1060 lines
36 KiB
HTML
1060 lines
36 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Language" content="en-us">
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<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
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<title>The Boost Format library</title>
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</head>
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<body bgcolor="white" text="black">
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<h1><img align="middle" alt="boost.png (6897 bytes)" height="86" src=
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"../../../boost.png" width="277">The Boost Format library</h1>
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<p>The <code><a href=
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"../../../boost/format.hpp"><boost/format.hpp></a></code> format
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class provides printf-like formatting, in a type-safe manner which allows
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output of user-defined types.<br></p>
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<ul>
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<li><a href="#synopsis">Synopsis</a></li>
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<li><a href="#how_it_works">How it works</a></li>
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<li><a href="#examples">Examples</a></li>
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<li>
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<a href="#syntax">Syntax</a>
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<ul>
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<li><a href="#printf_directives">printf format-specification
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syntax</a></li>
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<li><a href="#printf_differences">Incompatibilities with
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printf</a></li>
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</ul>
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</li>
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<li><a href="#manipulators">Manipulators and the internal stream
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state</a></li>
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<li><a href="#user-defined">User-defined types</a></li>
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<li><a href="#alternatives">Alternatives</a></li>
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<li><a href="#exceptions">Exceptions</a></li>
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<li><a href="#performance">Performance</a></li>
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<li><a href="#extract">Class Interface Extract</a></li>
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<li><a href="#rationale">Rationale</a></li>
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</ul><a name="synopsis" id="synopsis"></a>
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<hr>
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<h2>Synopsis</h2>
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<p>A format object is constructed from a format-string, and is then given
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arguments through repeated calls to <i>operator%</i>.<br>
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Each of those arguments are then converted to strings, who are in turn
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combined into one string, according to the format-string.</p>
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<blockquote>
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<pre>
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cout << boost::format("writing %1%, x=%2% : %3%-th try") % "toto" % 40.23 % 50;
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// prints "writing toto, x=40.230 : 50-th try"
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</pre>
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</blockquote><a name="how_it_works" id="how_it_works"></a>
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<hr>
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<h2>How it works</h2>
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<ol>
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<li>When you call <i>format(s)</i>, where s is the format-string, it
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constructs an object, which parses the format string and look for all
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directives in it and prepares internal structures for the next step.</li>
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<li>Then, either immediately, as in
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<blockquote>
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<pre>
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cout << format("%2% %1%") % 36 % 77;
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</pre>
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</blockquote>or later on, as in
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<blockquote>
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<pre>
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format fmter("%2% %1%");
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fmter % 36; fmter % 77;
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</pre>
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</blockquote>you <i>feed</i> variables into the formatter.<br>
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those variables are dumped into an internal stream, which state is set
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according to the given formatting options in the format-string -if
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there are any-, and the format object stores the string results for the
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last step.
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</li>
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<li>Once all arguments have been fed you can dump the format object to a
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stream, or get its string value by using the <i>str()</i> member
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function, or the free function <i>str(const format& )</i> in
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namespace <i>boost</i>. The result string stays accessible in the format
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object until another argument is passed, at which time it is
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reinitialised.
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<blockquote>
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<pre>
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// fmter was previously created and fed arguments, it can print the result :
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cout << fmter ;
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// You can take the string result :
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string s = fmter.str();
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// possibly several times :
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s = fmter.str( );
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// You can also do all steps at once :
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cout << boost::format("%2% %1%") % 36 % 77;
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// using the str free function :
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string s2 = str( format("%2% %1%") % 36 % 77 );
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</pre>
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</blockquote>
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</li>
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<li>Optionnally, after step 3, you can re-use a format object and restart
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at step2 : <i>fmter % 18 % 39;</i><br>
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to format new variables with the same format-string, saving the expensive
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processing involved at step 1.</li>
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</ol>All in all, the format class translates a format-string (with
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eventually printf-like directives) into operations on an internal stream,
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and finally returns the result of the formatting, as a string, or directly
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into an output stream. <a name="examples" id="examples"></a>
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<hr>
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<h2>Examples</h2>
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<blockquote>
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<pre>
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using namespace std;
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using boost::format;
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using boost::io::group;
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</pre>
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</blockquote>
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<ul>
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<li>Simple output, with reordering :
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<blockquote>
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<pre>
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cout << format("%1% %2% %3% %2% %1% \n") % "11" % "22" % "333"; // 'simple' style.
