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jwakely merged 1 commit into from
Aug 28, 2026
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New P/R for 4559 #616

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144 changes: 122 additions & 22 deletions xml/issue4559.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ Although the original paper <a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2846r6.pdf">P2846</a> gives a reason for not
providing <code>reserve_hint</code> for these three views — namely, uncertainty
about what the best approach is — it seems to me that providing them is indeed intuitively unambiguous. We
don't need to consider the inconsistency between <code>const</code> and non-<code>const</code> overloads because
don't need to consider the inconsistency between <code>const</code> and non-<code>const</code> overloads because
it's not within reasonable ranges.
<p/>
Given that we provided <code>reserve_hint</code> for <code>concat_view</code> to resolve NB comment <a
Expand All @@ -33,9 +33,7 @@ Set priority to 3 after reflector poll.
Concerns with `cartesian_product` as any overestimation will be amplified
exponentially. P0 votes for just the `zip` change.
</p>
</discussion>

<resolution>
<superseded>
<p>
This wording is relative to <paper num="N5032"/>.
</p>
Expand Down Expand Up @@ -133,22 +131,6 @@ namespace std::ranges {
</pre>
</blockquote>
[&hellip;]
<pre>
Comment thread
jwakely marked this conversation as resolved.
</pre>
<blockquote>
<p>
</p>
<blockquote><pre>
</pre></blockquote>
</blockquote>
<pre>
</pre>
<blockquote>
<p>
</p>
<blockquote><pre>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

Expand All @@ -163,7 +145,7 @@ namespace std::ranges {
template&lt;class... Vs&gt;
concept <i>cartesian-product-is-sized</i> = // <i>exposition only</i>
(sized_range&lt;Vs&gt; &amp;&amp; ...);

<ins>template&lt;class... Vs&gt;
concept <i>cartesian-product-is-approximately-sized</i> = // <i>exposition only</i>
(approximately_sized_range&lt;Vs&gt; &amp;&amp; ...);</ins>
Expand Down Expand Up @@ -199,7 +181,7 @@ constexpr <i>see below</i> size() const
-7- The return type is an implementation-defined unsigned-integer-like type.
<p/>
-8- <i>Recommended practice</i>: The return type should be the smallest unsigned-integer-like type that
is sufficiently wide to store the product of the maximum sizes of all the underlying ranges, if such
is sufficiently wide to store the product of the maximum sizes of all the underlying ranges, if such
a type exists.
<p/>
-9- Let <i>p</i> be the product of the sizes of all the ranges in <code><i>bases_</i></code>.
Expand Down Expand Up @@ -236,6 +218,124 @@ ranges, if such a type exists.</ins>

</ol>

</superseded>

<note>2026-08-27; Tim provides new wording</note>
<p> Dropping the `cartesian_product` change per reflector discussion.
<p/>
It seems to me that, unlike the previous resolution, and as the original paper alluded to,
we could provide `reserve_hint` for `zip` as long as at least one of the ranges
is approximately sized.
</p>

</discussion>

<resolution>
<p>
This wording is relative to <paper num="N5054"/>.
</p>

<ol>
<li>
<p>Modify <sref ref="[range.zip.view]"/> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
[&hellip;]
template&lt;input_range... Views&gt;
requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0)
class zip_view : public view_interface&lt;zip_view&lt;Views...&gt;&gt; {
[&hellip;]
constexpr auto size() requires (sized_range&lt;Views&gt; &amp;&amp; ...);
constexpr auto size() const requires (sized_range&lt;const Views&gt; &amp;&amp; ...);

<ins> constexpr auto reserve_hint() requires (approximately_sized_range&lt;Views&gt; || ...);</ins>
<ins> constexpr auto reserve_hint() const requires (approximately_sized_range&lt;const Views&gt; || ...);</ins>
};
[&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr auto size() requires (sized_range&lt;Views&gt; &amp;&amp; ...);
constexpr auto size() const requires (sized_range&lt;const Views&gt; &amp;&amp; ...);
</pre>
<blockquote>
<p>
-3- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
return apply([](auto... sizes) {
using CT = <i>make-unsigned-like-t</i>&lt;common_type_t&lt;decltype(sizes)...&gt;&gt;;
return ranges::min({CT(sizes)...});
}, <i>tuple-transform</i>(ranges::size, <i>views_</i>));
</pre></blockquote>
</blockquote>
<pre>
<ins>constexpr auto reserve_hint() requires (approximately_sized_range&lt;Views&gt; || ...);
constexpr auto reserve_hint() const requires (approximately_sized_range&lt;const Views&gt; || ...);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Let `Const` be `false` for the first overload and `true` for the second overload.
Let <i>I</i> be the largest set of indices such that
<tt>approximately_sized_range&lt;<i>maybe-const</i>&lt;Const, Views...[Is]&gt;&gt;</tt> is `true`,
and let `Is` be a pack of `size_t` values consisting of the elements of
<i>I</i> in ascending order. Equivalent to:</ins>
</p>
<blockquote><pre>
<ins>
using CT = <i>make-unsigned-like-t</i>&lt;common_type_t&lt;decltype(ranges::reserve_hint(get&lt;Is&gt;(<i>views_</i>)))...&gt;&gt;;
return ranges::min({CT(ranges::reserve_hint(get&lt;Is&gt;(<i>views_</i>)))...});</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li>
<p>Modify <sref ref="[range.zip.transform.view]"/> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
[&hellip;]
namespace std::ranges {
template&lt;move_constructible F, input_range... Views&gt;
requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
regular_invocable&lt;F&amp;, range_reference_t&lt;Views&gt;...&gt; &amp;&amp;
<i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;Views&gt;...&gt;&gt;
class zip_transform_view : public view_interface&lt;zip_transform_view&lt;F, Views...&gt;&gt; {
[&hellip;]
constexpr auto size() requires sized_range&lt;<i>InnerView</i>&gt; {
return <i>zip_</i>.size();
}

constexpr auto size() const requires sized_range&lt;const <i>InnerView</i>&gt; {
return <i>zip_</i>.size();
}

<ins> constexpr auto reserve_hint() requires approximately_sized_range&lt;<i>InnerView</i>&gt; {
return <i>zip_</i>.reserve_hint();
}

constexpr auto reserve_hint() const requires approximately_sized_range&lt;const <i>InnerView</i>&gt; {
return <i>zip_</i>.reserve_hint();
}</ins>
};
[&hellip;]
}
</pre>
</blockquote>
[&hellip;]
</blockquote>
</li>

</ol>

</resolution>

</issue>