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<!doctype html public "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<head>
<title>char8_t backward compatibility remediation</title>
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<body>
<table id="header">
<tr>
<th>Document Number:</th>
<td>P1423R2</td>
</tr>
<tr>
<th>Date:</th>
<td>2019-06-16</td>
</tr>
<tr>
<th>Audience:</th>
<td>Library Evolution Working Group<br/>
Library Working Group</td>
</tr>
<tr>
<th>Reply-to:</th>
<td>Tom Honermann <[email protected]></td>
</tr>
</table>
<h1>char8_t backward compatibility remediation</h1>
<ul>
<li><a href="#changes">
Changes since P1423R1</a></li>
<li><a href="#introduction">
Introduction</a></li>
<li><a href="#examples">
Examples</a></li>
<li><a href="#impact">
Anticipated impact</a></li>
<li><a href="#remediation">
Remediation approaches</a>
<ul>
<li><a href="#disable">
Disable <tt>char8_t</tt> support</a></li>
<li><a href="#overload">
Add overloads</a></li>
<li><a href="#ordinary">
Change <tt>u8</tt> literals to ordinary literals with escape sequences</a></li>
<li><a href="#reinterpret_cast">
reinterpret_cast <tt>u8</tt> literals to <tt>char</tt></a></li>
<li><a href="#emulate">
Emulate C++17 <tt>u8</tt> literals</a></li>
<li><a href="#array-subst">
Substitute class types for C arrays initialized with <tt>u8</tt> string literals</a></li>
</li>
<li><a href="#conversion_fns">
Use explicit conversion functions</a></li>
<li><a href="#tooling">
Tooling</a></li>
</ul>
</li>
<li><a href="#options">
Options considered to reduce backward compatibility impact</a>
<ul>
<li><a href="#option1">
1) Reinstate <tt>u8</tt> literals as type <tt>char</tt> and introduce a new literal prefix for <tt>char8_t</tt></a></li>
<li><a href="#option2">
2) Allow implicit conversions from <tt>char8_t</tt> to <tt>char</tt></a></li>
<li><a href="#option3">
3) Allow initializing an array of <tt>char</tt> with a <tt>u8</tt> string literal</a></li>
<li><a href="#option4">
4) Allow initializing an array with a reference to an array</a></li>
<li><a href="#option5">
5) Allow <tt>std::string</tt> to be initialized with <tt>char8_t</tt> based types</a></li>
<li><a href="#option6">
6) Allow implicit conversions from <tt>std::u8string</tt> to <tt>std::string</tt></a></li>
<li><a href="#option7">
7) Add deleted ostream inserters for <tt>char8_t</tt>, <tt>char16_t</tt>, and <tt>char32_t</tt></a></li>
<li><a href="#option8">
8) Allow <tt>std::filesystem::u8path</tt> to accept ranges and iterators with <tt>char8_t</tt> value types</a></li>
</ul>
</li>
<li><a href="#proposal">
Proposal</a></li>
<li><a href="#wording">
Wording</a>
<ul>
<li><a href="#library_wording">
Library wording</a></li>
<li><a href="#annex_c_wording">
Annex C Compatibility wording</a></li>
<li><a href="#annex_d_wording">
Annex D Compatibility features wording</a></li>
</ul>
</li>
<li><a href="#references">
References</a></li>
</ul>
<h1 id="changes">Changes since <a href="http://wg21.link/p1423r1">P1423R1</a></h1>
<ul>
<li>Addressed wording and feature feedback provided during the
2019-05-21 LWG telecon:
<ul>
<li>Added deleted overloads of
<tt>basic_ostream<wchar_t, ...>::operator<<</tt> for
<tt>char8_t</tt>, <tt>char16_t</tt>, and <tt>char32_t</tt> character
and string types. This maintains consistency of deleted overloads for
ordinary and wide streams.</li>
<li>Moved wording for the deleted <tt>ostream</tt> inserter overloads from
<a href="http://eel.is/c++draft/ostream.inserters.character">
29.7.5.2.4 [ostream.inserters.character]</a> to the <tt>ostream</tt>
synopsis in
<a href="http://eel.is/c++draft/ostream">29.7.5.1 [ostream]</a> and
removed parameter names.</li>
<li>Updated code comments in the new paragraph added to
<a href="http://eel.is/c++draft/diff.cpp17.input.output">
C.5.11 [diff.cpp17.input.output]</a> to avoid assumptions of which
<tt>ostream</tt> inserter overload would be invoked.</li>
</ul>
</li>
<li>Rebased wording on N4810.</li>
</ul>
<h1 id="introduction">Introduction</h1>
<p>The support for <tt>char8_t</tt> as adopted for C++20 via