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</pre>
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</blockquote>It prints : "11 22 333 22 11 \n"
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</li>
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<li>More precise formatting, with Posix-printf positional directives :
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<blockquote>
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<pre>
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cout << format("(x,y) = (%1$+5d,%2$+5d) \n") % -23 % 35; // Posix-Printf style
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</pre>
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</blockquote>It prints : "(x,y) = ( -23, +35) \n"
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</li>
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<li>classical printf directive, no reordering :
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<blockquote>
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<pre>
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cout << format("writing %s, x=%s : %d-th step \n") % "toto" % 40.23 % 50;
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</pre>
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</blockquote>It prints : "writing toto, x=40.23 : 50-th step \n"
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</li>
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<li>Several ways to express the same thing :
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<blockquote>
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<pre>
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cout << format("(x,y) = (%+5d,%+5d) \n") % -23 % 35;
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cout << format("(x,y) = (%|+5|,%|+5|) \n") % -23 % 35;
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cout << format("(x,y) = (%1$+5d,%2$+5d) \n") % -23 % 35;
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cout << format("(x,y) = (%|1$+5|,%|2$+5|) \n") % -23 % 35;
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</pre>
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</blockquote>all those print : "(x,y) = ( -23, +35) \n"
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</li>
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<li>Using manipulators to modify the format-string :
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<blockquote>
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<pre>
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format fmter("_%1$+5d_ %1$d \n");
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format fmter2("_%1%_ %1% \n");
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fmter2.modify_item(1, group(showpos, setw(5)) );
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cout << fmter % 101 ;
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cout << fmter2 % 101 ;
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</pre>
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</blockquote>Both print the same : "_ +101_ 101 \n"
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</li>
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<li>Using manipulators with arguments :
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<blockquote>
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<pre>
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cout << format("_%1%_ %1% \n") % group(showpos, setw(5), 101);
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</pre>
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</blockquote>The manipulators are applied at each occurrence of %1%, and
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thus it prints : "_ +101_ +101 \n"
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</li>
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<li>New formatting feature : 'absolute tabulations', useful inside loops,
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to insure a field is printed at the same position from one line to the
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next, even if the widthes of the previous arguments can vary a lot.
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<blockquote>
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<pre>
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for(unsigned int i=0; i < names.size(); ++i)
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cout << format("%1%, %2%, %|40t|%3%\n") % names[i] % surname[i] % tel[i];
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</pre>
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</blockquote>For some std::vector <i>names</i>, <i>surnames</i>, and
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<i>tel</i> (see sample_new_features.cpp) it prints :
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<blockquote>
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<pre>
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Marc-François Michel, Durand, +33 (0) 123 456 789
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Jean, de Lattre de Tassigny, +33 (0) 987 654 321
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</pre>
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</blockquote>
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</li>
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</ul>
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<hr>
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<h2>Sample Files</h2>
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<p>The program <a href=
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"../example/sample_formats.cpp">sample_formats.cpp</a> demonstrates simple
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uses of <b>format</b>.<br></p>
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<p><a href="../example/sample_new_features.cpp">sample_new_features.cpp</a>
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illustrates the few formatting features that were added to printf's syntax
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such as simple positional directives, centered alignment, and
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'tabulations'.<br></p>
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<p><a href="../example/sample_advanced.cpp">sample_advanced.cpp</a>
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demonstrates uses of advanced features, like reusing, and modifying, format
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objects, etc..<br></p>
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<p>And <a href="../example/sample_userType.cpp">sample_userType.cpp</a>
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shows the behaviour of the <b>format</b> library on user-defined
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types.</p><a name="syntax" id="syntax"></a>
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<hr>
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<h2>Syntax</h2>
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<p><b>boost::format(</b> format-string <b>) %</b> arg1 <b>%</b> arg2
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<b>%</b> ... <b>%</b> argN</p>
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<p>The <i>format-string</i> contains text in which special directives will
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be replaced by strings resulting from the formatting of the given
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arguments.<br>
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The legacy syntax in the C and C++ worlds is the one used by printf, and
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thus format can use directly printf format-strings, and produce the same
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result (in almost all cases. see <a href=
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"#printf_differences">Incompatibilities with printf</a> for details)<br>
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This core syntax was extended, to allow new features, but also to adapt to
