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137 lines
4.5 KiB
HTML
137 lines
4.5 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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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="#Syntax">Locale Support</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>
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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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MSVC.</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 uses <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. TODO: table of correspondence between
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printf and the new syntax
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</p>
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<h3><a name="Locale">Locale Support</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="References">References</a></h2>
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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
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</p>
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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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</p>
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</body>
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