3D"color: #660;" class=3D"styled-by-prettify"><</span><span =
style=3D"color: #008;" class=3D"styled-by-prettify">typename</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">,</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> T</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">...</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"> </span><span style=3D"color: #606;" class=3D"styled-by-p=
rettify">Args</span><span style=3D"color: #660;" class=3D"styled-by-prettif=
y">></span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br=
></span><span style=3D"color: #008;" class=3D"styled-by-prettify">constexpr=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T max</sp=
an><span style=3D"color: #660;" class=3D"styled-by-prettify">(</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: #000;" class=3D"styled-by-prettify">integer_sequence</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify"><</span><span style=3D"co=
lor: #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: #606;" class=3D"style=
d-by-prettify">Args</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">...>)</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"><br></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 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>return</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"> </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">???</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #606=
;" class=3D"styled-by-prettify">Args</span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">);</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"> </span><span style=3D"color: #800;" class=3D"styled-by-p=
rettify">// what operator should be here?</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">}</span></div></code></div><div><br></div><div>To =
solve that we need binary function <b>f(x,y)</b> returning either <b>x</b> =
or <b>y</b>. And there is no standard binary operator for doing this.</div>=
<div>But we already have these functions in algorithm library=C2=A0</div><d=
iv><br></div><div class=3D"prettyprint" style=3D"background-color: rgb(250,=
250, 250); border-color: rgb(187, 187, 187); border-style: solid; border-w=
idth: 1px; word-wrap: break-word;"><code class=3D"prettyprint"><div class=
=3D"subprettyprint"><span style=3D"color: #008;" class=3D"styled-by-prettif=
y">template</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
><</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </sp=
an><span style=3D"color: #008;" class=3D"styled-by-prettify">class</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> T </span><span sty=
le=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"styled-by-prettify">constexpr</span><span style=3D"color:=
#000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">const</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> T</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">&</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"> min</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">const</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">&</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> a</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">const</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> T</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">&</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
b </span><span style=3D"color: #660;" class=3D"styled-by-prettify">);</spa=
n><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><spa=
n style=3D"color: #008;" class=3D"styled-by-prettify">template</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">class</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> T </span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">></span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> <br></span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">constexpr</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prett=
ify">const</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> max</span><s=
pan 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">const</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> T</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">&</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> a</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">const<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">&</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> b </span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">);</span></div></code></div><di=
v><br></div><div>I suggest to extend the meaning of binary operator in fold=
expression. <b>Operator is a function</b>, so any <b>constexpr binary func=
tion</b> could be used in fold expression.</div><div>I've invented a li=
ttle hack to proof the idea</div><div><br></div><div class=3D"prettyprint" =
style=3D"background-color: rgb(250, 250, 250); border-color: rgb(187, 187, =
187); border-style: solid; border-width: 1px; word-wrap: break-word;"><code=
class=3D"prettyprint"><div class=3D"subprettyprint"><span style=3D"color: =
#800;" class=3D"styled-by-prettify">#include</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify"> </span><span style=3D"color: #080;" clas=
s=3D"styled-by-prettify"><iostream></span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"><br></span><span style=3D"color: #800;" clas=
s=3D"styled-by-prettify">#include</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> </span><span style=3D"color: #080;" class=3D"style=
d-by-prettify"><algorithm></span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br><br></span><span style=3D"color: #800;" class=
=3D"styled-by-prettify">// it's a user defined type convertible to int<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">enum</span><span =
style=3D"color: #000;" class=3D"styled-by-prettify"> dummy_int</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: #0=
08;" class=3D"styled-by-prettify">int</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">{};</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"> <br></span><span style=3D"color: #800;" class=3D"styled-by-pret=
tify">// now we can define operators for it</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #800;=
