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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: A type for UTF-8 characters and strings (Revision 6)</title>
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<body>
<table id="header">
<tr>
<th>Document Number:</th>
<td>P0482R6</td>
</tr>
<tr>
<th>Date:</th>
<td>2018-11-09</td>
</tr>
<tr>
<th>Audience:</th>
<td>Core Working Group<br/>
Library Working Group</td>
</tr>
<tr>
<th>Reply-to:</th>
<td>Tom Honermann <[email protected]></td>
</tr>
</table>
<h1>char8_t: A type for UTF-8 characters and strings (Revision 6)</h1>
<ul>
<li><a href="#changes_since_P0482R5">
Changes since P0482R5</a></li>
<li><a href="#introduction">
Introduction</a></li>
<li><a href="#motivation">
Motivation</a></li>
<li><a href="#proposal">
Proposal</a></li>
<li><a href="#design">
Design Considerations</a>
<ul>
<li><a href="#design_compat">
Backward compatibility
</a>
<ul>
<li><a href="#design_compat_core">
Core language backward compatibility
</a>
<ul>
<li><a href="#design_compat_core_init">
Initialization
</a></li>
<li><a href="#design_compat_core_implicit_conversion">
Implicit conversions
</a></li>
<li><a href="#design_compat_core_type_deduction">
Type deduction
</a></li>
<li><a href="#design_compat_core_overload_resolution">
Overload resolution
</a></li>
<li><a href="#design_compat_core_template_specialization">
Template specialization
</a></li>
</ul>
</li>
<li><a href="#design_compat_library">
Library backward compatibility
</a>
<ul>
<li><a href="#design_compat_library_u8string">
Return type of <tt>path::u8string</tt> and <tt>path::generic_u8string</tt>
</a></li>
<li><a href="#design_compat_library_literal_operators">
Return type of <tt>operator ""s</tt> and <tt>operator ""sv</tt>
</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#design_narrow_utf8">
Should UTF-8 literals continue to be referred to as narrow literals?
</a></li>
<li><a href="#design_char8_t_underlying_type">
What should be the underlying type of char8_t?
</a></li>
</ul>
</li>
<li><a href="#implementation_exp">
Implementation Experience</a></li>
<li><a href="#wording">
Formal Wording</a>
<ul>
<li><a href="#core_wording">
Core wording</a></li>
<li><a href="#library_wording">
Library wording</a></li>
<li><a href="#annex_a_wording">
Annex A Grammar summary 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="#acknowledgements">
Acknowledgements</a></li>
<li><a href="#references">
References</a></li>
</ul>
<h1 id="changes_since_P0482R5">Changes since <a href="http://wg21.link/p0482r5">P0482R5</a></h1>
<ul>
<li>Addressed CWG review feedback:
<ul>
<li>Updated the feature test macro values to 201811 and added a drafting
note that the final value will be chosen by the project editor to reflect
the date of approval.</li>
</ul>
</li>
<li>Addressed LWG review feedback:
<ul>
<li>Updated the specification for <tt>c8rtomb</tt> in 20.5.6 [c.mb.wcs] to
reflect that calls that pass a null pointer for argument <tt>buf</tt>
may introduce a data race.</li>
<li>Changed a use of <i>char8_t string literal</i> to
<i>UTF-8 string literal</i> in C.1.1 [diff.lex] paragraph 3 to
align with C terminology.</li>
<li>Added an example of silent backward compatibility impact to
C.5.? [input.output] concerning passing UTF-8 literals to
ostream inserters.</li>
<li>Corrected missing monospace markup for <tt>source</tt> in
D.?? [depr.fs.path.factory].</li>
</ul>
</li>
</ul>
<h1 id="introduction">Introduction</h1>
<p>C++11 introduced support for UTF-8, UTF-16, and UTF-32 encoded string
literals via
<a title="N2249: New Character Types in C++"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2249.html">
N2249</a>
<sup><a title="N2249: New Character Types in C++"
href="#ref_n2249">
[N2249]</a></sup>.
New <tt>char16_t</tt> and <tt>char32_t</tt> types were added to hold values of
code units for the UTF-16 and UTF-32 variants, but a new type was not added for
the UTF-8 variants. Instead, UTF-8 character literals (added in C++17 via
<a title="N4197: Adding u8 character literals"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4197.html">
N4197</a>
<sup><a title="N4197: Adding u8 character literals"
href="#ref_n4197">
[N4197]</a></sup>)
and UTF-8 string literals were defined in terms of the <tt>char</tt> type used
for the code unit type of ordinary character and string literals. UTF-8 is the
only text encoding mandated to be supported by the C++ standard for which there
is no distinct code unit type. Lack of a distinct type for UTF-8 encoded
character and string literals prevents the use of overloading and template
specialization in interfaces designed for interoperability with encoded text.