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the C++ streams context. Thus, format accepts several forms of directives
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in format-strings :</p>
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<ul>
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<li>Legacy printf format strings : <b>%</b><i>spec</i> where <i>spec</i>
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is a <a href="#printf_directives">printf format specification</a><br>
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<i>spec</i> passes formatting options, like width, alignment, numerical
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base used for formatting numbers, as well as other specific flags. But
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the classical <i>type-specification</i> flag of printf has a weaker
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meaning in format. It merely sets the appropriate flags on the internal
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stream, and/or formatting parameters, but does not require the
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corresponding argument to be of a specific type.<br>
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e.g. : the specification <i>2$x</i>, meaning "print argument number 2,
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which is an integral number, in hexa" for printf, merely means "print
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argument 2 with stream basefield flags set to <i>hex</i>" for
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format.</li>
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<li><b>%|</b><i>spec</i><b>|</b> where <i>spec</i> is a printf format
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specification.<br>
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This pipe-delimited syntax is introduced, to improve the readability of the
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format-string, but primarily, to make the <i>type-conversion
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character</i> optional in <i>spec</i>. This information is not necessary
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with C++ variables, but with direct printf syntax, it is necessary to
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always give a type-conversion character, merely because this character is
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crucial to determine the end of a format-specification.<br>
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e.g. : "%|-5|" will format the next variable with width set to 5, and
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left-alignment just like the following printf directives : "%-5g",
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"%-5f", "%-5s" ..</li>
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<li><b>%</b><i>N</i><b>%</b><br>
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This simple positional notation requests the formatting of the
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<i>N</i>-th argument - wihout any formatting option.<br>
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(It's merely a shortcut to Printf's positional directives (like
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"%<i>N</i>$s"), but a major benefit is that it's much more readable, and
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does not use a "type-conversion" character)</li>
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</ul>On top of the standard printf format specifications, new features were
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implemented, like centered alignment. See <a href="#new_directives">new
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format specification</a> for details. <a name="printf_directives" id=
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"printf_directives"></a>
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<h3>printf format specifications</h3>
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<p>The printf format specifications supported by Boost.format follows the
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Unix98 <a href=
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"http://www.opengroup.org/onlinepubs/7908799/xsh/fprintf.html">Open-group
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printf</a> precise syntax, rather than the standard C printf, which does
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not support positional arguments. (Common flags have the same meaning in
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both, so it should not be a headache for anybody)<br>
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<i>Note that it is an error to use positional format specifications</i>
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(e.g. <i>%3$+d</i>) <i>mixed with non-positional ones</i> (e.g. <i>%+d</i>)
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<i>in the same format string.</i><br>
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In the Open-group specification, referring to the same argument several
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times (e.g. <i>"%1$d %1$d"</i>) has undefined behaviour. Boost.format's
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behaviour in such cases is to allow each argument to be reffered to any
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number of times. The only constraint is that it expects exactly <i>P</i>
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arguments, <i>P</i> being the maximum argument number used in the format
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string. (e.g., for "%1$d %10$d", <i>P</i> == 10 ).<br>
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Supplying more, or less, than <i>P</i> arguments raises an exception.
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(unless it was set otherwise, see <a href="#exceptions">exceptions</a>)</p>
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<p>A specification <i>spec</i> has the form:
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<pre> [ <i>N</i><b>$</b> ] [ <i>flags</i> ] [ <i>width</i> ] [ <b>.</b> <i>precision</i> ] [ <i>argument-type</i> ] <i>conversion-specifier</i></pre>
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Fields inside square brackets are optional. Each of those fields are
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explained one by one in the following list:</p>
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<ul>
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<li><i>N</i> <b>$</b> (optional field) specifies that the format
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specification applies to the <i>N</i>-th argument (it is called a
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<i>positional format specification</i>).<br>
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If this is not present, arguments are taken one by one. (and it is then
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an error to later supply an argument number)</li><br />
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<li>
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<i>flags</i> is a sequence of any of these:
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<blockquote>
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<table border="1" cellpadding="5" summary="">
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<tr>
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<td><b>Flag</b></td>
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<td><b>Meaning</b></td>
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<td><b>effect on internal stream</b></td>
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</tr>
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<tr>
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<td><b>'-'</b></td>