<a title="char8_t: A type for UTF-8 characters and strings (Revision 6)"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0482r6.html">
P0482R6</a>
<sup><a title="char8_t: A type for UTF-8 characters and strings (Revision 6)"
href="#ref_p0482r6">
[P0482R6]</a></sup> affects backward
compatibility for existing C++17 programs in at least the following ways:
<ol>
<li>Introduction of a new <tt>char8_t</tt> keyword, new
<tt>std::u8string</tt>,
<tt>std::u8string_view</tt>,
<tt>std::u8streampos</tt> type aliases and
<tt>std::mbrtoc8</tt> and
<tt>std::c8rtomb</tt> functions; these names may conflict with existing
uses of these names.
</li>
<li>Change of return type for <tt>std::filesystem::path</tt> member functions
<tt>u8string</tt> and <tt>generic_u8string</tt>.
</li>
<li>Change of type for <tt>u8</tt> character and string literals and
<tt>u8R</tt> raw string literals.</li>
</ol>
</p>
<p>This paper does <em>not</em> further discuss case 1 above. Adding new
keywords and new members to the <tt>std</tt> namespace is business as usual;
see
<a title="SD-8: Standard Library Compatibility"
href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility">
SD-8</a>
<sup><a title="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility"
href="#ref_sd8">
[SD-8]</a></sup>. It is
acknowledged that these additions will affect some code bases. Code surveys
have found that these names have generally been used to emulate the set of
features introduced with the adoption of
<a title="char8_t: A type for UTF-8 characters and strings (Revision 6)"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0482r6.html">
P0482R6</a>
<sup><a title="char8_t: A type for UTF-8 characters and strings (Revision 6)"
href="#ref_p0482r6">
[P0482R6]</a></sup>. In some cases, existing code has already been updated to
adapt to the new standard features. For example,
<a href="https://github.com/electronicarts/EASTL">EASTL</a> will now use the
the standard provided <tt>char8_t</tt> type when available instead of the type
alias previously used. The pull request for this change can be found at
<a href="https://github.com/electronicarts/EASTL/pull/239">
https://github.com/electronicarts/EASTL/pull/239</a>.
</p>
<p>Case 2 above is a change that does <em>not</em> fit into the set of standard
library rights reserved in
<a title="SD-8: Standard Library Compatibility"
href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility">
SD-8</a>
<sup><a title="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility"
href="#ref_sd8">
[SD-8]</a></sup>.
This is a cause for concern, but is somewhat mitigated by the fact that
<tt>std::filesystem</tt> is new with C++17 and therefore does not have a long
history of use. Some options for dealing with this change are discussed later
in this paper.
</p>
<p>Case 3 above is the change responsible for most of the backward
compatibility impact.
</p>
<p>This paper is motivated by three goals:
<ul>
<li>To document a set of options available to programmers to facilitate
migration of existing code to C++20. Where possible, options are
presented for writing code that is compatible with both C++17 and C++20.
</li>
<li>To ensure that WG21 members are aware of the backward compatibility
issues and anticipated impact, and find the set of options available to
mitigate the impact acceptable.
</li>
<li>To consider options available to reduce backward compatibility impact.
This paper documents a number of such options, but only proposes two
small standard library changes intended to remove backward compatibility
impact that was not intended by the adoption of P0482R6.
</li>
</ul>
</p>
<h1 id="examples">Examples</h1>
<p>The following table presents examples of well-formed C++17 code that is
either ill-formed or behaves differently in C++20. The table also reflects the
intended changes proposed in this paper. Note that most of these examples
remain ill-formed with this proposal. This is intentional as the examples
reflect problematic code that leads to mojibake in C++17 code due to use of the
same type (<tt>char</tt>) for multiple encodings (execution encoding and UTF-8).