The inability to infer an encoding for narrow characters and strings limits
design possibilities and hinders the production of elegant interfaces that work
seemlessly in generic code. Library authors must choose to limit encoding
support, design interfaces that require users to explicitly specify encodings,
or provide distinct interfaces for, at least, the implementation defined
execution and UTF-8 encodings.</p>
<p>Whether <tt>char</tt> is a signed or unsigned type is implementation defined
and implementations that use an 8-bit signed char are at a disadvantage with
respect to working with UTF-8 encoded text due to the necessity of having to
rely on conversions to unsigned types in order to correctly process leading and
continuation code units of multi-byte encoded code points.</p>
<p>The lack of a distinct type and the use of a code unit type with a range that
does not portably include the full unsigned range of UTF-8 code units presents
challenges for working with UTF-8 encoded text that are not present when working
with UTF-16 or UTF-32 encoded text. Enclosed is a proposal for a new
<tt>char8_t</tt> fundamental type and related library enhancements intended to
remove barriers to working with UTF-8 encoded text and to enable generic
interfaces that work with all five of the standard mandated text encodings in a
consistent manner.</p>
<h1 id="motivation">Motivation</h1>
<p>Consider the following string literal expressions, all of which encode
<tt>U+0123</tt>, <tt>LATIN SMALL LETTER G WITH CEDILLA</tt>:
<fieldset>
<pre><code class="c++">u8"\u0123" // UTF-8: const char[]: 0xC4 0xA3 0x00
u"\u0123" // UTF-16: const char16_t[]: 0x0123 0x0000
U"\u0123" // UTF-32: const char32_t[]: 0x00000123 0x00000000
"\u0123" // ???: const char[]: ???
L"\u0123" // ???: const wchar_t[]: ???
</code></pre>
</fieldset>
</p>
<p>The UTF-8, UTF-16, and UTF-32 string literals have well-defined and portable
sequences of code unit values. The ordinary and wide string literal code unit
sequences depend on the implementation defined execution and execution wide
encodings respectively. Code that is designed to work with text encodings must
be able to differentiate these strings. This is straight forward for wide,
UTF-16, and UTF-32 string literals since they each have a distinct code unit
type suitable for differentiation via function overloading or template
specialization. But for ordinary and UTF-8 string literals, differentiating
between them requires additional information since they have the same code unit
type. That additional information might be provided implicitly via differently
named functions, or explicitly via additional function or template
arguments. For example:
<fieldset>
<pre><code class="c++">// Differentiation by function name:
void do_x(const char *);
void do_x_utf8(const char *);
void do_x(const wchar_t *);
void do_x(const char16_t *);
void do_x(const char32_t *);
// Differentiation by suffix for user-defined literals:
int operator ""_udl(const char *s, std::size_t);
int operator ""_udl_utf8(const char *s, std::size_t);
int operator ""_udl(const wchar_t *s, std::size_t);
int operator ""_udl(const char16_t *s, std::size_t);
int operator ""_udl(const char32_t *s, std::size_t);
// Differentiation by function parameter:
void do_x2(const char *, bool is_utf8);
void do_x2(const wchar_t *);
void do_x2(const char16_t *);
void do_x2(const char32_t *);
// Differentiation by template parameter:
template<bool IsUTF8>
void do_x3(const char *);
</code></pre>
</fieldset>
</p>
<p>The requirement to, in some way, specify the text encoding, other than
through the type of the string, limits the ability to provide elegant encoding
sensitive interfaces. Consider the following invocations of the
<tt>make_text_view</tt> function proposed in
<a title="P0244R2: Text_view: A C++ concepts and range based character encoding
and code point enumeration library"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0244r2.html">
P0244R2</a>
<sup><a title="P0244R2: Text_view: A C++ concepts and range based character
encoding and code point enumeration library"
href="#ref_p0244r2">
[P0244R2]</a></sup>:
<fieldset>
<pre><code class="c++">make_text_view<execution_character_encoding>("text")
make_text_view<execution_wide_character_encoding>(L"text")
make_text_view<utf8_encoding>(u8"text")
make_text_view<utf16_encoding>(u"text")
make_text_view<utf32_encoding>(U"text")
</code></pre>
</fieldset>
</p>
<p>For each invocation, the encoding of the string literal is known at compile
time, so having to explicitly specify the encoding tag is redundant. If
UTF-8 string literals had a distinct type, then the encoding type could be
inferred, while still allowing an overriding tag to be supplied:
<fieldset>
<pre><code class="c++">make_text_view("text") // defaults to execution_character_encoding.
make_text_view(L"text") // defaults to execution_wide_character_encoding.
make_text_view(u8"text") // defaults to utf8_encoding.
make_text_view(u"text") // defaults to utf16_encoding.
make_text_view(U"text") // defaults to utf32_encoding.
make_text_view<utf16be_encoding>("\0t\0e\0x\0t\0") // Default overridden to select UTF-16BE.
