220 35828 <e9825fff-7825-4b8e-85c4-bd006ceccc54@isocpp.org> article
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Subject: Is there any proposal in the pipeline to allow any
 parameter in a template?
Date: Sat, 9 Dec 2017 12:12:35 -0800 (PST)
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The problem with templates is that you must specify a particular item you 
want as a parameter, such as a `typename`, a `template` or a `value`. 
 However, you can't say, specialize the template based on if the next 
parameter is any of those three.

I propose that there be a way to specialize based on these parameters like 
in the following example:
    
    template <...Xs>
    struct X;

    template <template <typename...> class TT>
    struct X<TT> {};

    template <typename T>
    struct X<T> {};

    template <typename T, T I>
    struct X<I> {};

This would represent a template that takes a parameter pack of unspecified 
parameters, and is only accessible though specialization.

This allows the ability to have more control over template specialization 
while adding flexibility by allowing any parameter list. This on turn would 
allowing the creation of clearer and cleaner iinterfaces.

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<div dir=3D"ltr"><div><span style=3D"font-size: 29.5346px;">The problem wit=
h templates is that you must specify a particular item you want as a parame=
ter, such as a `typename`, a `template` or a `value`. =C2=A0However, you ca=
n&#39;t say, specialize the template based on if the next parameter is any =
of those three.</span></div><div><span style=3D"font-size: 29.5346px;"><br>=
</span></div><div><span style=3D"font-size: 29.5346px;">I propose that ther=
e be a way to specialize based on these parameters like in the following ex=
ample:</span></div><div><span style=3D"font-size: 29.5346px;">=C2=A0 =C2=A0=
 </span></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: #000;" class=3D"styled-by-prettif=
y">=C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-pre=
ttify">template</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;=
....</span><span style=3D"color: #606;" class=3D"styled-by-prettify">Xs</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">struct</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"> X</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">;</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br><br>=C2=A0 =C2=A0 </span><span=
 style=3D"color: #008;" class=3D"styled-by-prettify">template</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: =
#008;" class=3D"styled-by-prettify">template</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">&lt;</span><span style=3D"color: #008;" class=3D"s=
tyled-by-prettify">typename</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">...&gt;</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prett=
ify">class</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
 TT</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</s=
pan><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">str=
uct</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> X</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify">TT</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">&gt;</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;" clas=
s=3D"styled-by-prettify"><br><br>=C2=A0 =C2=A0 </span><span style=3D"color:=
 #008;" class=3D"styled-by-prettify">template</span><span style=3D"color: #=
000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">&lt;</span><span style=3D"color: #008;" class=3D"=
styled-by-prettify">typename</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"> T</span><span style=3D"color: #660;" class=3D"styled-by-=
prettify">&gt;</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-b=
y-prettify">struct</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> X</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
&lt;</span><span style=3D"color: #000;" class=3D"styled-by-prettify">T</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</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"col=
or: #000;" class=3D"styled-by-prettify"><br><br>=C2=A0 =C2=A0 </span><span =
style=3D"color: #008;" class=3D"styled-by-prettify">template</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #=
008;" class=3D"styled-by-prettify">typename</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"styl=
ed-by-prettify"> T I</span><span style=3D"color: #660;" class=3D"styled-by-=
prettify">&gt;</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-b=
y-prettify">struct</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> X</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
&lt;</span><span style=3D"color: #000;" class=3D"styled-by-prettify">I</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">{};</span></div></code></div=
><div><br></div><div><span style=3D"font-size: 29.5346px;">This would repre=
sent a template that takes a parameter pack of unspecified parameters, and =
is only accessible though specialization.</span></div><div><span style=3D"f=
ont-size: 29.5346px;"><br></span></div><div><span style=3D"font-size: 29.53=
46px;">This allows the ability to have more control over template specializ=
ation while adding flexibility by allowing any parameter list. This on turn=
 would allowing the creation of clearer and cleaner iinterfaces.</span></di=
v><div><br></div></div>

<p></p>

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