</p>
<p>
<table border="1">
<tr>
<th>Code</th>
<th>C++17</th>
<th>C++20 with P0482R6</th>
<th>C++20 with this proposal</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">const char *p = u8"text";</code></pre>
</fieldset>
</td>
<td>Initializes <tt>p</tt> with the address of the UTF-8 encoded string.</td>
<td>Ill-formed.</td>
<td>Ill-formed.</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">char a[] = u8"text";</code></pre>
</fieldset>
</td>
<td>Initializes <tt>a</tt> with the UTF-8 encoded string.</td>
<td>Ill-formed.</td>
<td>Ill-formed.</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">int operator ""_udl(const char*, unsigned long);
int v = u8"text"_udl;</code></pre>
</fieldset>
</td>
<td>Initializes <tt>v</tt> with the result of calling
<tt>operator ""_udl</tt> with the UTF-8 encoded string literal.
</td>
<td>Ill-formed.</td>
<td>Ill-formed.</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">std::string s(u8"text");</code></pre>
</fieldset>
</td>
<td>Initializes <tt>s</tt> with the UTF-8 encoded string.</td>
<td>Ill-formed.</td>
<td>Ill-formed.</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">std::filesystem::path p = ...;
std::string s = p.u8string();</code></pre>
</fieldset>
</td>
<td>Initializes <tt>s</tt> with the UTF-8 encoded representation
of the file path stored in <tt>p</tt>.
</td>
<td>Ill-formed.</td>
<td>Ill-formed.</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">std::cout << u8'x';
std::cout << u8"text";</code></pre>
</fieldset>
</td>
<td>Writes a sequence of UTF-8 code units as characters to stdout.<br/>
(mojibake if the execution character encoding is not UTF-8)
</td>
<td>Writes an integer or pointer value to stdout.<br/>
(consistent with handling of char16_t and char32_t)
</td>
<td>Ill-formed.<br/>
(for all of char8_t, char16_t, and char32_t)
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">std::filesystem::u8path(u8"filename");</code></pre>
</fieldset>
</td>
<td>Constructs a <tt>std::filesystem::path</tt> object from the UTF-8
encoded string.</td>
<td>Ill-formed.</td>
<td>Constructs a <tt>std::filesystem::path</tt> object from the UTF-8
encoded string.</td>
</tr>
</table>
</p>
<h1 id="impact">Anticipated impact</h1>
<p><tt>u8</tt> string literals were added in C++11, but support for <tt>u8</tt>
character literals was only added in C++17.
</p>
<p>Code surveys have so far revealed little use of <tt>u8</tt> literals.
Google and Facebook have both reported less than 1000 occurrences in their
code bases, approximately half of which occur in test code. Representatives
of both organizations have stated that, given the actual size of their code
base, this is an insignificant number of occurrences. Likewise, Firefox
contains around 100 occurrences and Mozilla engineers have reviewed this paper
and have no concerns regarding addressing the existing uses using the
remediation approaches discussed here.
</p>
<p>Code surveys have been attempted on github, but github search doesn't
facilitate distinguishing uses of <tt>u8</tt> as identifiers (which is quite
common) vs use as a UTF-8 literal. Further, github doesn't provide a search
that filters out duplicate hits for the same source code in different
repositories. As a result, finding instances of <tt>u8</tt> literals is
challenging. Most cases that were identified were in tests included in clones
of Clang and gcc.
</p>
<p>Searches on Debian code search found uses in only a few packages and, with
one exception discussed below, only a small number of uses (mostly single digit
counts), most of which occurred in tests.
</p>
<p>Survey results for Debian code search
(<a href="https://codesearch.debian.net">https://codesearch.debian.net</a>)
follow. These exclude hits to gcc, clang, packages like fmt and eastl that
have already been conditionalized for <tt>char8_t</tt> in C++20, and C code.
Additionally, packages bundled with other packages are omitted to avoid
double counting. Search hits reflect number of lines matching the search,
not number of occurrences. Search results are categorized by hits in
program source files vs hits in test source files.
