327 lines
11 KiB
HTML
327 lines
11 KiB
HTML
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html;charset=US-ASCII">
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<title>Text Formatting</title>
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</head>
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<body>
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<h1>Text Formatting</h1>
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<p>
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2016-08-19
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</p>
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<address>
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Victor Zverovich, victor.zverovich@gmail.com
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</address>
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<p>
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<a href="#Introduction">Introduction</a><br>
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<a href="#Design">Design</a><br>
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<a href="#Syntax">Format String Syntax</a><br>
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<a href="#Extensibility">Extensibility</a><br>
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<a href="#Safety">Safety</a><br>
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<a href="#Locale">Locale Support</a><br>
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<a href="#PosArguments">Positional Arguments</a><br>
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<a href="#Wording">Wording</a><br>
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<a href="#References">References</a><br>
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</p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>
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This paper proposes a new text formatting functionality that can be used as a
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safe and extensible alternative to the <code>printf</code> family of functions.
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It is intended to complement the existing C++ I/O streams library and reuse
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some of its infrastructure such as overloaded insertion operators for
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user-defined types.
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</p>
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<p>
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Example:
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<pre class="example">
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<code>std::string message = std::format("The answer is {}.", 42);</code>
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</pre>
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<h2><a name="Design">Design</a></h2>
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<h3><a name="Syntax">Format String Syntax</a></h3>
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<p>
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Variations of the printf format string syntax are arguably the most popular
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among the programming languages and C++ itself inherits <code>printf</code>
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from C <a href="#1">[1]</a>. The advantage of the printf syntax is that many
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programmers are familiar with it. However, in its current form it has a number
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of issues:
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</p>
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<ul>
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<li>Many format specifiers like <code>hh</code>, <code>h</code>, <code>l</code>,
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<code>j</code>, etc. are used only to convey type information.
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They are redundant in type-safe formatting and would unnecessarily
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complicate specification and parsing.</li>
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<li>There is no standard way to extend the syntax for user-defined types.</li>
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<li>There are subtle differences between different implementations. For example,
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POSIX positional arguments <a href="#2">[2]</a> are not supported on
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some systems <a href="#6">[6]</a>.</li>
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<li>Using <code>'%'</code> in a custom format specifier, e.g. for
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<code>put_time</code>-like time formatting, poses difficulties.</li>
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</ul>
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<p>
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Although it is possible to address these issues, this will break compatibility
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and can potentially be more confusing to users than introducing a different
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syntax.
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</p>
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</p>
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Therefore we propose a new syntax based on the ones used in Python
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<a href="#3">[3]</a>, the .NET family of languages <a href="#4">[4]</a>,
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and Rust <a href="#5">[5]</a>. This syntax employs <code>'{'</code> and
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<code>'}'</code> as replacement field delimiters instead of <code>'%'</code>
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and it is described in details in TODO:link. Here are some of the advantages:
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</p>
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<ul>
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<li>Consistent and easy to parse mini-language focused on formatting rather
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than conveying type information</li>
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<li>Extensibility and support for custom format strings for user-defined
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types</li>
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<li>Positional arguments</li>
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<li>Support for both locale-specific and locale-independent formatting (see
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<a href="#Locale">Locale Support</a>)</li>
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<li>Minor formatting improvements such as center alignment and binary format
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</ul>
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<p>
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The syntax is expressive enough to enable translation, possibly automated,
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of most printf format strings. The correspondence between <code>printf</code>
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and the new syntax is given in the following table.
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</p>
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<table>
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<thead>
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<tr><th>printf</th><th>new</th><th>comment</th></tr>
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</thead>
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<tbody>
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<tr><td>-</td><td><</td><td>left alignment</td></tr>
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<tr><td>+</td><td>+</td><td></td></tr>
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<tr><td><em>space</em></td><td><em>space</em></td><td></td></tr>
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<tr><td>#</td><td>#</td><td></td></tr>
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<tr><td>0</td><td>0</td><td></td></tr>
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<tr><td>hh</td><td>unused</td><td></td></tr>
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<tr><td>h</td><td>unused</td><td></td></tr>
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<tr><td>l</td><td>unused</td><td></td></tr>
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<tr><td>ll</td><td>unused</td><td></td></tr>
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<tr><td>j</td><td>unused</td><td></td></tr>
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<tr><td>z</td><td>unused</td><td></td></tr>
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<tr><td>t</td><td>unused</td><td></td></tr>
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<tr><td>L</td><td>unused</td><td></td></tr>
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<tr><td>c</td><td>c (optional)</td><td></td></tr>
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<tr><td>s</td><td>s (optional)</td><td></td></tr>
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<tr><td>d</td><td>d (optional)</td><td></td></tr>
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<tr><td>i</td><td>d (optional)</td><td></td></tr>
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<tr><td>o</td><td>o</td><td></td></tr>
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<tr><td>x</td><td>x</td><td></td></tr>
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<tr><td>X</td><td>X</td><td></td></tr>
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<tr><td>u</td><td>d (optional)</td><td></td></tr>
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<tr><td>f</td><td>f</td><td></td></tr>
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<tr><td>F</td><td>F</td><td></td></tr>
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<tr><td>e</td><td>e</td><td></td></tr>
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<tr><td>E</td><td>E</td><td></td></tr>
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<tr><td>a</td><td>a</td><td></td></tr>
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<tr><td>A</td><td>A</td><td></td></tr>
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<tr><td>g</td><td>g (optional)</td><td></td></tr>
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<tr><td>G</td><td>G</td><td></td></tr>
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<tr><td>n</td><td>unused</td><td></td></tr>
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<tr><td>p</td><td>p (optional)</td><td></td></tr>
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</tbody>
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</table>
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<p>
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Width and precision are represented similarly in <code>printf</code> and the
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proposed syntax with the only difference that runtime value is specified by
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<code>*</code> in the former and <code>{}</code> in the latter, possibly with
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the index of the argument inside the braces.
