Topic: Extending N4371 (incomplete type support for
Author: Zhihao Yuan <zy@miator.net>
Date: Thu, 19 Jul 2018 15:32:57 -0400
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That use case is not very strong, because extending
std::variant is easier. I have my own implementation
variant which supports an =E2=80=9CIncompleteType[]=E2=80=9D syntax
to spell the recursive wrapper. A recursive wrapper,
after all, has an identifiable type in the type list. The
incomplete type support for containers are =E2=80=9Cdirect,=E2=80=9D
indistinguishable from the complete element types.
--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
_______________________________________________
From: itsjimporter@gmail.com <itsjimporter@gmail.com>
Sent: Thursday, July 19, 2018 2:08 PM
struct json : std::variant<
double,
std::string,
std::vector<json>,
std::map<std::string, json>// not allowed (yet)...
>{
using std::variant</*...*/>::variant;
};
This would eliminate the need to use things like Boost's `make_recursive_va=
riant` for common cases like JSON (and similar) data types without requirin=
g full recursive-variant support in the standard. I'm happy to help push th=
is forward, but my understanding is that the main concern is what standard =
library implementers think of this.
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<html><head></head><body><div class=3D"WordSection1"><p class=3D"MsoNormal"=
>That use case is not very strong, because extending<br/>std::variant is ea=
sier.=C2=A0 I have my own implementation<br/>variant which supports an =E2=
=80=9CIncompleteType[]=E2=80=9D syntax<br/>to spell the recursive wrapper.=
=C2=A0 A recursive wrapper,<br/>after all, has an identifiable type in the =
type list.=C2=A0 The<br/>incomplete type support for containers are =E2=80=
=9Cdirect,=E2=80=9D<br/>indistinguishable from the complete element types.<=
br/><br/><o:p></o:p></p><p class=3D"MsoNormal">--<br/>Zhihao Yuan, ID lichr=
ay<br/>The best way to predict the future is to invent it.<br/>____________=
___________________________________<o:p></o:p></p><p class=3D"MsoNormal"><o=
:p>=C2=A0</o:p></p><div style=3D"border:none;border-left:solid blue 1.5pt;p=
adding:0in 0in 0in 4.0pt"><div><div style=3D"border:none;border-top:solid #=
E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in"><p class=3D"MsoNormal"><b>From:</b>=
itsjimporter@gmail.com <itsjimporter@gmail.com> <br/><b>Sent:</b> Th=
ursday, July 19, 2018 2:08 PM<o:p></o:p></p></div></div><div><div><p class=
=3D"MsoNormal"><o:p>=C2=A0</o:p></p></div><div style=3D"border:solid #BBBBB=
B 1.0pt;padding:0in 0in 0in 0in;overflow-wrap: break-word"><div><p class=3D=
"MsoNormal" style=3D"background:#FAFAFA"><span class=3D"styled-by-prettify"=
><span style=3D"font-size:10.0pt;font-family:"Courier New";color:#0=
00088">struct</span></span><span class=3D"styled-by-prettify"><span style=
=3D"font-size:10.0pt;font-family:"Courier New";color:black"> json <=
/span></span><span class=3D"styled-by-prettify"><span style=3D"font-size:10=
..0pt;font-family:"Courier New";color:#666600">:</span></span><span =
class=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-family:&#=
34;Courier New";color:black"> std</span></span><span class=3D"styled-by=
-prettify"><span style=3D"font-size:10.0pt;font-family:"Courier New"=
;;color:#666600">::</span></span><span class=3D"styled-by-prettify"><span s=
tyle=3D"font-size:10.0pt;font-family:"Courier New";color:black">var=
iant</span></span><span class=3D"styled-by-prettify"><span style=3D"font-si=
ze:10.0pt;font-family:"Courier New";color:#666600"><</span></spa=
n><span style=3D"font-size:10.0pt;font-family:"Courier New";color:b=
lack"><br/><span class=3D"styled-by-prettify">=C2=A0 </span></span><span cl=
ass=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-family:"=
;Courier New";color:#000088">double</span></span><span class=3D"styled-=
by-prettify"><span style=3D"font-size:10.0pt;font-family:"Courier New&#=
34;;color:#666600">,</span></span><span style=3D"font-size:10.0pt;font-fami=
ly:"Courier New";color:black"><br/><span class=3D"styled-by-prettif=
y">=C2=A0 std</span></span><span class=3D"styled-by-prettify"><span style=
=3D"font-size:10.0pt;font-family:"Courier New";color:#666600">::</s=
pan></span><span class=3D"styled-by-prettify"><span style=3D"font-size:10.0=
pt;font-family:"Courier New";color:#000088">string</span></span><sp=
an class=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-family=
:"Courier New";color:#666600">,</span></span><span style=3D"font-si=
ze:10.0pt;font-family:"Courier New";color:black"><br/><span class=
=3D"styled-by-prettify">=C2=A0 std</span></span><span class=3D"styled-by-pr=
