Topic: Perfectly forwarding initializer_list, in cases other
Author: Arthur O'Dwyer <arthur.j.odwyer@gmail.com>
Date: Sun, 27 Nov 2016 01:33:38 -0800 (PST)
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Now that std::any, std::optional, and std::variant have blessed the idea of=
=20
splitting each "perfectly forwarding emplacement function" into a pair of=
=20
functions, one taking initializer_list and the other not, I wonder if=20
there's any significant interest in retrofitting that solution onto the=20
rest of the STL containers.
For example, to take this current wording in [forwardlist.modifiers]
template <class... Args> reference emplace_front(Args&&... args);
Effects: Inserts an object of type value_type constructed with value_type(s=
td::forward<Args>(=20
args)...) at the beginning of the list.=20
and replace it with this wording instead:
template <class... Args> reference emplace_front(Args&&... args);
Effects: Inserts an object of type value_type constructed with value_type(s=
td::forward<Args>(=20
args)...) at the beginning of the list.=20
Remarks: This function shall not participate in overload resolution unless =
is_constructible_v<T,=20
Args...> is true.
template <class U, class... Args> reference emplace_front(initializer_list<=
U> il, Args&&... args);
Effects: Inserts an object of type value_type constructed with value_type(i=
l,=20
std::forward<Args>( args)...) at the beginning of the list.=20
Remarks: This function shall not participate in overload resolution unless =
is_constructible_v<T,=20
std::initializer_list<U>&, Args...> is true.
and so on for std::vector, std::list, std::deque,...
Is there a technical reason this retrofitting hasn't happened, or is it=20
just one of those "nobody's written the paper yet" cases?
Ville's N4462=20
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4462.html> seems=
=20
to be a discussion of the "how to forward initializer_list" problem in=20
general, but AFAICT it dates from before the adoption of the two-function=
=20
approach. I don't know where the two-function approach originally came from=
=20
=E2=80=94 not that I've tried very hard to find out.
=E2=80=93Arthur
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<div dir=3D"ltr"><div>Now that std::any, std::optional, and std::variant ha=
ve blessed the idea of splitting each "perfectly forwarding emplacemen=
t function" into a pair of functions, one taking initializer_list and =
the other not, I wonder if there's any significant interest in retrofit=
ting that solution onto the rest of the STL containers.</div><div><br></div=
><div>For example, to take this current wording in [forwardlist.modifiers]<=
/div><div><br></div><div>
=09
=09
=09
<div class=3D"page" title=3D"Page 899">
<div class=3D"layoutArea">
<div class=3D"column">
<pre><span style=3D"font-family: LMMono9;"><font size=3D"4">template &=
lt;class... Args> reference emplace_front(Args&&... args);
</font></span></pre>
</div>
</div>
</div></div><div>
=09
=09
=09
<div class=3D"page" title=3D"Page 899">
<div class=3D"layoutArea">
<div class=3D"column">
<p><font size=3D"4"><span style=3D"font-family: LMRoman10; font-style:=
italic;">Effects: </span><span style=3D"font-family: LMRoman10;">Inserts a=
n object of type </span><span style=3D"font-family: LMMono10;">value_type <=
/span><span style=3D"font-family: LMRoman10;">constructed with </span><span=
style=3D"font-family: LMMono10;">value_type(std::forward<Args>(
args)...) </span><span style=3D"font-family: LMRoman10;">at the beginning o=
f the list.=C2=A0</span></font></p>
</div>
</div>
</div></div><div><br></div><div>and replace it with this wording instead:=
</div><div><br></div><div><div><div class=3D"page" title=3D"Page 899"><div =