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<td>left alignment</td>
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<td>N/A (applied later on the string)</td>
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</tr>
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<tr>
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<td><b>'='</b></td>
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<td>centered alignment</td>
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<td>N/A (applied later on the string)<br>
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<i>- note : added feature, not in printf -</i></td>
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</tr>
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<tr>
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<td><b>'_'</b></td>
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<td>internal alignment</td>
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<td>sets internal alignment<br>
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<i>- note : added feature, not in printf -</i></td>
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</tr>
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<tr>
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<td><b>'+'</b></td>
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<td>show sign even for positive numbers</td>
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<td>sets <i>showpos</i></td>
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</tr>
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<tr>
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<td><b>'#'</b></td>
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<td>show numerical base, and decimal point</td>
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<td>sets <i>showbase</i> and <i>showpoint</i></td>
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</tr>
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<tr>
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<td><b>'0'</b></td>
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<td>pad with 0's (inserted after sign or base indicator)</td>
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<td>if not left-aligned, calls <i>setfill('0')</i> and sets
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<i>internal</i><br>
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Extra actions are taken after stream conversion to handle
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<a href="#user-defined">user-defined output</a>.</td>
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</tr>
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<tr>
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<td><b>' '</b></td>
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<td>if the string does not begin with <i>+</i> or <i>-</i>,
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insert a <i>space</i> before the converted string</td>
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<td>N/A (applied later on the string)<br>
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Different to printf's behaviour : it is not affected by internal
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alignment</td>
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</tr>
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</table>
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</blockquote><br />
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</li>
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<li><i>width</i> specifies a minimal width for the string resulting form
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the conversion. If necessary, the string will be padded with alignment
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and fill characters either set on the stream via manipulators, or
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specified by the format-string (e.g. flags '0', '-', ..)<br>
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Note that width is not just set on the conversion stream. To support
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output of <a href="#user-defined">user-defined types</a> (that might call
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<i>operator<<</i> many times on several members), the width is
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handled after stream conversion of the whole argument object, in the
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format class code.</li><br />
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<li>
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<i>precision</i> (preceded by a point), sets the stream's
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<i>precision</i>
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<ul>
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<li>When outputting a floatting type number, it sets the maximum
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number of digits
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<ul>
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<li>after decimal point when in fixed or scientific mode</li>
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<li>in total when in default mode ('<i>general mode</i>', like
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<i>%g</i>)</li>
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</ul>
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</li>
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<li>When used with type-char <b>s</b> or <b>S</b> it takes another
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meaning : the conversion string is truncated to the <i>precision</i>
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first chars. (Note that the eventual padding to <i>width</i> is done
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after truncation.)</li>
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</ul><br />
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</li>
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<li>
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<i>argument-type</i> is used by the printf family to properly process
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the arguments being passed in through varargs. With <code>boost::format</code>
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the arguments are fed into format through <code>operator %</code> which
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allows the template to carry the argument type. Therefore the classical
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printf style argument type is consumed and ignored.
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Argument-types <code>hh</code>, <code>h</code>, <code>l</code>, <code>ll</code>, <code>j</code>,
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<code>z</code>, and <code>L</code> are recognized, as are the Microsoft extensions <code>w</code>,
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<code>I</code> (capital i), <code>I32</code>, and <code>I64</code>. Argument-type <code>t</code>
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from the ISO C99 standard is not recognized as an argument type, as it has been in use as a
|
|
conversion specifier for tabular output for many years in <code>boost::format</code>.
|
|
</li><br />
|
|
|
|
<li>
|
|
<i>conversion-specifier</i> does <b>not</b> impose the concerned argument
|
|
to be of a restricted set of types, but merely sets the ios flags that are
|
|
associated with a type specification:
|
|
|
|
<blockquote>
|
|
<table border="1" cellpadding="5" summary="">
|
|
<tr>
|
|
<td><b>conversion-specifier</b></td>
|
|
|
|
<td><b>Meaning</b></td>
|
|
|
|
<td><b>effect on stream</b></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>b</b></td>
|
|
|
|
<td>boolean string output</td>
|
|
|
|
<td>sets <i>boolalpha</i>; only works for type <code>bool</code><br />
|
|
To customize the resulting string, see
|
|
<a href="http://en.cppreference.com/w/cpp/locale/numpunct/truefalsename">std::numpunct</a>.
