220 20298 <d8e90a20-a383-4ffc-ae18-7b8012e1d41b@isocpp.org> article
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From: Columbo <r.hl@gmx.net>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: Progress for universal template parameters
Date: Sat, 5 Sep 2015 11:41:13 -0700 (PDT)
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A is not accepting everything. A is solely accepting class or alias=20
templates; However, it does not require any specific signature. It's just a=
=20
feature that helps to implement arbitrary.

Am Samstag, 5. September 2015 20:36:17 UTC+2 schrieb Nicol Bolas:
>
>
>
> On Saturday, September 5, 2015 at 2:00:44 PM UTC-4, Columbo wrote:
>>
>> Am Samstag, 5. September 2015 18:53:10 UTC+2 schrieb Nicol Bolas:
>>>
>>> Are you talking about template parameters which could be values, types,=
=20
>>> or templates? That seems syntactically... difficult to reconcile; it so=
unds=20
>>> only one step removed from full-frontal macros.
>>>
>> I thought about introducing the following notation
>> template <template class> struct A {};
>> =E2=80=A6such that A accepts *any* template.
>>
>
> OK, so... what does `A` do with "any template"? If the template argument=
=20
> to `A` can be a template, class, or value, what exactly is `A` going to d=
o=20
> with it?
>
> You can't have A do something like this:
>
> template<stuff argName> struct A { argName a; };
>
> Because the user could pass a value or a template And if they did, the=20
> above would be syntactic nonsense.
>
> Similarly, you can't do this:
>
> template<stuff argName> struct A { argName<foo> a; };
>
> If `argName` were a class or value, this is likewise syntactic nonsense.=
=20
> And so forth.
>
> I do not know of a use of `argName` which would be equally valid C++ for=
=20
> values, templates, and concrete classes. Can you provide an example? One=
=20
> that actually does something with the template argument?
>

--=20

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<div dir=3D"ltr"><font face=3D"courier new, monospace">A</font> is not acce=
pting everything. <font face=3D"courier new, monospace">A</font> is solely =
accepting class or alias templates; However, it does not require any specif=
ic signature. It&#39;s just a feature that helps to implement <font face=3D=
"courier new, monospace">arbitrary</font>.<br><br>Am Samstag, 5. September =
2015 20:36:17 UTC+2 schrieb Nicol Bolas:<blockquote class=3D"gmail_quote" s=
tyle=3D"margin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-le=
ft: 1ex;"><br><br>On Saturday, September 5, 2015 at 2:00:44 PM UTC-4, Colum=
bo wrote:<blockquote class=3D"gmail_quote" style=3D"margin:0;margin-left:0.=
8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Am Samsta=
g, 5. September 2015 18:53:10 UTC+2 schrieb Nicol Bolas:<blockquote class=
=3D"gmail_quote" style=3D"margin:0;margin-left:0.8ex;border-left:1px #ccc s=
olid;padding-left:1ex"><div dir=3D"ltr">Are you talking about template para=
meters which could be values, types, or templates? That seems syntactically=
.... difficult to reconcile; it sounds only one step removed from full-front=
al macros.<br></div></blockquote><div>I thought about introducing the follo=
wing notation</div><div><div style=3D"border:1px solid rgb(187,187,187);wor=
d-wrap:break-word;background-color:rgb(250,250,250)"><code><div><span style=
=3D"color:#008">template</span><span style=3D"color:#000"> </span><span sty=
le=3D"color:#660">&lt;</span><span style=3D"color:#008">template</span><spa=
n style=3D"color:#000"> </span><span style=3D"color:#008">class</span><span=
 style=3D"color:#660">&gt;</span><span style=3D"color:#000"> </span><span s=
tyle=3D"color:#008">struct</span><span style=3D"color:#000"> A </span><span=
 style=3D"color:#660">{};</span></div></code></div></div><div><font face=3D=
"arial, sans-serif">=E2=80=A6</font><font face=3D"arial, sans-serif">such t=
hat</font><font face=3D"courier new, monospace"> A=C2=A0</font>accepts <i>a=
ny</i> template.</div></div></blockquote><div><br>OK, so... what does `A` d=
o with &quot;any template&quot;? If the template argument to `A` can be a t=
emplate, class, or value, what exactly is `A` going to do with it?<br><br>Y=
ou can&#39;t have A do something like this:<br><br><div style=3D"background=
-color:rgb(250,250,250);border-color:rgb(187,187,187);border-style:solid;bo=
rder-width:1px;word-wrap:break-word"><code><div><span style=3D"color:#008">=
template</span><span style=3D"color:#660">&lt;</span><span style=3D"color:#=
000">stuff argName</span><span style=3D"color:#660">&gt;</span><span style=
=3D"color:#000"> </span><span style=3D"color:#008">struct</span><span style=
=3D"color:#000"> A </span><span style=3D"color:#660">{</span><span style=3D=
"color:#000"> argName a</span><span style=3D"color:#660">;</span><span styl=
e=3D"color:#000"> </span><span style=3D"color:#660">};</span></div></code><=
/div><br>Because the user could pass a value or a template And if they did,=
 the above would be syntactic nonsense.<br><br>Similarly, you can&#39;t do =
this:<br><br><div style=3D"background-color:rgb(250,250,250);border-color:r=
gb(187,187,187);border-style:solid;border-width:1px;word-wrap:break-word"><=
code><div><span style=3D"color:#008">template</span><span style=3D"color:#6=
60">&lt;</span><span style=3D"color:#000">stuff argName</span><span style=
=3D"color:#660">&gt;</span><span style=3D"color:#000"> </span><span style=
=3D"color:#008">struct</span><span style=3D"color:#000"> A </span><span sty=
le=3D"color:#660">{</span><span style=3D"color:#000"> argName</span><span s=
tyle=3D"color:#080">&lt;foo&gt;</span><span style=3D"color:#000"> a</span><=
span style=3D"color:#660">;</span><span style=3D"color:#000"> </span><span =
style=3D"color:#660">};</span></div></code></div><br>If `argName` were a cl=
ass or value, this is likewise syntactic nonsense. And so forth.<br><br>I d=
o not know of a use of `argName` which would be equally valid C++ for value=
s, templates, and concrete classes. Can you provide an example? One that ac=
tually does something with the template argument?<br></div></blockquote></d=
iv>

<p></p>

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