</p>
<p>There is one clear outlier in these search results. Chromium has two orders
of magnitude more uses of <tt>u8</tt> literals than any other package. The
Chromium team was contacted to discuss this. Almost all of the existing uses
appear in bulk data source files that the Chromium team has determined can be
mechanically addressed.
</p>
<table border="1">
<tr>
<th>Searched for</th>
<th>Debian packages (out of ~18000)</th>
</tr>
<tr>
<td><tt>char8_t</tt></td>
<td>spring (emulates its own <tt>char8_t</tt> support)</td>
</tr>
<tr>
<td><tt>u8string</tt></td>
<td>libopenmpt (defines a <tt>mpt::u8string</tt> typedef of
<tt>std::basic_string</tt> with a custom <tt>char_traits</tt>)<br/>
spring (defines a <tt>std::u8string</tt> class that derives from
<tt>std::string</tt>)</td>
</tr>
<tr>
<td><tt>u8string_view</tt></td>
<td><none></td>
</tr>
<tr>
<td><tt>u8streampos</tt></td>
<td><none></td>
</tr>
<tr>
<td><tt>mbrtoc8</tt></td>
<td><none></td>
</tr>
<tr>
<td><tt>c8rtomb</tt></td>
<td><none></td>
</tr>
<tr>
<td><tt>u8path(u8"text")</tt></td>
<td><none></td>
</tr>
<tr>
<td><tt>u8"text"</tt></td>
<td>
chromium (~10310, ~102 files)<br/>
firefox (97 hits, 1 files, 3 test files)<br/>
icu (83 hits, 1 files, 5 test files)<br/>
qbs (56 hits, 1 file, 1 test file)<br/>
mongodb (30 hits, 2 test files)<br/>
aseba (28 hits, 1 file)<br/>
monero (26 hits, 1 file, 1 test file)<br/>
nlohmann-json (21 hits, 1 file, 3 test files)<br/>
bambootracker (20 hits, 5 files)<br/>
capnproto (18 hits, 1 file, 1 test file)<br/>
lgogdownloader (11 hits, 1 file)<br/>
libosmium (10 hits, 3 test files)<br/>
cbmc (8 hits, 3 test files)<br/>
maim (8 hits, 1 file)<br/>
octave-ltfat (8 hits, 1 file)<br/>
praat (8 hits, 2 files)<br/>
slop (8 hits, 1 file)<br/>
mame (7 hits, 3 files)<br/>
nlohmann-json3 (7 hits, 3 test files)<br/>
sdcc (7 hits, 1 test file)<br/>
antlr4 (3 hits, 1 file)<br/>
keyman-keyboardprocessor (3 hits, 2 test files)<br/>
scram (3 hits, 2 test files)<br/>
tesseract (3 hits, 1 test file)<br/>
boost1.67 (2 hits, 1 test file)<br/>
freeorion (2 hits, 1 file)<br/>
supertux (2 hits, 1 file)<br/>
cjs (1 hit, 1 test file)<br/>
cpp-hocon (1 hit, 1 test file)<br/>
efl (1 hit, 1 file)<br/>
gjs (1 hit, 1 test file)<br/>
kate4 (1 hit, 1 example file)<br/>
kodi (1 hit, 1 test file)<br/>
libtcod (1 hit, 1 test file)<br/>
retroarch (1 hit, 1 file)<br/>
rtags (1 hit, 1 test file)<br/>
</td>
</tr>
<tr>
<td><tt>u8R"(text)"</tt></td>
<td>
chromium (940, 2 files, 2 test files)<br/>
kate4 (2 hits, 1 example file, 1 test file)<br/>
nlohmann-json (2 hits, 1 test file)<br/>
nlohmann-json3 (2 hits, 1 test file)<br/>
ksyntax-highlighting (1 hit, 1 test file)<br/>
ktexteditor (1 hit, 1 test file)<br/>
</td>
</tr>
</table>
<h1 id="remediation">Remediation approaches</h1>
<p>A single approach to addressing backward compatibility impact is unlikely to
be the best approach for all projects. This section presents a number of
options to address various types of backward compatibility impact. In some
cases, the best solution may involve a mix of these options.