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>p or x</b></td>
|
|
|
|
<td>hexadecimal output</td>
|
|
|
|
<td>sets <i>hex</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>o</b></td>
|
|
|
|
<td>octal output</td>
|
|
|
|
<td>sets <i>oct</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>a</b></td>
|
|
|
|
<td>hexadecimal exponent notation</td>
|
|
|
|
<td>sets floatfield bits to <i>scientific</i> | <i>fixed</i> (which is equivalent to <i>hexfloat</i>)</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>e</b></td>
|
|
|
|
<td>scientific float format</td>
|
|
|
|
<td>sets floatfield bits to <i>scientific</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>f</b></td>
|
|
|
|
<td>fixed float format</td>
|
|
|
|
<td>sets floatfield bits to <i>fixed</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>g</b></td>
|
|
|
|
<td>general -default- float format</td>
|
|
|
|
<td><b>unset</b> all floatfield bits</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>X, A, E, F</b> or <b>G</b></td>
|
|
|
|
<td>same effect as their lowercase counterparts, but using
|
|
uppercase letters for number outputs. (exponents, hex digits,
|
|
..)</td>
|
|
|
|
<td>same effects as <i>'x'</i>, <i>'a'</i>, <i>'e'</i>, <i>'f'</i>, or <i>'g'</i> respectively,
|
|
<b>plus</b> <i>uppercase</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>d, i</b> or <b>u</b></td>
|
|
|
|
<td><b>decimal</b> type output</td>
|
|
|
|
<td>sets basefield bits to <i>dec</i></td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>s</b> or <b>S</b></td>
|
|
|
|
<td>string output</td>
|
|
|
|
<td><i>precision</i> specification is unset, and its value goes
|
|
to an internal field for later 'truncation'. (see
|
|
<i>precision</i> explanation above)</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>c</b> or <b>C</b></td>
|
|
|
|
<td>1-character output</td>
|
|
|
|
<td>only the first character of the conversion string is
|
|
used.</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td><b>%</b></td>
|
|
|
|
<td>print the character <i>%</i></td>
|
|
|
|
<td>N/A</td>
|
|
</tr>
|
|
</table>
|
|
</blockquote>
|
|
|
|
<p>Note that the 'n' conversion-specifier is ignored (and so is the
|
|
corresponding argument), because it does not fit in this context.</p>
|
|
</li>
|
|
</ul><a name="new_directives" id="new_directives"></a>
|
|
|
|
<h3>new format-specifications</h3>
|
|
|
|
<ul>
|
|
<li>as stated in the flags table, centered and internal alignment flags
|
|
(' <i>=</i> ', and ' <i>_</i> ') were added.</li>
|
|
|
|
<li><i><b>%{</b>n</i><b>t}</b> , where <i>n</i> is a positive number,
|
|
inserts an <i>absolute tabulation</i>. It means that format will, if
|
|
needed, fill the string with characters, until the length of the string
|
|
created so far reaches <i>n</i> characters. (see <a href=
|
|
"#examples">examples</a> )</li>
|
|
|
|
<li><b>%|</b><i>n</i><b>T</b><i>X</i><b>|</b> inserts a tabulation in the
|
|
same way, but using <i>X</i> as fill character instead of the current
|
|
'fill' char of the stream (which is <i>space</i> for a stream in default
|
|
state)</li>
|
|
</ul><a name="printf_differences" id="printf_differences"></a>
|
|
|
|
<h2>Differences of behaviour vs printf</h2>Suppose you have variables
|
|
<i>x1, x2</i> (built_in types, supported by C's printf),<br>
|
|
and a format string <i>s</i> intended for use with a printf function this
|
|
way :
|
|
|
|
<blockquote>
|
|
<pre>
|
|
printf(s, x1, x2);
|
|
</pre>
|
|
</blockquote><br>
|
|
In almost all cases, the result will be the same as with this command :
|
|
|
|
<blockquote>
|
|
<pre>
|
|
cout << format(s) % x1 % x2;
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>But because some printf format specifications don't translate well into
|
|
stream formatting options, there are a few notable imperfections in the way
|
|
Boost.format emulates printf.<br>
|
|
In any case, the <i>format</i> class should quietly ignore the unsupported
|
|
options, so that printf format-strings are always accepted by format and
|
|
produce almost the same output as printf.</p><br>
|
|
Here is the full list of such differences :
|
|
|
|
<ul>
|
|
<li><b>'0'</b> and <b>' '</b> options : printf ignores these options for
|
|
non numeric conversions, but format applies them to all types of
|
|
variables. (so it is possible to use those options on user-defined types,
|
|
e.g. a Rational class, etc..)</li>
|
|
|
|
<li><b>precision</b> for integral types arguments has a special meaning
|
|
for printf :<br>
|
|
<i>printf( "(%5.3d)" , 7 ) ;</i> prints « ( 007) »<br>
|
|
While format, like streams, ignores the precision parameter for integral
|
|
types conversions.</li>
|
|
|
|
<li>the <b>'</b> printf option (<i>format with thousands grouping
|
|
characters)</i>) has no effect in format.</li>
|
|
|
|
<li>Width or precision set to asterisk (<i>*</i>) are used by printf to
|
|
read this field from an argument. e.g.