" class=3D"styled-by-prettify">// it's a constexpr wrapper around min f=
unction</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br=
></span><span style=3D"color: #008;" class=3D"styled-by-prettify">constexpr=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> dummy_int=
</span><span style=3D"color: #008;" class=3D"styled-by-prettify">const</sp=
an><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: #008;" class=3D"styled-by-prettify">operator</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"> </span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify">dummy_int </span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">const</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">&</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> lhs</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">,</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> dummy_=
int </span><span style=3D"color: #008;" class=3D"styled-by-prettify">const<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">&</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"> rhs</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=
: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" =
class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"color:=
#008;" class=3D"styled-by-prettify">return</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> std</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">::</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify">min</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y">lhs</span><span style=3D"color: #660;" class=3D"styled-by-prettify">,</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> rhs</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">);</span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">}</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #800;=
" class=3D"styled-by-prettify">// it's a constexpr wrapper around max f=
unction</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br=
></span><span style=3D"color: #008;" class=3D"styled-by-prettify">constexpr=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> dummy_int=
</span><span style=3D"color: #008;" class=3D"styled-by-prettify">const</sp=
an><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: #008;" class=3D"styled-by-prettify">operator</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"> </span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify">dummy_int </span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">const</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">&</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> lhs</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">,</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> dummy_=
int </span><span style=3D"color: #008;" class=3D"styled-by-prettify">const<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">&</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"> rhs</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=
: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" =
class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"color:=
#008;" class=3D"styled-by-prettify">return</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> std</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">::</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify">max</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y">lhs</span><span style=3D"color: #660;" class=3D"styled-by-prettify">,</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> rhs</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">);</span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">}</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #008;=
" class=3D"styled-by-prettify">template</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify"><</span><span style=3D"color: #008;" class=
=3D"styled-by-prettify">typename</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> T</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"> 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: #606;" class=3D"styled-by-prettify">Args</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"co=
lor: #008;" class=3D"styled-by-prettify">constexpr</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"> T min</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify">std</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">::</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify">integer_sequence</span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify"><</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify">T</span><span style=3D"color: #660;" class=3D"styled-by-prettif=
y">,</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </spa=
n><span style=3D"color: #606;" class=3D"styled-by-prettify">Args</span><spa=
n 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: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">(...</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify"><<</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> dummy_int</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">(</span><span style=3D"color: #606;" class=3D"styled-by-pr=
ettify">Args</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">));</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </sp=
an><span style=3D"color: #800;" class=3D"styled-by-prettify">// explicitly =
convert to dummy_int</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br><=
br></span><span style=3D"color: #008;" class=3D"styled-by-prettify">templat=
e</span><span style=3D"color: #660;" class=3D"styled-by-prettify"><</spa=
n><span style=3D"color: #008;" class=3D"styled-by-prettify">typename</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">...</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"> </span><span style=3D"color: #606;" class=3D"sty=
led-by-prettify">Args</span><span style=3D"color: #660;" class=3D"styled-by=
-prettify">></span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"><br></span><span style=3D"color: #008;" class=3D"styled-by-prettify">c=
onstexpr</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T=
max</span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</spa=