</code></pre>
</fieldset>
</p>
<p>The inability to infer an encoding for narrow strings doesn't just limit the
interfaces of new features under consideration. Compromised interfaces are
already present in the standard library.</p>
<p>Consider the design of the <tt>codecvt</tt> class template. The standard
specifies the following specializations of <tt>codecvt</tt> be provided to
enable transcoding text from one encoding to another.
<fieldset>
<pre><code class="c++">codecvt<char, char, mbstate_t> <em>// #1</em>
codecvt<wchar_t, char, mbstate_t> <em>// #2</em>
codecvt<char16_t, char, mbstate_t> <em>// #3</em>
codecvt<char32_t, char, mbstate_t> <em>// #4</em>
</code></pre>
</fieldset>
</p>
<p>#1 performs no conversions. #2 converts between strings encoded in the
implementation defined wide and narrow encodings. #3 and #4 convert between
either the UTF-16 or UTF-32 encoding and the UTF-8 encoding. Specializations
are not currently specified for conversion between the implementation defined
narrow and wide encodings and any of the UTF-8, UTF-16, or UTF-32 encodings.
However, if support for such conversions were to be added, the desired
interfaces are already taken by #1, #3 and #4.</p>
<p>The file system interface adopted for C++17 via
<a title="P0218R1: Adopt the File System TS for C++17"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0372r0.html">
P0218R1</a>
<sup><a title="P0218R1: Adopt the File System TS for C++17"
href="#ref_p0218r1">
[P0218R1]</a></sup>
provides an example of a feature that supports all five of the standard mandated
encodings, but does so with an asymetric interface due to the inability to
overload functions for UTF-8 encoded strings. Class
<tt>std::filesystem::path</tt> provides the following constructors to initialize
a <tt>path</tt> object based on a range of code unit values where the encoding
is inferred based on the value type of the range.
<fieldset>
<pre><code class="c++">template <class Source>
path(const Source& source);
template <class InputIterator>
path(InputIterator first, InputIterator last);
</code></pre>
</fieldset>
<p>§ 30.11.7.2.2 [fs.path.type.cvt] describes how the source encoding is
determined based on whether the source range value type is <tt>char</tt>,
<tt>wchar_t</tt>, <tt>char16_t</tt>, or <tt>char32_t</tt>. A range with value
type <tt>char</tt> is interpreted using the implementation defined execution
encoding. It is not possible to construct a path object from UTF-8
encoded text using these constructors.
<p>To accommodate UTF-8 encoded text, the file system library specifies the
following factory functions. Matching factory functions are not provided for
other encodings.
<fieldset>
<pre><code class="c++">template <class Source>
path u8path(const Source& source);
template <class InputIterator>
path u8path(InputIterator first, InputIterator last);
</code></pre>
</fieldset>
<p>The requirement to construct <tt>path</tt> objects using one interface for
UTF-8 strings vs another interface for all other supported encodings creates
unnecessary difficulties for portable code. Consider an application that uses
UTF-8 as its internal encoding on POSIX systems, but uses UTF-16 on Windows.
Conditional compilation or other abstractions must be implemented and used
in otherwise platform neutral code to construct <tt>path</tt> objects.</p>
<p>The inability to infer an encoding based on string type is not the only
challenge posed by use of <tt>char</tt> as the UTF-8 code unit type. The
following code exhibits implementation defined behavior.
<fieldset>
<pre><code class="c++">bool is_utf8_multibyte_code_unit(char c) {
return c >= 0x80;
}
</code></pre>
</fieldset>
</p>
<p>UTF-8 leading and continuation code units have values in the range 128
(0x80) to 255 (0xFF). In the common case where <tt>char</tt> is implemented
as a signed 8-bit type with a two's complement representation and a range of
-128 (-0x80) to 127 (0x7F), these values exceed the unsigned range of the
<tt>char</tt> type. Such implementations typically encode such code units as
unsigned values which are then reinterpreted as signed values when read. In
the code above, integral promotion rules result in <tt>c</tt> being promoted to
type <tt>int</tt> for comparison to the <tt>0x80</tt> operand. if <tt>c</tt>
holds a value corresponding to a leading or continuation code unit value, then
its value will be interpreted as negative and the promoted value of type
<tt>int</tt> will likewise be negative. The result is that the comparison
is always false for these implementations.</p>
<p>To correct the code above, explicit conversions are required. For example:
<fieldset>
<pre><code class="c++">bool is_utf8_multibyte_code_unit(char c) {
return static_cast<unsigned char>(c) >= 0x80;
}
</code></pre>
</fieldset>
</p>
<p>Finally, processing of UTF-8 strings is currently subject to an optimization
pessimization due to glvalue expressions of type <tt>char</tt> potentially
aliasing objects of other types. Use of a distinct type that does not share
this aliasing behavior may allow for further compiler optimizations.</p>
<p>As of November 2017,
<a title="Usage of UTF-8 for websites"
href="https://w3techs.com/technologies/details/en-utf8/all/all">
UTF-8 is now used by more than 90% of all websites</a>
<sup><a title="Usage of UTF-8 for websites"
href="#ref_w3techs">
[W3Techs]</a></sup>.
