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New issue from Jan Schultke: "basic_string accidentally fails to meet…
… the reversible container requirements"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4029" status="New"> | ||
| <title><tt>basic_string</tt> accidentally fails to meet the reversible container requirements</title> | ||
| <section><sref ref="[basic.string.general]"/></section> | ||
| <submitter>Jan Schultke</submitter> | ||
| <date>18 Dec 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| The complexity requirements for a reversible container (<sref ref="[container.rev.reqmts]"/>) are that each | ||
| function must have constant complexity. The corresponding member functions in <sref ref="[string.iterators]"/> | ||
| have no complexity requirements, and <tt>basic_string</tt> unintentionally is not a reversible container | ||
| (unless the implementation coincidentally provides constant complexity member functions). | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4971"/>. | ||
| </p> | ||
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| <blockquote class="note"> | ||
| <p> | ||
| [<i>Drafting Note:</i> The proposed wording is similar to the specification in <sref ref="[vector.overview]"/> p2 | ||
| and suggests to simply strike <sref ref="[string.iterators]"/>, because it doesn't say anything new compared to | ||
| <sref ref="[container.reqmts]"/> and <sref ref="[container.rev.reqmts]"/>. | ||
| <p/> | ||
| Alternatively, one could add a | ||
| <p/> | ||
| <ins><i>Complexity</i>: Constant.</ins> | ||
| <p/> | ||
| paragraph to each function in <sref ref="[string.iterators]"/>, but that would be less clearer and would not | ||
| explicitly say <tt>basic_string</tt> meets the reversible container requirements. | ||
| ] | ||
| </p> | ||
| </blockquote> | ||
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| <ol> | ||
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| <li><p>Modify <sref ref="[basic.string.general]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <p> | ||
| -2- A specialization of <tt>basic_string</tt> is a contiguous container <ins>and reversible container</ins> | ||
| (<sref ref="[container.reqmts]"/>). | ||
| <p/> | ||
| -3- In all cases, <tt>[data(), data() + size()]</tt> is a valid range, <tt>data() + size()</tt> points at | ||
| an object with value <tt>charT()</tt> (a "null terminator"), and <tt>size() <= capacity()</tt> is <tt>true</tt>. | ||
| </p> | ||
| <blockquote><pre> | ||
| namespace std { | ||
| template<class charT, class traits = char_traits<charT>, | ||
| class Allocator = allocator<charT>> | ||
| class basic_string { | ||
| […] | ||
| // <i><del><sref ref="[string.iterators]"/>,</del> iterators</i> | ||
| constexpr iterator begin() noexcept; | ||
| constexpr const_iterator begin() const noexcept; | ||
| constexpr iterator end() noexcept; | ||
| constexpr const_iterator end() const noexcept; | ||
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| constexpr reverse_iterator rbegin() noexcept; | ||
| constexpr const_reverse_iterator rbegin() const noexcept; | ||
| constexpr reverse_iterator rend() noexcept; | ||
| constexpr const_reverse_iterator rend() const noexcept; | ||
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| constexpr const_iterator cbegin() const noexcept; | ||
| constexpr const_iterator cend() const noexcept; | ||
| constexpr const_reverse_iterator crbegin() const noexcept; | ||
| constexpr const_reverse_iterator crend() const noexcept; | ||
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| // <i><sref ref="[string.capacity]"/>, capacity</i> | ||
| […] | ||
| }; | ||
| […] | ||
| } | ||
| </pre></blockquote> | ||
| […] | ||
| </blockquote> | ||
| </li> | ||
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| <li><p>Remove subclause <sref ref="[string.iterators]"/> in its entirety:</p> | ||
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| <blockquote> | ||
| <p> | ||
| <del><b>23.4.3.4 Iterator support [string.iterators]</b></del> | ||
| </p> | ||
| <pre> | ||
| <del>constexpr iterator begin() noexcept; | ||
| constexpr const_iterator begin() const noexcept; | ||
| constexpr const_iterator cbegin() const noexcept;</del> | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| <del>-1- <i>Returns</i>: An iterator referring to the first character in the string.</del> | ||
| </p> | ||
| </blockquote> | ||
| […] | ||
| <pre> | ||
| <del>constexpr reverse_iterator rend() noexcept; | ||
| constexpr const_reverse_iterator rend() const noexcept; | ||
| constexpr const_reverse_iterator crend() const noexcept;</del> | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| <del>-4- <i>Returns</i>: An iterator which is semantically equivalent to <tt>reverse_iterator(begin())</tt>.</del> | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| </ol> | ||
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| </resolution> | ||
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| </issue> |