n><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"color: #000;" class=3D"styled-by-prettify">integer_sequence</span><span=
style=3D"color: #660;" class=3D"styled-by-prettify"><</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: #606;" class=3D"=
styled-by-prettify">Args</span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">...>)</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">{</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br=
>=C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prett=
ify">return</span><span style=3D"color: #000;" class=3D"styled-by-prettify"=
> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">(...</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">>></span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> dummy_int</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"co=
lor: #606;" class=3D"styled-by-prettify">Args</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">));</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"> </span><span style=3D"color: #800;" class=3D"s=
tyled-by-prettify">// explicitly convert to dummy_int</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color:=
#660;" class=3D"styled-by-prettify">}</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"><br><br></span><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">int</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"> main</span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"><br></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 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>typedef</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> s=
td</span><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify">integer_sequence=
</span><span style=3D"color: #660;" class=3D"styled-by-prettify"><</span=
><span style=3D"color: #008;" class=3D"styled-by-prettify">int</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: #0=
66;" class=3D"styled-by-prettify">3</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"> </span><span style=3D"color: #066;" class=3D"styled-by-pre=
ttify">4</span><span style=3D"color: #660;" class=3D"styled-by-prettify">,<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><sp=
an style=3D"color: #066;" class=3D"styled-by-prettify">1</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: #066;" =
class=3D"styled-by-prettify">2</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">></span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> ints</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">;</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
<br>=C2=A0 =C2=A0 </span><span style=3D"color: #800;" class=3D"styled-by-pr=
ettify">// to be sure in compile-time evaluation use the result as template=
parameter</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
<br>=C2=A0 =C2=A0 std</span><span style=3D"color: #660;" class=3D"styled-by=
-prettify">::</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y">cout </span><span style=3D"color: #660;" class=3D"styled-by-prettify">&l=
t;<</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"color: #000;" class=3D"styled-by-prettify">integral_constant<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify"><</span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">int</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify"> min</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify">ints</span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">())>::</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify">value </span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify"><<</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> std</span><span style=3D"color: #660;" class=3D"styled-by-pr=
ettify">::</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
endl</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;</spa=
n><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=
=A0 std</span><span style=3D"color: #660;" class=3D"styled-by-prettify">::<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify">cout </span=
><span style=3D"color: #660;" class=3D"styled-by-prettify"><<</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> std</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify">integral_constant</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify"><</span><span style=3D=
"color: #008;" class=3D"styled-by-prettify">int</span><span style=3D"color:=
#660;" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"> max</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify">ints</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">())>::</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify">value </span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy"><<</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"color: #000;" class=3D"styled-by-prettify">endl</span><=
span style=3D"color: #660;" class=3D"styled-by-prettify">;</span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><s=
pan style=3D"color: #008;" class=3D"styled-by-prettify">return</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: #6=
60;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">}</span></div></code></div><div><br></div><div>The code =
would be simpler if I can write it like this</div><div><br></div><div class=
=3D"prettyprint" style=3D"background-color: rgb(250, 250, 250); border-colo=
r: rgb(187, 187, 187); border-style: solid; border-width: 1px; word-wrap: b=
reak-word;"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span=
style=3D"color: #008;" class=3D"styled-by-prettify">template</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify"><</span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">typename</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">,</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"sty=
led-by-prettify"> </span><span style=3D"color: #606;" class=3D"styled-by-pr=
ettify">Args</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"styled-by-prettify">constexpr<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T min</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify">std</span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"color: =
#000;" class=3D"styled-by-prettify">integer_sequence</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify"><</span><span style=3D"color: =