</p>
<p>Each of these approaches assumes a requirement for continued use of UTF-8
encoded literals with <tt>char</tt> based types. For most projects, such a
requirement is expected to be temporary while the project is fully migrated to
C++20. However, some projects may retain a sustained need for such literals.
For those projects, the <a href="#emulate">Emulate C++17 <tt>u8</tt>
literals</a> approach is able to address most cases of backward compatibility
impact.
</p>
<h2 id="disable">Disable <tt>char8_t</tt> support</h2>
<p>The simplest possible solution in the short term is to simply disable the
new features completely. Clang and gcc will allow disabling <tt>char8_t</tt>
features in both the language and standard library, via a <tt>-fno-char8_t</tt>
option. It is expected that Microsoft and EDG based compilers will offer a
similar option.
</p>
<p>This option should be considered a short-term solution to enable testing
existing C++17 code compiled as C++20 with minimal effort. This isn't a
viable long-term option as continued use would potentially complicate
composition with code that depends on the new features.
</p>
<h2 id="overload">Add overloads</h2>
<p>Adding function overloads that accept <tt>char8_t</tt> based types is an
effective step towards full migration to C++20. Ideally, older <tt>char</tt>
based functions would eventually be removed.
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">int ft(const char*);
ft(u8"text");</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">int ft(const char*);
<ins>#if defined(__cpp_char8_t)
int ft(const char8_t*);
#endif</ins>
ft(u8"text"); <ins>// C++17 or C++20</ins></code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">int operator ""_udl(const char*, unsigned long);
int v = u8"text"_udl;</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">int operator ""_udl(const char*, unsigned long);
<ins>#if defined(__cpp_char8_t)
int operator ""_udl(const char8_t*, unsigned long);
#endif</ins>
int v = u8"text"_udl; <ins>// C++17 or C++20</ins></code></pre>
</fieldset>
</td>
</tr>
</table>
<h2 id="ordinary">Change <tt>u8</tt> literals to ordinary literals with escape sequences</h2>
<p>This approach may be a reasonable option when the execution encoding is
ASCII based (but not UTF-8; otherwise just use ordinary literals) and
characters outside the basic source character set are infrequently used in
existing <tt>u8</tt> literals. This approach matches how code using UTF-8
had to be written prior to C++11.
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">u8"\u00E1"<br/></code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++"><ins>"\xC3\xA1" // U+00E1</ins></code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">u8"á"<br/>(assuming source encoding is UTF-8)</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++"><ins>"\xC3\xA1" // U+00E1</ins><br/>(works with any source encoding)</code></pre>
</fieldset>
</td>
</tr>
</table>
<h2 id="reinterpret_cast">reinterpret_cast <tt>u8</tt> literals to <tt>char</tt></h2>
<p>Common uses of <tt>u8</tt> literals can be handled in a backward compatible
manner through use of <tt>reinterpret_cast</tt>. Note that use of
<tt>reinterpret_cast</tt> is well-formed in these situations since
<a href="http://eel.is/c++draft/expr#basic.lval-11">lvalues of type
<tt>char</tt> may be used to access values of other types</a>. Such code is
valid in both C++17 and C++20.
</p>
<p>This approach may suffice when there are just a few uses of UTF-8 literals
that need to be addressed and the uses do not appear in <tt>constexpr</tt>
context. In general, sprinkling <tt>reinterpret_cast</tt> all over a code
base is not desirable.
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">const char &r = u8’x';</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">const char &r = <ins>reinterpret_cast<const char &>(</ins>u8’x'<ins>)</ins>; <ins>// C++17 or C++20</ins></code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">const char *p = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">const char *p = <ins>reinterpret_cast<const char *>(</ins>u8"text"<ins>)</ins>; <ins>// C++17 or C++20</ins></code></pre>
</fieldset>
</td>
</tr>
</table>
<h2 id="emulate">Emulate C++17 <tt>u8</tt> literals</h2>
<p>The techniques applied here are also applicable to the examples illustrated
in the prior section regarding use of <tt>reinterpret_cast</tt>. This approach
makes use of
<a title="Class Types in Non-Type Template Parameters"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0732r2.pdf">
P0732R2</a>
<sup><a title="Class Types in Non-Type Template Parameters"
href="#ref_p0732r2">
[P0732R2]</a></sup>
to enable constexpr UTF-8 encoded <tt>char</tt> based literals using a user
defined literal. The example code below defines overloaded character and
string UDL operators named <tt>_as_char</tt>. These UDLs can then be used in
place of existing UTF-8 character and string literals.