ettify"><span style=3D"font-size:10.0pt;font-family:"Courier New";c=
olor:#666600">::</span></span><span class=3D"styled-by-prettify"><span styl=
e=3D"font-size:10.0pt;font-family:"Courier New";color:black">vector=
</span></span><span class=3D"styled-by-prettify"><span style=3D"font-size:1=
0.0pt;font-family:"Courier New";color:#008800"><json></span><=
/span><span class=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;fo=
nt-family:"Courier New";color:#666600">,</span></span><span style=
=3D"font-size:10.0pt;font-family:"Courier New";color:black"><br/><s=
pan class=3D"styled-by-prettify">=C2=A0 std</span></span><span class=3D"sty=
led-by-prettify"><span style=3D"font-size:10.0pt;font-family:"Courier N=
ew";color:#666600">::</span></span><span class=3D"styled-by-prettify"><=
span style=3D"font-size:10.0pt;font-family:"Courier New";color:blac=
k">map</span></span><span class=3D"styled-by-prettify"><span style=3D"font-=
size:10.0pt;font-family:"Courier New";color:#666600"><</span></s=
pan><span class=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font=
-family:"Courier New";color:black">std</span></span><span class=3D"=
styled-by-prettify"><span style=3D"font-size:10.0pt;font-family:"Courie=
r New";color:#666600">::</span></span><span class=3D"styled-by-prettify=
"><span style=3D"font-size:10.0pt;font-family:"Courier New";color:#=
000088">string</span></span><span class=3D"styled-by-prettify"><span style=
=3D"font-size:10.0pt;font-family:"Courier New";color:#666600">,</sp=
an></span><span class=3D"styled-by-prettify"><span style=3D"font-size:10.0p=
t;font-family:"Courier New";color:black"> json</span></span><span c=
lass=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-family:=
4;Courier New";color:#666600">></span></span><span class=3D"styled-b=
y-prettify"><span style=3D"font-size:10.0pt;font-family:"Courier New=
4;;color:black"> </span></span><span class=3D"styled-by-prettify"><span sty=
le=3D"font-size:10.0pt;font-family:"Courier New";color:#880000">// =
not allowed (yet)...</span></span><span style=3D"font-size:10.0pt;font-fami=
ly:"Courier New";color:black"><br/></span><span class=3D"styled-by-=
prettify"><span style=3D"font-size:10.0pt;font-family:"Courier New"=
;color:#666600">></span></span><span class=3D"styled-by-prettify"><span =
style=3D"font-size:10.0pt;font-family:"Courier New";color:black"> <=
/span></span><span class=3D"styled-by-prettify"><span style=3D"font-size:10=
..0pt;font-family:"Courier New";color:#666600">{</span></span><span =
style=3D"font-size:10.0pt;font-family:"Courier New";color:black"><b=
r/><span class=3D"styled-by-prettify">=C2=A0 =C2=A0</span></span><span clas=
s=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-family:"C=
ourier New";color:#000088">using</span></span><span class=3D"styled-by-=
prettify"><span style=3D"font-size:10.0pt;font-family:"Courier New"=
;color:black"> std</span></span><span class=3D"styled-by-prettify"><span st=
yle=3D"font-size:10.0pt;font-family:"Courier New";color:#666600">::=
</span></span><span class=3D"styled-by-prettify"><span style=3D"font-size:1=
0.0pt;font-family:"Courier New";color:black">variant</span></span><=
span class=3D"styled-by-prettify"><span style=3D"font-size:10.0pt;font-fami=
ly:"Courier New";color:#666600"><</span></span><span class=3D"st=
yled-by-prettify"><span style=3D"font-size:10.0pt;font-family:"Courier =
New";color:#880000">/*...*/</span></span><span class=3D"styled-by-prett=
ify"><span style=3D"font-size:10.0pt;font-family:"Courier New";colo=
r:#666600">>::</span></span><span class=3D"styled-by-prettify"><span sty=
le=3D"font-size:10.0pt;font-family:"Courier New";color:black">varia=
nt</span></span><span class=3D"styled-by-prettify"><span style=3D"font-size=
:10.0pt;font-family:"Courier New";color:#666600">;</span></span><sp=
an style=3D"font-size:10.0pt;font-family:"Courier New";color:black"=
><br/></span><span class=3D"styled-by-prettify"><span style=3D"font-size:10=
..0pt;font-family:"Courier New";color:#666600">};</span></span><span=
style=3D"font-size:10.0pt;font-family:"Courier New""><o:p></o:p></=
span></p></div></div><div><p class=3D"MsoNormal"><o:p>=C2=A0</o:p></p></div=
><div><p class=3D"MsoNormal">This would eliminate the need to use things li=
ke Boost's `make_recursive_variant` for common cases like JSON (and sim=
ilar) data types without requiring full recursive-variant support in the st=
andard. I'm happy to help push this forward, but my understanding is th=
at the main concern is what standard library implementers think of this.<o:=
p></o:p></p></div></div><p class=3D"MsoNormal"><o:p>=C2=A0</o:p></p></div><=
/div></body></html>
<p></p>
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