#000;" class=3D"styled-by-prettify">T</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"> </span><span style=3D"color: #606;" class=3D"styled-by-p=
rettify">Args</span><span style=3D"color: #660;" class=3D"styled-by-prettif=
y">...>)</span><span style=3D"color: #000;" class=3D"styled-by-prettify"=
><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=
=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">return=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">(...</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> std</span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"color: =
#000;" class=3D"styled-by-prettify">min </span><span style=3D"color: #606;"=
class=3D"styled-by-prettify">Args</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">);</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-=
prettify">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"=
><br><br></span><span style=3D"color: #008;" class=3D"styled-by-prettify">t=
emplate</span><span style=3D"color: #660;" class=3D"styled-by-prettify"><=
;</span><span style=3D"color: #008;" class=3D"styled-by-prettify">typename<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">...</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"> </span><span style=3D"color: #606;" class=
=3D"styled-by-prettify">Args</span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">></span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"><br></span><span style=3D"color: #008;" class=3D"styled-by-pret=
tify">constexpr</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> T max</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>(</span><span style=3D"color: #000;" class=3D"styled-by-prettify">std</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span><span =
style=3D"color: #000;" class=3D"styled-by-prettify">integer_sequence</span>=
<span style=3D"color: #660;" class=3D"styled-by-prettify"><</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: #0=
00;" class=3D"styled-by-prettify"> </span><span style=3D"color: #606;" clas=
s=3D"styled-by-prettify">Args</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">...>)</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"><br></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 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">return</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">(..=
..</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> std</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span><span =
style=3D"color: #000;" class=3D"styled-by-prettify">max </span><span style=
=3D"color: #606;" class=3D"styled-by-prettify">Args</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">);</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> <br></span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify">}</span></div></code></div><div><br></div></div=
>
<p></p>
-- <br />
You received this message because you are subscribed to the Google Groups &=
quot;ISO C++ Standard - Future Proposals" group.<br />
To unsubscribe from this group and stop receiving emails from it, send an e=
mail to <a href=3D"mailto:std-proposals+unsubscribe@isocpp.org">std-proposa=
ls+unsubscribe@isocpp.org</a>.<br />
To post to this group, send email to <a href=3D"mailto:std-proposals@isocpp=
..org">std-proposals@isocpp.org</a>.<br />
To view this discussion on the web visit <a href=3D"https://groups.google.c=
om/a/isocpp.org/d/msgid/std-proposals/c2b847b0-dd26-4d79-aad7-8c808008c0a4%=
40isocpp.org?utm_medium=3Demail&utm_source=3Dfooter">https://groups.google.=
com/a/isocpp.org/d/msgid/std-proposals/c2b847b0-dd26-4d79-aad7-8c808008c0a4=
%40isocpp.org</a>.<br />
------=_Part_370_1023903247.1489000286872--
------=_Part_369_1573466444.1489000286871--
.
Author: Arthur O'Dwyer <arthur.j.odwyer@gmail.com>
Date: Wed, 8 Mar 2017 14:58:17 -0800 (PST)
Raw View
------=_Part_2273_1087352596.1489013897864
Content-Type: multipart/alternative;
boundary="----=_Part_2274_525504152.1489013897865"
------=_Part_2274_525504152.1489013897865
Content-Type: text/plain; charset=UTF-8
On Wednesday, March 8, 2017 at 11:11:27 AM UTC-8, Roman Orlov wrote:
[TLDR: folding over arbitrary binary functions]
>
> I suggest to extend the meaning of binary operator in fold expression. *Operator
> is a function*, so any *constexpr binary function* could be used in fold
> expression.
> I've invented a little hack to proof the idea
>
(Minor point: constexpr is completely irrelevant to your use-case. You
should drop the mention of a constexpr requirement.)
This hack is not new. See e.g.
https://ngathanasiou.wordpress.com/2015/12/15/182/
http://stackoverflow.com/a/42587567/1424877
> template<typename T, T... Args>
> constexpr T min(std::integer_sequence<T, Args...>)
> {
> return (... << dummy_int(Args)); // explicitly convert to dummy_int
> }
>
Using my version <http://stackoverflow.com/a/42587567/1424877> of the hack,
you would write
template<typename T, T... Args>
constexpr T max(std::integer_sequence<T, Args...>)
{
auto mmax = make_named_operator([](auto&& a, auto&& b){ return std::max(
a,b); });
return (... + mmax(Args));
}
Nick Athanasiou's original blog post takes the more functional-programming
approach: that what you should really write is
return foldl([](auto&& a, auto&& b){ return std::max(a,b); }, Args...);
The code would be simpler if I can write it like this
>
> template<typename T, T... Args>
> constexpr T max(std::integer_sequence<T, Args...>)
> {
> return (... std::max Args);
> }
>
Agreed that that might look simpler (and you could add a code comment with
that syntax if you think it'd help the reader); but I don't see how you
expect the compiler to figure out what you mean by that syntax,
unambiguously. For example, what is
return (... std::max (Args));
return (... std::max<int> Args);
return (... func_ptr Args);
return (... (*func_ptr) Args);
return (... func_ptr*Args);
return (... (*Args) Args);
Any time you take a piece of the grammar that currently accepts a closed
set of operators and propose changing it to accept an open set of
expressions, that's a red flag; or at least an indication that you need to
think really hard about whether there might be any corner cases.