class=3D"layoutArea"><div class=3D"column"><pre><span style=3D"font-family:=
LMMono9;"><font size=3D"4">template <class... Args> reference emplac=
e_front(Args&&... args);
</font></span></pre></div></div></div></div><div><div class=3D"page" title=
=3D"Page 899"><div class=3D"layoutArea"><div class=3D"column"><p><font size=
=3D"4"><span style=3D"font-family: LMRoman10; font-style: italic;">Effects:=
=C2=A0</span><span style=3D"font-family: LMRoman10;">Inserts an object of t=
ype=C2=A0</span><span style=3D"font-family: LMMono10;">value_type=C2=A0</sp=
an><span style=3D"font-family: LMRoman10;">constructed with=C2=A0</span><sp=
an style=3D"font-family: LMMono10;">value_type(std::forward<Args>( ar=
gs)...)=C2=A0</span><span style=3D"font-family: LMRoman10;">at the beginnin=
g of the list.=C2=A0</span></font></p><div>
=09
=09
=09
<div class=3D"page" title=3D"Page 589">
<div class=3D"layoutArea">
<div class=3D"column">
=09
=09
=09
=09
<p><font size=3D"4"><span style=3D"font-family: LMRoman10; font-style: it=
alic;">Remarks:=C2=A0</span><span style=3D"font-family: LMRoman10;">This fu=
nction shall not participate in overload resolution unless </span><span sty=
le=3D"font-family: LMMono10;">is_constructible_v<T,
Args...> </span><span style=3D"font-family: LMRoman10;">is </span><span =
style=3D"font-family: LMMono10;">true.</span></font></p><p><span style=3D"f=
ont-family: LMMono10;"><font size=3D"4"><br></font></span></p>
</div>
</div>
</div></div><div><div><div class=3D"page" title=3D"Page 899"><div class=
=3D"layoutArea"><div class=3D"column"><pre><span style=3D"font-family: LMMo=
no9;"><font size=3D"4">template <class U, class... Args> reference em=
place_front(initializer_list<U> il, Args&&... args);
</font></span></pre></div></div></div></div><div><div class=3D"page" title=
=3D"Page 899"><div class=3D"layoutArea"><div class=3D"column"><p><font size=
=3D"4"><span style=3D"font-family: LMRoman10; font-style: italic;">Effects:=
=C2=A0</span><span style=3D"font-family: LMRoman10;">Inserts an object of t=
ype=C2=A0</span><span style=3D"font-family: LMMono10;">value_type=C2=A0</sp=
an><span style=3D"font-family: LMRoman10;">constructed with=C2=A0</span><sp=
an style=3D"font-family: LMMono10;">value_type(il, std::forward<Args>=
( args)...)=C2=A0</span><span style=3D"font-family: LMRoman10;">at the begi=
nning of the list.=C2=A0</span></font></p><div><font size=3D"4"><span style=
=3D"font-family: LMRoman10; font-style: italic;">Remarks:=C2=A0</span><span=
style=3D"font-family: LMRoman10;">This function shall not participate in o=
verload resolution unless=C2=A0</span><span style=3D"font-family: LMMono10;=
">is_constructible_v<T, std::initializer_list<U>&, Args...>=
=C2=A0</span><span style=3D"font-family: LMRoman10;">is=C2=A0</span><span s=
tyle=3D"font-family: LMMono10;">true.</span><span style=3D"font-family: LMR=
oman10;"><br></span></font></div><div><br><br>and so on for std::vector, st=
d::list, std::deque,...<br>Is there a technical reason this retrofitting ha=
sn't happened, or is it just one of those "nobody's written th=
e paper yet" cases?<br></div></div></div></div></div></div></div></div=
></div></div></div><div><br></div><div>Ville's <a href=3D"http://www.op=
en-std.org/jtc1/sc22/wg21/docs/papers/2015/n4462.html">N4462</a>=C2=A0seems=
to be a discussion of the "how to forward initializer_list" prob=
lem in general, but AFAICT it dates from before the adoption of the two-fun=
ction approach. I don't know where the two-function approach originally=
came from =E2=80=94 not that I've tried very hard to find out.</div><d=
iv><br></div><div>=E2=80=93Arthur</div></div>
<p></p>
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