The C++ standard must improve support for UTF-8 by removing the existing
barriers that result in redundant tagging of character encodings, non-generic
UTF-8 specific workarounds like <tt>u8path</tt>, and the need for static
casts to examine UTF-8 code unit values.
</p>
<h1 id="proposal">Proposal</h1>
<p>The proposed changes are intended to bring the standard to the state the
author believes it would likely be in had <tt>char8_t</tt> been added at the
same time that <tt>char16_t</tt> and <tt>char32_t</tt> were added. This
includes the ability to differentiate ordinary and UTF-8 literals in function
overloading, template specializations, and user-defined literal operator
signatures. The following core language changes are proposed in order to
facilitate these capabilities:
<ul>
<li>A new fundamental type named <tt>char8_t</tt>. This integral type has
the same signedness, size, alignment, and integer conversion rank as
<tt>unsigned char</tt>, but does not alias with any other type
(e.g., this proposal does not add <tt>char8_t</tt> to the list of
aliasing types in § 8.2.1 [basic.lval] paragraph 11 (11.8)).</li>
<li>The type of UTF-8 string literals is changed from array of
<tt>const char</tt> to array of <tt>const char8_t</tt>.</li>
<li>The type of UTF-8 character literals is changed from <tt>char</tt>
to <tt>char8_t</tt>.</li>
<li>New <tt>char8_t</tt> based signatures for user-defined literal
operators.</li>
</ul></p>
<p>The following library changes are proposed to address concerns like those
raised in the motivation section above, and to take advantage of the new
core features:
<ul>
<li>New <tt>char8_t</tt> based specializations of <tt>atomic</tt>,
<tt>numeric_limits</tt>, <tt>hash</tt>, <tt>char_traits</tt>,
<tt>basic_string</tt>, and <tt>basic_string_view</tt>.</li>
<li>New <tt>u8streampos</tt>, <tt>u8string</tt>, <tt>u8string_view</tt>
type aliases.</li>
<li>New <tt>operator ""s</tt> and <tt>operator ""sv</tt> <tt>char8_t</tt>
based overloads for UTF-8 literals.</li>
<li>New <tt>char8_t</tt> based specializations of <tt>codecvt</tt> and
<tt>codecvt_byname</tt> for converting between UTF-16, UTF-32, and
UTF-8. The existing <tt>char</tt> based specializations are deprecated.
The new specializations are functionally identical to the deprecated
ones.</li>
<li>The return type of the <tt>u8string</tt> and <tt>generic_u8string</tt>
member functions of the filesystem <tt>path</tt> class are changed
from <tt>string</tt> to <tt>u8string</tt>.</li>
<li>Filesystem <tt>path</tt> objects may now be constructed with UTF-8
strings using the existing <tt>path</tt> constructors used for
construction with other encodings. The existing <tt>u8path</tt>
factory functions are deprecated.</li>
</ul></p>
<p>These changes necessarily impact backward compatibility as described in
the <a href="#design_compat">Backward compatibility</a> section.</p>
<h1 id="design">Design Considerations</h1>
<h2 id="design_compat">Backward compatibility</h2>
<p>This proposal does not specify any backward compatibility features other than
to retain interfaces that it deprecates. The author believes such features are
necessary, but that a single set of such features would unnecessarily compromise
the goals of this proposal. Rather, the expectation is that implementations
will provide options to enable more fine grained compatibility features.</p>
<p>The following sections discuss backward compatibility impact.</p>
<h3 id="design_compat_core">Core language backward compatibility</h3>
<h4 id="design_compat_core_init">Initialization</h4>
<p>Declarations of arrays of <tt>char</tt> may currently be initialized with
UTF-8 string literals. Under this proposal, such initializations would
become ill-formed. This is intended to maintain consistency with
initialization of arrays of <tt>wchar_t</tt>, <tt>char16_t</tt>, and
<tt>char32_t</tt>, all of which require the initializing string literal to
have a matching element type as specified in § 11.6.2 [dcl.init.string].
