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Given a std::multimap<K,V> m, how do you iterate over all the elements with
some key k0?

Am I right in believing that the cleanest way is:

    auto er = m.equal_range(k0);
    for (auto it = er.first; it != er.second; ++it) {
      const auto& [k, v] = *it;

      // do stuff with k,v
    }

Or is there a better way?

What would we think about having a way to write something like:

   for (const auto& [k,v] : m[k0]) {
      // do stuff with k,v
   }

ie std::multimap::operator[] would produce some sort of container view of
the .equal_range?

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<div dir=3D"ltr">Given a std::multimap&lt;K,V&gt; m, how do you iterate ove=
r all the elements with some key k0?<div><br></div><div>Am I right in belie=
ving that the cleanest way is:</div><div><br></div><div>=C2=A0 =C2=A0 auto =
er =3D m.equal_range(k0);</div><div>=C2=A0 =C2=A0 for (auto it =3D er.first=
; it !=3D er.second;=C2=A0++it) {</div><div>=C2=A0 =C2=A0 =C2=A0 const auto=
&amp; [k, v] =3D *it;</div><div><br></div><div>=C2=A0 =C2=A0 =C2=A0 // do s=
tuff with k,v</div><div>=C2=A0 =C2=A0 }</div><div><br></div><div>Or is ther=
e a better way?</div><div><br></div><div>What would we think about having a=
 way to write something like:</div><div><br></div><div>=C2=A0 =C2=A0for (co=
nst auto&amp; [k,v] : m[k0]) {</div><div>=C2=A0 =C2=A0 =C2=A0 // do stuff w=
ith k,v</div><div>=C2=A0 =C2=A0}</div><div><br></div><div>ie std::multimap:=
:operator[] would produce some sort of container view of the .equal_range?<=
/div><div><br></div></div>

<p></p>

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On Tuesday, 25 December 2018 11:24:18 UTC, Andrew Tomazos wrote:
>
> Given a std::multimap<K,V> m, how do you iterate over all the elements
> with some key k0?
>
> Am I right in believing that the cleanest way is:
>
>     auto er = m.equal_range(k0);
>     for (auto it = er.first; it != er.second; ++it) {
>       const auto& [k, v] = *it;
>
>       // do stuff with k,v
>     }
>
> Or is there a better way?
>
> What would we think about having a way to write something like:
>
>    for (const auto& [k,v] : m[k0]) {
>       // do stuff with k,v
>    }
>
> ie std::multimap::operator[] would produce some sort of container view of
> the .equal_range?
>

It looks like all you need is a kind of "iterator range" adaptor that can
be constructed from a pair. The one in Boost may not be quite it, though
you could have your own library function that emits a boost::iterator_range
(or similar type) once and for all, and then just use:

*    for (const auto& [k, v] : make_range(m.equal_range(key)**)**) { /* ...
*/ }*

That seems like a sufficiently accessible solution to a rare problem that
might be preferable to growing the already large interface of
std::{,unordered_}multi{map,set}. Not that it would be hard to grow the
interface, it just doesn't seem like that would buy us a massive
improvement in usability. The putative new interface would need to be
taught and understood (e.g. what does "m[key] = x;" mean?), whereas my
proposed approach uses entirely known and familiar patterns.

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<div dir=3D"ltr">On Tuesday, 25 December 2018 11:24:18 UTC, Andrew Tomazos =
 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">Given =
a std::multimap&lt;K,V&gt; m, how do you iterate over all the elements with=
 some key k0?<div><br></div><div>Am I right in believing that the cleanest =
way is:</div><div><br></div><div>=C2=A0 =C2=A0 auto er =3D m.equal_range(k0=
);</div><div>=C2=A0 =C2=A0 for (auto it =3D er.first; it !=3D er.second;=C2=
=A0++it) {</div><div>=C2=A0 =C2=A0 =C2=A0 const auto&amp; [k, v] =3D *it;</=
div><div><br></div><div>=C2=A0 =C2=A0 =C2=A0 // do stuff with k,v</div><div=
>=C2=A0 =C2=A0 }</div><div><br></div><div>Or is there a better way?</div><d=
iv><br></div><div>What would we think about having a way to write something=
 like:</div><div><br></div><div>=C2=A0 =C2=A0for (const auto&amp; [k,v] : m=
[k0]) {</div><div>=C2=A0 =C2=A0 =C2=A0 // do stuff with k,v</div><div>=C2=
=A0 =C2=A0}</div><div><br></div><div>ie std::multimap::operator[] would pro=
duce some sort of container view of the .equal_range?</div></div></blockquo=
te><div><br></div><div>It looks like all you need is a kind of &quot;iterat=
or range&quot; adaptor that can be constructed from a pair. The one in Boos=
t may not be quite it, though you could have your own library function that=
 emits a boost::iterator_range (or similar type) once and for all, and then=
 just use:</div><div><br></div><div><font face=3D"courier new, monospace"><=
b>=C2=A0 =C2=A0 for (const auto&amp; [k, v] : <font color=3D"#38761d">make_=
range(</font>m.equal_range(key)</b></font><b style=3D"font-family: &quot;co=
urier new&quot;, monospace;"><font color=3D"#38761d">)</font></b><font face=
=3D"courier new, monospace"><b>)=C2=A0{ /* ... */ }</b></font></div><div><b=
r></div><div>That seems like a sufficiently accessible solution to a rare p=
roblem that might be preferable to growing the already large interface of s=
td::{,unordered_}multi{map,set}. Not that it would be hard to grow the inte=
rface, it just doesn&#39;t seem like that would buy us a massive improvemen=
t in usability. The putative new interface would need to be taught and unde=
rstood (e.g. what does &quot;m[key] =3D x;&quot; mean?), whereas my propose=
d approach uses entirely known and familiar patterns.</div></div>

<p></p>

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40isocpp.org?utm_medium=3Demail&utm_source=3Dfooter">https://groups.google.=
com/a/isocpp.org/d/msgid/std-proposals/8568a673-2342-447d-9eeb-6cc335eeb942=
%40isocpp.org</a>.<br />

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If you change the spelling from

make_range(m.equal_range(key))

to

std::ranges::subrange(m.equal_range(key))

I believe that this will already work in C++20, there's a deduction guide for pair like. http://eel.is/c++draft/range.subrange

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