FYI, Barry Revzin's P0573R0
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0573r0.html>
(which I am generally against) mentions (but does not propose) the syntax
[]std::max as a shortcut way of writing [](auto&&... args) ->
decltype(auto) { return std::max(args...); }. If we had that, not only
would the two alternatives presented above get a lot shorter, but you
wouldn't have to worry quite so much about the fact that the identifier
std::max by itself doesn't denote any particular *entity* (but rather just
an overload set of a bunch of otherwise unrelated entities). You could
design your syntax to work cleanly with []foo by design.
my $.02,
Arthur
--
You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/dd6502e6-274b-4de4-b31f-1fe9783456eb%40isocpp.org.
------=_Part_2274_525504152.1489013897865
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
<div dir=3D"ltr">On Wednesday, March 8, 2017 at 11:11:27 AM UTC-8, Roman Or=
lov wrote:<div>[TLDR: folding over arbitrary binary functions]=C2=A0<blockq=
uote class=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex;border-lef=
t: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"ltr"><div>I suggest to ex=
tend the meaning of binary operator in fold expression. <b>Operator is a fu=
nction</b>, so any <b>constexpr binary function</b> could be used in fold e=
xpression.</div><div>I've invented a little hack to proof the idea</div=
></div></blockquote><div><br></div><div>(Minor point: constexpr is complete=
ly irrelevant to your use-case. You should drop the mention of a constexpr =
requirement.)</div><div><br></div><div>This hack is not new. See e.g.</div>=
<div><a href=3D"https://ngathanasiou.wordpress.com/2015/12/15/182/">https:/=
/ngathanasiou.wordpress.com/2015/12/15/182/</a><br></div><div><a href=3D"ht=
tp://stackoverflow.com/a/42587567/1424877">http://stackoverflow.com/a/42587=
567/1424877</a><br></div><div>=C2=A0<br></div><blockquote class=3D"gmail_qu=
ote" style=3D"margin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padd=
ing-left: 1ex;"><div dir=3D"ltr"><div style=3D"background-color:rgb(250,250=
,250);border-color:rgb(187,187,187);border-style:solid;border-width:1px;wor=
d-wrap:break-word"><code><div><span style=3D"color:#008">template</span><sp=
an style=3D"color:#660"><</span><span style=3D"color:#008">typename</spa=
n><span style=3D"color:#000"> T</span><span style=3D"color:#660">,</span><s=
pan style=3D"color:#000"> T</span><span style=3D"color:#660">...</span><spa=
n style=3D"color:#000"> </span><span style=3D"color:#606">Args</span><span =
style=3D"color:#660">></span><span style=3D"color:#000"><br></span><span=
style=3D"color:#008">constexpr</span><span style=3D"color:#000"> T min</sp=
an><span style=3D"color:#660">(</span><span style=3D"color:#000">std</span>=
<span style=3D"color:#660">::</span><span style=3D"color:#000">integer_sequ=
ence</span><span style=3D"color:#660"><</span><span style=3D"color:#000"=
>T</span><span style=3D"color:#660">,</span><span style=3D"color:#000"> </s=
pan><span style=3D"color:#606">Args</span><span style=3D"color:#660">...>=
;)</span><span style=3D"color:#000"><br></span><span style=3D"color:#660">{=
</span><span style=3D"color:#000"><br>=C2=A0 =C2=A0 </span><span style=3D"c=
olor:#008">return</span><span style=3D"color:#000"> </span><span style=3D"c=
olor:#660">(...</span><span style=3D"color:#000"> </span><span style=3D"col=
or:#660"><<</span><span style=3D"color:#000"> dummy_int</span><span s=
tyle=3D"color:#660">(</span><span style=3D"color:#606">Args</span><span sty=
le=3D"color:#660">));</span><span style=3D"color:#000"> </span><span style=
=3D"color:#800">// explicitly convert to dummy_int</span><span style=3D"col=
or:#000"><br></span><span style=3D"color:#660">}</span></div></code></div><=
/div></blockquote><div><br></div><div>Using <a href=3D"http://stackoverflow=
..com/a/42587567/1424877">my version</a> of the hack, you would write</div><=
div><br></div><div><div class=3D"prettyprint" style=3D"background-color: rg=
b(250, 250, 250); border: 1px solid rgb(187, 187, 187); word-wrap: break-wo=
rd;"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span style=
=3D"color: #008;" class=3D"styled-by-prettify">template</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify"><</span><span style=3D"co=
lor: #008;" class=3D"styled-by-prettify">typename</span><span style=3D"colo=