<fieldset>
<pre><code class="c++">char ca[] = u8"text"; // C++17: Ok.
// This proposal: Ill-formed.
char8_t c8a[] = "text"; // C++17: N/A (char8_t is not a type specifier).
// This proposal: Ill-formed.
</code></pre>
</fieldset>
</p>
<p>Implementations are encouraged to add options to allow the above
initializations (with a warning) to assist users in migrating their code.</p>
<p>Declarations of variables of type <tt>char</tt> initialized with a UTF-8
character literal remain well-formed and are initialized following the
standard conversion rules.
<fieldset>
<pre><code class="c++">char c = u8'c'; // C++17: Ok.
// This proposal: Ok (no change from C++17).
char8_t c8 = 'c'; // C++17: N/A (char8_t is not a type specifier).
// This proposal: Ok; c8 is assigned the value of the 'c'
// character in the execution character set.
</code></pre>
</fieldset>
</p>
<h4 id="design_compat_core_implicit_conversion">Implicit conversions</h4>
<p>Under this proposal, UTF-8 string literals no longer bind to references
to array of type <tt>const char</tt> nor do they implicitly convert to pointer
to <tt>const char</tt>. The following code is currently well-formed, but would
become ill-formed under this proposal:
<fieldset>
<pre><code class="c++">const char (&u8r)[] = u8"text"; // C++17: Ok.
// This proposal: Ill-formed.
const char *u8p = u8"text"; // C++17: Ok.
// This proposal: Ill-formed.
</code></pre>
</fieldset>
</p>
<p>Implementations are encouraged to add options to allow the above
conversions (with a warning) to assist users in migrating their code.
Such options would require allowing aliasing of <tt>char</tt> and
<tt>char8_t</tt>. Note that it may be useful to permit these conversions
only for UTF-8 string literals and not for general expressions of array
of <tt>char8_t</tt> type.</p>
<h4 id="design_compat_core_type_deduction">Type deduction</h4>
<p>Under this proposal, UTF-8 string and character literals have type array of
<tt>const char8_t</tt> and <tt>char8_t</tt> respectively. This affects the
types deduced for placeholder types and template parameter types.
<fieldset>
<pre><code class="c++">template<typename T1, typename T2>
void ft(T1, T2);
ft(u8"text", u8'c'); // C++17: T1 deduced to const char*, T2 deduced to char.
// This proposal: T1 deduced to const char8_t*, T2 deduced to char8_t.
auto u8p = u8"text"; // C++17: Type deduced to const char*.
// This proposal: Type deduced to const char8_t*.
auto u8c = u8'c'; // C++17: Type deduced to char.
// This proposal: Type deduced to char8_t.
</code></pre>
</fieldset>
</p>
<p>This change in behavior is a primary objective of this proposal.
Implementations are encouraged to add options to disable <tt>char8_t</tt>
support entirely when necessary to preserve compatibility with C++17.</p>
<h4 id="design_compat_core_overload_resolution">Overload resolution</h4>
<p>The following code is currently well-formed, and would remain well-formed
under this proposal, but would behave differently:
<fieldset>
<pre><code class="c++">template<typename T> void f(const T*);
void f(const char*);
f(u8"text"); // C++17: Calls f(const char*).
// This proposal: Calls f<char8_t>(const char8_t*).
</code></pre>
</fieldset>
</p>
<p>The following code is currently well-formed, but would become ill-formed
under this proposal:
<fieldset>
<pre><code class="c++">void f(const char*);
f(u8"text"); // C++17: Ok.
// This proposal: Ill-formed; no matching function found.
int operator ""_udl(const char*, size_t);
auto x = u8"text"_udl; // C++17: Ok
// This proposal: Ill-formed; no matching literal operator found.
</code></pre>
</fieldset>
</p>
<p>These changes in behavior are a primary objective of this proposal.
Implementations are encouraged to add options to disable <tt>char8_t</tt>
support entirely when necessary to preserve compatibility with C++17.</p>
<h4 id="design_compat_core_template_specialization">Template specialization</h4>
<p>The following code is currently well-formed, and would remain well-formed
under this proposal, but would behave differently:
<fieldset>
<pre><code class="c++">template<typename T> struct ct { static constexpr bool value = false; };
template<> struct ct<char> { static constexpr bool value = true; };
template<typename T> bool ft(const T*) { return ct<T>::value; }
ft(u8"text"); // C++17: returns true.
// This proposal: returns false.
</code></pre>
</fieldset>
</p>
<p>This change in behavior is a primary objective of this proposal.
