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From: George Makrydakis <irrequietus@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: Re: Generalizing the equivalence of auto and
 template type deduction
Date: Thu, 02 Oct 2014 12:05:04 +0300
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To: std-proposals@isocpp.org,Tomasz <tomaszkam@gmail.com>
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Again, this has already been discussed. See previous emails on this thread on why you would have problems with that as it is. You do not even gain total equivalence over canonical template syntax, if that is your goal.

On October 2, 2014 11:43:58 AM EEST, Tomasz <tomaszkam@gmail.com> wrote:
>My proposal is that every use of placholder (today we have only auto,
>but 
>concept name would be a possible extension) would introduce separate 
>typename, so:
>std::pair<auto, auto> p = f();
>
>Would use equivalent deduction as:
>template<typename T1, typename T2>
>void foo(std::pair<T1, T2>);
>
>And two different types can be deduced. To achieve the pair of the same
>
>types I would use syntax proposed in 
>http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3878.pdf 
><http://n3878>, that allow us to introduce indentyfier for a type
>deduced 
>from placeholder. So if we want to have pair of same types we would
>write:
>std::pair<auto{T}, T> p = f();
>Which will use deduction as:
>template<typename T>
>void foo(std::pair<T,T>);
>
>We will benefit from being able to refer to type T without use of 
>std::decay_t<decltype(p)>::first_type and have same treatment of
>multiple 
>placeholders in every context.
>
>This syntaxt can be extended to use concept names as placeholders:
>std::pair<Iterator, Iterator> f = ...;
>Will allow pair of two interator with unrelated types that fullfills 
>iterator concept. To have pair of iterator with same type only, we
>could 
>write
>std::pair<Iterator{I}, I> f = ...;
>
>-- 
>
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<html><head></head><body><p dir=3D"ltr">Again, this has already been discus=
sed. See previous emails on this thread on why you would have problems with=
 that as it is. You do not even gain total equivalence over canonical templ=
ate syntax, if that is your goal.</p>
<br><br><div class=3D"gmail_quote">On October 2, 2014 11:43:58 AM EEST, Tom=
asz &lt;tomaszkam@gmail.com&gt; wrote:<blockquote class=3D"gmail_quote" sty=
le=3D"margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204);=
 padding-left: 1ex;">
<div dir=3D"ltr">My proposal is that every use of placholder (today we have=
 only auto, but concept name would be a possible extension) would introduce=
 separate typename, so:<br />std::pair&lt;auto, auto&gt; p =3D f();<br /><b=
r />Would use equivalent deduction as:<br />template&lt;typename T1, typena=
me T2&gt;<br />void foo(std::pair&lt;T1, T2&gt;);<br /><br />And two differ=
ent types can be deduced. To achieve the pair of the same types I would use=
 syntax proposed in <a href=3D"http://n3878">http://www.open-std.org/JTC1/S=
C22/WG21/docs/papers/2014/n3878.pdf</a>, that allow us to introduce indenty=
fier for a type deduced from placeholder. So if we want to have pair of sam=
e types we would write:<br />std::pair&lt;auto{T}, T&gt; p =3D f();<br />Wh=
ich will use deduction as:<br />template&lt;typename T&gt;<br />void foo(st=
d::pair&lt;T,T&gt;);<br /><br />We will benefit from being able to refer to=
 type T without use of std::decay_t&lt;decltype(p)&gt;::first_type and have=
 same treatment of multiple
placeholders in every context.<br /><br />This syntaxt can be extended to u=
se concept names as placeholders:<br />std::pair&lt;Iterator, Iterator&gt; =
f =3D ...;<br />Will allow pair of two interator with unrelated types that =
fullfills iterator concept. To have pair of iterator with same type only, w=
e could write<br />std::pair&lt;Iterator{I}, I&gt; f =3D ...;<br /><br /></=
div>

<p></p>
</blockquote></div></body></html>

<p></p>

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