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</p>
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<p>
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As can be seen from the table above, most of the specifiers remain the same
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which simplifies migration from <code>printf</code>. Notable difference is
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in the alignment specification. The proposed syntax allows left, center,
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and right alignment represented by <code>'<'</code>, <code>'^'</code>,
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and <code>'>'</code> respectively which is more expressive than the
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corresponding <code>printf</code> syntax. The latter only supports left and
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right (the default) alignment.
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</p>
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<p>
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The following example uses center alignment and <code>'*'</code> as a fill
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character:
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</p>
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<pre class="example">
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<code>std::format("{:*^30}", "centered");</code>
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</pre>
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<p>
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resulting in <code>"***********centered***********"</code>.
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The same formatting cannot be easily achieved with <code>printf</code>.
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</p>
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<h3><a name="Extensibility">Extensibility</a></h3>
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<p>
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Both the format string syntax and the API are designed with extensibility in
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mind. The mini-language can be extended for user-defined types and users can
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provide functions that do parsing and formatting for such types.
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</p>
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<p>The general syntax of a replacement field in a format string is
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<dl>
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<dt><em>replacement-field</em>:</dt>
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<dd>
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<code>{</code> <em>integer<sub>opt</sub></em> <code>}</code><br/>
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<code>{</code> <em>integer<sub>opt</sub></em>
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<code>:</code> <em>format-spec</em> <code>}</code>
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</dd>
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</dl>
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<p>
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where <em>format-spec</em> is predefined for built-in types, but can be
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customized for user-defined types. For example, the syntax can be extended
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for <code>put_time</code>-like date and time formatting
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</p>
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<pre class="example">
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<code>std::time_t t = std::time(nullptr);
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std::string date = std::format("The date is {0:%Y-%m-%d}.", *std::localtime(&t));</code>
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</pre>
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<p>by providing an overload of <code>std::format_arg</code> for
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<code>std::tm</code>:</p>
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TODO: example
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<h3><a name="Safety">Safety</a></h3>
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Formatting functions rely on variadic templates instead of the mechanism
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provided by <code><cstdarg></code>. The type information is captured
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automatically and passed to formatters guaranteeing type safety and making
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many of the <code>printf</code> specifiers redundant (see <a href="#Syntax">
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Format String Syntax</a>). Buffer management is also automatic to prevent
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buffer overflow errors common to <code>printf</code>.
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<h3><a name="Locale">Locale Support</a></h3>
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<p>
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As pointed out in
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0067r1.html">
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P0067R1: Elementary string conversions</a> there is a number of use
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cases that do not require internationalization support, but do require high
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throughput when produced by a server. These include various text-based
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interchange formats such as JSON or XML. The need for locale-independent
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functions for conversions between integers and strings and between
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floating-point numbers and strings has also been highlighted in
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<a href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4412.html">
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N4412: Shortcomings of iostreams</a>. Therefore a user should be able to
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easily control whether to use locales or not during formatting.
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</p>
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<p>
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We follow Python's approach <a href="#3">[3]</a> and designate a separate format
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specifier <code>'n'</code> for locale-aware numeric formatting. It applies to
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all integral and floating-point types. All other specifiers produce output
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unaffected by locale settings.
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</p>
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<h3><a name="PosArguments">Positional Arguments</a></h3>
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<p>TODO</p>
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<h3><a name="Locale">Performance</a></h3>
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<p>TODO</p>
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<h2><a name="Wording">Wording</a></h2>
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<p>TODO</p>
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<h2><a name="Implementation">Implementation</a></h2>
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<p>
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The ideas proposed in this paper have been implemented in the open-source fmt
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library. TODO: link and mention other implementations (Boost Format, FastFormat)
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</p>
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<h2><a name="References">References</a></h2>
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<p>
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<a name="1">[1]</a>
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<cite>The <code>fprintf</code> function. ISO/IEC 9899:2011. 7.21.6.1.</cite><br/>
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<a name="2">[2]</a>
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<cite><a href="http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html">
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fprintf, printf, snprintf, sprintf - print formatted output</a>. The Open
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Group Base Specifications Issue 6 IEEE Std 1003.1, 2004 Edition.</cite><br/>
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<a name="3">[3]</a>
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<cite><a href="https://docs.python.org/3/library/string.html#format-string-syntax">
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6.1.3. Format String Syntax</a>. Python 3.5.2 documentation.</cite><br/>
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<a name="4">[4]</a>
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<cite><a href="https://msdn.microsoft.com/en-us/library/system.string.format(v=vs.110).aspx">
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String.Format Method</a>. .NET Framework Class Library.</cite><br/>
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<a name="5">[5]</a>
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<cite><a href="https://doc.rust-lang.org/std/fmt/">
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Module <code>std::fmt</code></a>. The Rust Standard Library.</cite><br/>
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<a name="6">[6]</a>
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<cite><a href="https://msdn.microsoft.com/en-us/library/56e442dc(v=vs.120).aspx">
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Format Specification Syntax: printf and wprintf Functions</a>. C++ Language and
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Standard Libraries.</cite><br/>
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</p>
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</body>
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