Implementations are encouraged to add options to disable <tt>char8_t</tt>
support entirely when necessary to preserve compatibility with C++17.</p>
<h3 id="design_compat_library">Library backward compatibility</h3>
<h4 id="design_compat_library_u8string">
Return type of <tt>path::u8string</tt> and <tt>path::generic_u8string</tt></h4>
<p>This proposal includes a new specialization of <tt>std::basic_string</tt>
for the new <tt>char8_t</tt> type, a new <tt>std::u8string</tt> type alias,
and changes to the <tt>u8string</tt> and <tt>generic_u8string</tt> member
functions of <tt>filesystem::path</tt> to return <tt>std::u8string</tt>
instead of <tt>std::string</tt>. This change renders ill-formed the following
code that is currently well-formed.
<fieldset>
<pre><code class="c++">void f(std::filesystem::path p) {
std::string s;
s = p.u8string(); // C++17: Ok.
// This proposal: ill-formed.
}
</code></pre>
</fieldset>
</p>
<p>Implementations are encouraged to add an option that allows implicit
conversion of <tt>std::u8string</tt> to <tt>std::string</tt> to assist in
a gradual migration of code that calls these functions.</p>
<h4 id="design_compat_library_literal_operators">
Return type of <tt>operator ""s</tt> and <tt>operator ""sv</tt></h4>
<p>This proposal includes new overloads of <tt>operator ""s</tt> and
<tt>operator ""sv</tt> that return <tt>char8_t</tt> specializations of
<tt>std::basic_string</tt> and <tt>std::basic_string_view</tt> respectively.
This change renders ill-formed the following code that is currently well-formed.
<fieldset>
<pre><code class="c++">std::string s;
s = u8"text"s; // C++17: Ok.
// This proposal: ill-formed.
s = u8"text"sv; // C++17: Ok.
// This proposal: ill-formed.
</code></pre>
</fieldset>
</p>
<p>Implementations are encouraged to add an option that allows implicit
conversion of <tt>std::u8string</tt> to <tt>std::string</tt> to assist in
a gradual migration of code that calls these functions.</p>
<h2 id="design_narrow_utf8">
Should UTF-8 literals continue to be referred to as narrow literals?</h2>
<p>UTF-8 literals are maintained as narrow literals in this proposal.</p>
<h2 id="design_char8_t_underlying_type">
What should be the underlying type of char8_t?</h2>
<p>There are several choices for the underlying type of <tt>char8_t</tt>.
Use of <tt>unsigned char</tt> closely aligns with historical use. Use of
<tt>uint_least8_t</tt> would maintain consistency with how the underlying
types of <tt>char16_t</tt> and <tt>char32_t</tt> are specified.</p>
<p>This proposal specifies <tt>unsigned char</tt> as the underlying type as
noted in the changes to § 6.7.1 <tt>[basic.fundamental]</tt> paragraph 5.</p>
<h1 id="implementation_exp">Implementation Experience</h1>
<p>An implementation is available in the <tt>char8_t</tt> branch of a gcc
fork hosted on GitHub at
<a href="https://github.com/tahonermann/gcc/tree/char8_t">
https://github.com/tahonermann/gcc/tree/char8_t</a>. This implementation is
believed to be complete for both the proposed core language and library
features with the exception of the proposed <tt>mbrtoc8</tt> and
<tt>c8rtomb</tt> transcoding functions (the author expects to complete these
shortly). New <tt>-fchar8_t</tt> and <tt>-fno-char8_t</tt> compiler options
support enabling and disabling the new features. No backward compatibility
features are currently implemented.</p>
<p>Richard Smith implemented support for the proposed core wording changes
and they are present in the release of Clang 7. The changes are guarded by new
<tt>-fchar8_t</tt> and <tt>-fno-char8_t</tt> options matching the gcc
implementation. No backward compatibility features are currently implemented.