r: #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"> T</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">...</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span><span style=3D"color: #606;" class=3D"styled-by-prettify">=
Args</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"styled-by-prettify">constexpr</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> T max</span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">(</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"color: #000;" c=
lass=3D"styled-by-prettify">integer_sequence</span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify"><</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">T</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: #606;" class=3D"styled-by-prettify"=
>Args</span><span style=3D"color: #660;" class=3D"styled-by-prettify">...&g=
t;)</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span><span=
style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </sp=
an><span style=3D"color: #008;" class=3D"styled-by-prettify">auto</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"> mmax </span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> make_named_operator</span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">([](</span><span styl=
e=3D"color: #008;" class=3D"styled-by-prettify">auto</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">&&</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify"> a</span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"styl=
ed-by-prettify">auto</span><span style=3D"color: #660;" class=3D"styled-by-=
prettify">&&</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"> b</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">){</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </spa=
n><span style=3D"color: #008;" class=3D"styled-by-prettify">return</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> std</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify">max</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify">a</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify">b</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 st=
yle=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">return</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">(...</span><span style=3D"co=
lor: #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"> mmax</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">(</span><span style=3D"color: #606;" class=3D"styled-by-=
prettify">Args</span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">));</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> <b=
r></span><span style=3D"color: #660;" class=3D"styled-by-prettify">}</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span></div>=
</code></div><div><br></div></div><div>Nick Athanasiou's original blog =
post takes the more functional-programming approach: that what you should r=
eally write is<br></div><div><br></div><div><div class=3D"prettyprint" styl=
e=3D"background-color: rgb(250, 250, 250); border: 1px solid rgb(187, 187, =
187); word-wrap: break-word;"><code class=3D"prettyprint"><div class=3D"sub=
prettyprint"><span style=3D"color: #000;" class=3D"styled-by-prettify">=C2=
=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>return</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> fo=
ldl</span><span style=3D"color: #660;" class=3D"styled-by-prettify">([](</s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">auto</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">&&</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"> a</span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;"=
class=3D"styled-by-prettify">auto</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">&&</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> b</span><span style=3D"color: #660;" class=3D"styl=
ed-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">r=
eturn</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> std<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify">max</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify">a</span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify">b</span><span style=3D"color: #660;" class=3D"style=
d-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: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #606;" class=3D"styled-by-prettify">Args</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">...);</span></div></code>=
</div><br></div><div><br></div><blockquote class=3D"gmail_quote" style=3D"m=
argin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;"=