</p>
<p>
<fieldset>
<pre><code class="c++">#include <utility>
template<std::size_t N>
struct char8_t_string_literal {
static constexpr inline std::size_t size = N;
template<std::size_t... I>
constexpr char8_t_string_literal(
const char8_t (&r)[N],
std::index_sequence<I...>)
:
s{r[I]...}
{}
constexpr char8_t_string_literal(
const char8_t (&r)[N])
:
char8_t_string_literal(r, std::make_index_sequence<N>())
{}
auto operator <=>(const char8_t_string_literal&) = default;
char8_t s[N];
};
template<char8_t_string_literal L, std::size_t... I>
constexpr inline const char as_char_buffer[sizeof...(I)] =
{ static_cast<char>(L.s[I])... };
template<char8_t_string_literal L, std::size_t... I>
constexpr auto& make_as_char_buffer(std::index_sequence<I...>) {
return as_char_buffer<L, I...>;
}
constexpr char operator ""_as_char(char8_t c) {
return c;
}
template<char8_t_string_literal L>
constexpr auto& operator""_as_char() {
return make_as_char_buffer<L>(std::make_index_sequence<decltype(L)::size>());
}
</code></pre>
</fieldset>
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr const char &r = u8’x';</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr const char &r = u8’x'<ins>_as_char</ins>; <ins>// C++20 only</ins</code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr const char *p = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr const char *p = u8"text"<ins>_as_char</ins>; <ins>// C++20 only</ins</code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">// gcc extension in C++17; standard C++ doesn't permit conversion
// to arrays of unknown bound.
constexpr const char (&r)[] = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">// Ok in C++20 with <a title="Class Types in Non-Type Template Parameters" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0388r2.html">P0388R2</a> <sup><a title="Class Types in Non-Type Template Parameters" href="#ref_p0388r2">[P0388R2]</a></sup>
constexpr const char (&r)[] = u8"text"<ins>_as_char</ins>; <ins>// C++20 only</ins</code></pre>
</fieldset>
</td>
</tr>
</table>
<p>When wrapped in macros, the above UDL can be used to retain source
compatibility across C++17 and C++20 for all known scenarios except for
array initialization.
<fieldset><pre><code class="c++">#if defined(__cpp_char8_t)
#define U8(x) u8##x##_as_char
#else
#define U8(x) u8##x
#endif
</code></pre></fieldset>
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr const char &r = u8’x';</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr const char &r = <ins>U8(’x')</ins>; <ins>// C++17 or C++20</ins</code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr const char *p = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr const char *p = <ins>U8("text")</ins>; <ins>// C++17 or C++20</ins</code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">// gcc extension in C++17; standard C++ doesn't permit conversion
// to arrays of unknown bound.
constexpr const char (&r)[] = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">// Ok in C++20 with <a title="Class Types in Non-Type Template Parameters" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0388r2.html">P0388R2</a> <sup><a title="Class Types in Non-Type Template Parameters" href="#ref_p0388r2">[P0388R2]</a></sup>
constexpr const char (&r)[] = <ins>U8("text")</ins>; <ins>// C++17 or C++20</ins</code></pre>
</fieldset>
</td>
</tr>
</table>
<h2 id="array-subst">Substitute class types for C arrays initialized with <tt>u8</tt> string literals</h2>
<p>In C++17, arrays of <tt>char</tt> may be initialized with <tt>u8</tt> string
literals, but such initialization is ill-formed in C++20. C++17 behavior can
be emulated by substituting a class type with appropriate class template
argument deduction guides.