Support for the proposed library features has not yet been implemented in
libc++. Richard's changes can be found at
<a href="http://llvm.org/viewvc/llvm-project?view=revision&revision=331244">
http://llvm.org/viewvc/llvm-project?view=revision&revision=331244</a>
</p>
<h1 id="wording">Formal Wording</h1>
<input type="checkbox" id="hidedel">Hide deleted text</input>
<p>These changes are relative to
<a title="Working Draft, Standard for Programming Language C++"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf">
N4762</a>
<sup><a title="Working Draft, Standard for Programming Language C++"
href="#ref_n4762">
[N4762]</a></sup></p>
<h2 id="core_wording">Core wording</h2>
<p>Change in
<a href="http://eel.is/c++draft/lex.key#1">
table 5 of 5.11 [lex.key] paragraph 1</a>:
<blockquote>
[…]<br/>
<div style="margin-left: 1em;">
<table>
<tr>
<td align="center">
Table 5 — Keywords
</td>
</tr>
<tr>
<td align="center">
<table border="1">
<tr><td>[…]</td></tr>
<tr><td>char</td></tr>
<tr><td><ins>char8_t</ins></td></tr>
<tr><td>char16_t</td></tr>
<tr><td>char32_t</td></tr>
<tr><td>[…]</td></tr>
</table>
</td>
</tr>
</table>
</div>
[…]<br/>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.literal#lex.ccon-3">
5.13.3 [lex.ccon] paragraph 3</a>:
<blockquote>
A character literal that begins with <tt>u8</tt>, such as <tt>u8'w'</tt>, is a
character literal of type <del><tt>char</tt></del><ins><tt>char8_t</tt></ins>,
known as a <em>UTF-8 character literal</em>.[…]
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.literal#lex.string-6">
5.13.5 [lex.string] paragraph 6</a>:
<blockquote>
After translation phase 6, a <em>string-literal</em> that does not begin with
an <em>encoding-prefix</em> is an <em>ordinary string literal</em><ins>. An
ordinary string literal has type "<em>array</em> of <em>n</em>
<tt>const char</tt>" where <em>n</em> is the size of the string as defined
below, has static storage duration
<a href="http://eel.is/c++draft/basic.stc">(6.6.4)</a></ins>, and is initialized
with the given characters.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.literal#lex.string-7">
5.13.5 [lex.string] paragraph 7</a>:
<blockquote>
A <em>string-literal</em> that begins with <tt>u8</tt>, such as
<tt>u8"asdf"</tt>, is a <em>UTF-8 string literal</em><ins>, also referred to as
a <tt>char8_t</tt> string literal. A <tt>char8_t</tt> string literal has type
"<em>array</em> of <em>n</em> <tt>const char8_t</tt>", where <em>n</em> is
the size of the string as defined below; each successive element of the object
representation
<a href="http://eel.is/c++draft/basic.types">(6.7)</a> has the value of the
corresponding code unit of the UTF-8 encoding of the string.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.literal#lex.string-8">
5.13.5 [lex.string] paragraph 8</a>:
<blockquote>
Ordinary string literals and UTF-8 string literals are also referred to as
narrow string literals. <del>A narrow string literal has type "<em>array</em>
of <em>n</em> <tt>const char</tt>", where <em>n</em> is the size of the string
as defined below, and has static storage duration
<a href="http://eel.is/c++draft/basic.stc">(6.6.4)</a>.</del>
</blockquote>
</p>
<p><em>Drafting note: The deleted paragraph 8 content was incorporated in the
changes to paragraphs 6 and 7.</em></p>
<p>Remove
<a href="http://eel.is/c++draft/lex.literal#lex.string-9">
5.13.5 [lex.string] paragraph 9</a>:
<blockquote class=stddel>
For a UTF-8 string literal, each successive element of the object
representation
<a href="http://eel.is/c++draft/basic.types">(6.7)</a>
has the value of the corresponding code unit of the UTF-8
encoding of the string.
</blockquote>
</p>
<p><em>Drafting note: The paragraph 9 content was incorporated in the changes
to paragraph 7.</em></p>
<p>Change in
<a href="http://eel.is/c++draft/lex.literal#lex.string-15">
5.13.5 [lex.string] paragraph 15</a>:
<blockquote>
[…] In a narrow string literal, a <em>universal-character-name</em>
may map to more than one <tt>char</tt> <ins>or <tt>char8_t</tt></ins> element
due to <em>multibyte encoding</em>. […]
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/basic.align#6">
6.6.5 [basic.align] paragraph 6</a>:
<blockquote>
The alignment requirement of a complete type can be queried using an
<tt>alignof</tt> expression
<a href="http://eel.is/c++draft/expr.alignof">(7.6.2.6)</a>.
Furthermore, the narrow character types
<a href="http://eel.is/c++draft/basic.fundamental">(6.7.1)</a>
shall
have the weakest alignment requirement. [ <em>Note</em>: This enables the
<del>narrow</del><ins>ordinary</ins> character types to be used as the
underlying type for an aligned memory area
<a href="http://eel.is/c++draft/dcl.align">(9.11.2)</a>. —
<em>end note</em> ]
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/basic.fundamental#1">
6.7.1 [basic.fundamental] paragraph 1</a>:
<blockquote>
Objects declared <del>as characters</del><ins>with type </ins>
<del>(</del><tt>char</tt><del>)</del> shall be large enough to store any member
of the implementation’s basic character set. If a character from this set is
stored in a character object, the integral value of that character object is
equal to the value of the single character literal form of that character. It is
implementation-defined whether a <tt>char</tt> object can hold negative values.