><div dir=3D"ltr"><div>The code would be simpler if I can write it like thi=
s</div><div><br></div><div style=3D"background-color:rgb(250,250,250);borde=
r-color:rgb(187,187,187);border-style:solid;border-width:1px;word-wrap:brea=
k-word"><code><div><span style=3D"color:#008">template</span><span style=3D=
"color:#660"><</span><span style=3D"color:#008">typename</span><span sty=
le=3D"color:#000"> T</span><span style=3D"color:#660">,</span><span style=
=3D"color:#000"> T</span><span style=3D"color:#660">...</span><span style=
=3D"color:#000"> </span><span style=3D"color:#606">Args</span><span style=
=3D"color:#660">></span><span style=3D"color:#000"><br></span><span styl=
e=3D"color:#008">constexpr</span><span style=3D"color:#000"> T max</span><s=
pan style=3D"color:#660">(</span><span style=3D"color:#000">std</span><span=
style=3D"color:#660">::</span><span style=3D"color:#000">integer_sequence<=
/span><span style=3D"color:#660"><</span><span style=3D"color:#000">T</s=
pan><span style=3D"color:#660">,</span><span style=3D"color:#000"> </span><=
span style=3D"color:#606">Args</span><span style=3D"color:#660">...>)</s=
pan><span style=3D"color:#000"><br></span><span style=3D"color:#660">{</spa=
n><span style=3D"color:#000"><br>=C2=A0 =C2=A0 </span><span style=3D"color:=
#008">return</span><span style=3D"color:#000"> </span><span style=3D"color:=
#660">(...</span><span style=3D"color:#000"> std</span><span style=3D"color=
:#660">::</span><span style=3D"color:#000">max </span><span style=3D"color:=
#606">Args</span><span style=3D"color:#660">);</span><span style=3D"color:#=
000"> <br></span><span style=3D"color:#660">}</span></div></code></div></di=
v></blockquote><div><br></div><div>Agreed that that might look simpler (and=
you could add a code comment with that syntax if you think it'd help t=
he reader); but I don't see how you expect the compiler to figure out w=
hat you mean by that syntax, unambiguously. For example, what is</div></div=
><div><br></div><div>=C2=A0 =C2=A0 return (... std::max (Args));</div><div>=
=C2=A0 =C2=A0 return (... std::max<int> Args);</div><div>=C2=A0 =C2=
=A0 return (... func_ptr Args);</div><div>=C2=A0 =C2=A0 return (... (*func_=
ptr) Args);<br></div><div><div>=C2=A0 =C2=A0 return (... func_ptr*Args);</d=
iv><div>=C2=A0 =C2=A0 return (... (*Args) Args);<br></div></div><div><br></=
div><div>Any time you take a piece of the grammar that currently accepts a =
closed set of operators and propose changing it to accept an open set of ex=
pressions, that's a red flag; or at least an indication that you need t=
o think really hard about whether there might be any corner cases.</div><di=
v><br></div><div>FYI, Barry Revzin's <a href=3D"http://www.open-std.org=
/jtc1/sc22/wg21/docs/papers/2017/p0573r0.html">P0573R0</a> (which I am gene=
rally against) mentions (but does not propose) the syntax <font face=3D"cou=
rier new, monospace">[]std::max</font> as a shortcut way of writing <font f=
ace=3D"courier new, monospace">[](auto&&... args) -> decltype(au=
to) { return std::max(args...); }</font>. If we had that, not only would th=
e two alternatives presented above get a lot shorter, but you wouldn't =
have to worry quite so much about the fact that the identifier <font face=
=3D"courier new, monospace">std::max</font> by itself doesn't denote an=
y particular <i>entity</i>=C2=A0(but rather just an overload set of a bunch=
of otherwise unrelated entities). You could design your syntax to work cle=
anly with <font face=3D"courier new, monospace">[]foo</font> by design.</di=
v><div><br></div><div>my $.02,</div><div>Arthur</div></div>
<p></p>
-- <br />
You received this message because you are subscribed to the Google Groups &=
quot;ISO C++ Standard - Future Proposals" group.<br />
To unsubscribe from this group and stop receiving emails from it, send an e=
mail to <a href=3D"mailto:std-proposals+unsubscribe@isocpp.org">std-proposa=
ls+unsubscribe@isocpp.org</a>.<br />
To post to this group, send email to <a href=3D"mailto:std-proposals@isocpp=
..org">std-proposals@isocpp.org</a>.<br />
To view this discussion on the web visit <a href=3D"https://groups.google.c=
om/a/isocpp.org/d/msgid/std-proposals/dd6502e6-274b-4de4-b31f-1fe9783456eb%=
40isocpp.org?utm_medium=3Demail&utm_source=3Dfooter">https://groups.google.=
com/a/isocpp.org/d/msgid/std-proposals/dd6502e6-274b-4de4-b31f-1fe9783456eb=
%40isocpp.org</a>.<br />
------=_Part_2274_525504152.1489013897865--
------=_Part_2273_1087352596.1489013897864--
.