</p>
<p>
<fieldset>
<pre><code class="c++">#include <utility>
template<std::size_t N>
struct char_array {
template<std::size_t P, std::size_t... I>
constexpr char_array(
const char (&r)[P],
std::index_sequence<I...>)
:
data{(I<P?r[I]:'\0')...}
{}
template<std::size_t P, typename = std::enable_if_t<(P<=N)>>
constexpr char_array(const char(&r)[P])
: char_array(r, std::make_index_sequence<N>())
{}
#if defined(__cpp_char8_t)
template<std::size_t P, std::size_t... I>
constexpr char_array(
const char8_t (&r)[P],
std::index_sequence<I...>)
:
data{(I<P?static_cast<char>(r[I]):'\0')...}
{}
template<std::size_t P, typename = std::enable_if_t<(P<=N)>>
constexpr char_array(const char8_t(&r)[P])
: char_array(r, std::make_index_sequence<N>())
{}
#endif
constexpr (&operator const char() const)[N] {
return data;
}
constexpr (&operator char())[N] {
return data;
}
char data[N];
};
template<std::size_t N>
char_array(const char(&)[N]) -> char_array<N>;
#if defined(__cpp_char8_t)
template<std::size_t N>
char_array(const char8_t(&)[N]) -> char_array<N>;
#endif
</code></pre>
</fieldset>
</p>
<table border="1">
<tr>
<th>Before</th>
<th>After</th>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">char a[] = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++"><ins>char_array</ins> a = u8"text"; <ins>// Ok, initialized with "text\0"</ins></code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr char a[] = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr <ins>char_array</ins> a = u8"text"; <ins>// Ok, initialized with "text\0"</ins></code></pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr char a[3] = u8"text"; // ill-formed</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr <ins>char_array<3></ins> a = u8"text"; // ill-formed (too many initializers)</pre>
</fieldset>
</td>
</tr>
<tr>
<td>
<fieldset><pre><code class="c++">constexpr char a[6] = u8"text";</code></pre>
</fieldset>
</td>
<td>
<fieldset><pre><code class="c++">constexpr <ins>char_array<6></ins> a = u8"text"; <ins>// Ok, initialized with "text\0\0"</code></pre>
</fieldset>
</td>
</tr>
</table>
<h2 id="conversion_fns">Use explicit conversion functions</h2>
<p>Explicit conversion functions can be used, in a C++17 compatible manner,
to cope with the change of return type to the <tt>std::filesystem::path</tt>
member functions when a UTF-8 encoded path is desired in an object of type
<tt>std::string</tt>. For example:
</p>
<p>
<fieldset><pre><code class="c++">std::string from_u8string(const std::string &s) {
return s;
}
std::string from_u8string(std::string &&s) {
return std::move(s);
}
#if defined(__cpp_lib_char8_t)
std::string from_u8string(const std::u8string &s) {
return std::string(s.begin(), s.end());
}
#endif
std::filesystem::path p = ...;
std::string s = from_u8string(p.u8string()); // C++17 or C++20</code></pre>
</fieldset>
</p>
<p>This naturally incurs a cost when building with <tt>char8_t</tt> support
enabled due to the need to copy the path contents.
</p>
<h2 id="tooling">Tooling</h2>
<p>Tooling could potentially assist programmers in migrating code. Several of
the approaches discussed above could be applied mechanically to an existing
code base. For example, re-writing existing <tt>u8</tt> literals to ordinary
literals with escape sequences, or adding an <tt>_as_char</tt> UDL suffix to
existing literals (inserting include directives as needed).
</p>
<h1 id="options">Options considered to reduce backward compatibility impact</h1>
<p>The following sections summarize options that have been considered to
reduce backward compatibility impact. Most of these options are <em>not</em>
proposed in this paper because they would actively interfere with goals of
the <tt>char8_t</tt> proposal; to enable the type system to protect against
inadvertent mixing of UTF-8 data and the execution encoding. However, some
of these options may be useful for some code bases and could be provided by
implementations as opt-in extensions.
</p>
<p>Only two of these options (7 and 8) are proposed for inclusion in the
standard. In both of these cases, the concern that is addressed was not
specifically intended by the changes adopted in P0482R6. These are
effectively bug fixes.
</p>