Characters <ins>declared with type <tt>char</tt> </ins>can be explicitly
declared <tt>unsigned</tt> or <tt>signed</tt>. Plain <tt>char</tt>,
<tt>signed char</tt>, and <tt>unsigned char</tt> are three distinct types,
collectively called <em><del>narrow</del><ins>ordinary</ins> character
types</em>. <ins>The ordinary character types and <tt>char8_t</tt> are
collectively called <em>narrow character types</em>.</ins> A <tt>char</tt>, a
<tt>signed char</tt>, <del>and </del>an <tt>unsigned char</tt><ins>, and a
<tt>char8_t</tt></ins> occupy the same amount of storage and have the same
alignment requirements
<a href="http://eel.is/c++draft/basic.align">(6.6.5)</a>; that is, they have
the same object representation. For narrow character types, all bits of the
object representation participate in the value representation. [ <em>Note</em>:
A bit-field of narrow character type whose length is larger than the number of
bits in the object representation of that type has padding bits; see
<a href="http://eel.is/c++draft/basic.types">6.7</a>.
— <em>end note</em> ] For unsigned narrow character types,
each possible bit pattern of the value representation
represents a distinct number. These requirements do not hold for other types.
In any particular implementation, a plain <tt>char</tt> object can
take on either the same values as a <tt>signed char</tt> or an
<tt>unsigned char</tt>; which one is implementation-defined. For each value
<em>i</em> <del>of type <tt>unsigned char</tt> </del>in the range 0 to 255
inclusive<ins> of type <tt>unsigned char</tt> or <tt>char8_t</tt></ins>, there
exists a value <em>j</em> of type <tt>char</tt>
such that the result of an integral conversion
<a href="http://eel.is/c++draft/conv.integral">(7.3.8)</a>
from <em>i</em> to
<tt>char</tt> is <em>j</em>, and the result of an integral conversion from
<em>j</em> to <tt>unsigned char</tt><ins> or <tt>char8_t</tt></ins> is
<em>i</em>.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/basic.fundamental#5">
6.7.1 [basic.fundamental] paragraph 5</a>:
<blockquote>
[…] Type <tt>wchar_t</tt> shall have the same size, signedness, and
alignment requirements
<a href="http://eel.is/c++draft/basic.align">(6.6.5)</a>
as one of the other integral types, called its
underlying type. <ins>Type <tt>char8_t</tt> denotes a distinct type with the
same size, signedness, and alignment as <tt>unsigned char</tt>, called its
underlying type.</ins> Types <tt>char16_t</tt> and <tt>char32_t</tt> denote
distinct types with the same size, signedness, and alignment as
<tt>uint_least16_t</tt> and <tt>uint_least32_t</tt>, respectively, in
<tt><cstdint></tt>, called the underlying types.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/basic.fundamental#7">
6.7.1 [basic.fundamental] paragraph 7</a>:
<blockquote>
Types <tt>bool</tt>, <tt>char</tt>, <ins><tt>char8_t</tt>, </ins>
<tt>char16_t</tt>, <tt>char32_t</tt>, <tt>wchar_t</tt>, and the signed and
unsigned integer types are collectively called integral types. […]
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/conv.rank#1.8">
6.7.4 [conv.rank] subparagraph (1.8)</a>:
<blockquote>
[…]<br/>
(1.8) — The ranks of <ins><tt>char8_t</tt>, </ins><tt>char16_t</tt>,
<tt>char32_t</tt>, and <tt>wchar_t</tt> shall equal the ranks of their
underlying types <a href="http://eel.is/c++draft/basic.fundamental">(6.7.1)</a>.
<br/>[…]
</blockquote>
</p>
<p>Change to
<a href="http://eel.is/c++draft/expr.arith.conv#footnote-65">
footnote 65 associated with 7.4 [expr.arith.conv] subparagraph (1.5)</a>:
<blockquote>
As a consequence, operands of type <tt>bool</tt>, <ins><tt>char8_t</tt>, </ins>
<tt>char16_t</tt>, <tt>char32_t</tt>, <tt>wchar_t</tt>, or an enumerated type
are converted to some integral type.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/expr.sizeof#1">
7.6.2.3 [expr.sizeof] paragraph 1</a>:
<blockquote>
[…] <del><tt>sizeof(char)</tt>, <tt>sizeof(signed char)</tt>
and <tt>sizeof(unsigned char)</tt> are 1</del><ins>The result of <tt>sizeof</tt>
applied to any of the narrow character types is 1</ins>. The result of
<tt>sizeof</tt> applied to any other fundamental type
<a href="http://eel.is/c++draft/basic.fundamental">(6.7.1)</a> is
implementation-defined.