|
|
<i>printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);</i><br>
|
|
This class does not support this mechanism for now. so such precision or
|
|
width fields are quietly ignored by the parsing.</li>
|
|
|
|
<li>argument-type is ignored</li>
|
|
</ul>Also, note that the special <b>'n'</b> type-specification (used to
|
|
tell printf to save in a variable the number of characters output by the
|
|
formatting) has no effect in format.<br>
|
|
Thus format strings containing this type-specification should produce the
|
|
same converted string by printf or format. It will not cause differences in
|
|
the formatted strings between printf and format.<br>
|
|
To get the number of characters in the formatted string using Boost.Format,
|
|
you can use the <i>size()</i> member function :
|
|
|
|
<blockquote>
|
|
<pre>
|
|
format formatter("%+5d");
|
|
cout << formatter % x;
|
|
unsigned int n = formatter.size();
|
|
</pre>
|
|
</blockquote><a name="user-defined" id="user-defined"></a>
|
|
<hr>
|
|
|
|
<h2>User-defined types output</h2>
|
|
|
|
<p>All flags which are translated into modification to the stream state act
|
|
recursively within user-defined types. ( the flags remain active, and so
|
|
does the desired format option, for each of the '<<' operations that
|
|
might be called by the user-defined class)</p>e.g., with a Rational class,
|
|
we would have something like :
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Rational ratio(16,9);
|
|
cerr << format("%#x \n") % ratio; // -> "0x10/0x9 \n"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>It's a different story for other formatting options. For example,
|
|
setting width applies to the final output produced by the object, not to
|
|
each of its internal outputs, and that's fortunate :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
cerr << format("%-8d") % ratio; // -> "16/9 " and not "16 /9 "
|
|
cerr << format("%=8d") % ratio; // -> " 16/9 " and not " 16 / 9 "
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p><br>
|
|
But so does the 0 and ' ' options (contrarily to '+' which is directly
|
|
translated to the stream state by <i>showpos</i>. But no such flags exist
|
|
for the zero and space printf options)<br>
|
|
and that is less natural :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
cerr << format("%+08d \n") % ratio; // -> "+00016/9"
|
|
cerr << format("% 08d \n") % ratio; // -> "000 16/9"
|
|
</pre>
|
|
</blockquote>It is possible to obtain a better behaviour by carefully
|
|
designing the Rational's <i>operator<<</i> to handle the stream's
|
|
width, alignment and <i>showpos</i> paramaters by itself. This is
|
|
demonstrated in <a href=
|
|
"../example/sample_userType.cpp">sample_userType.cpp</a>. <a name=
|
|
"manipulators" id="manipulators"></a>
|
|
<hr>
|
|
|
|
<h3>Manipulators, and internal stream state</h3>
|
|
|
|
<p>The internal stream state of <b>format</b> is saved before and restored
|
|
after output of an argument; therefore, the modifiers are not sticky and
|
|
affect only the argument they are applied to.<br>
|
|
The default state for streams, as stated by the standard, is : precision 6,
|
|
width 0, right alignment, and decimal flag set.</p>
|
|
|
|
<p>The state of the internal <b>format</b> stream can be changed by
|
|
manipulators passed along with the argument; via the <i>group</i> function,
|
|
like that :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
cout << format("%1% %2% %1%\n") % group(hex, showbase, 40) % 50; // prints "0x28 50 0x28\n"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p><br>
|
|
When passing N items inside a 'group' Boost.format needs to process
|
|
manipulators diferently from regular argument, and thus using group is
|
|
subject to the following constraints :</p>
|
|
|
|
<ol>
|
|
<li>the object to be printed must be passed as the last item in the
|
|
group</li>
|
|
|
|
<li>the first N-1 items are treated as manipulators, and if they do
|
|
produce output, it is discarded</li>
|
|
</ol>
|
|
|
|
<p>Such manipulators are passed to the streams right before the following
|
|
argument, at every occurrence. Note that formatting options specified within
|
|
the format string are overridden by stream state modifiers passed this way.
|
|
For instance in the following code, the <i>hex</i> manipulator has priority
|
|
over the <i>d</i> type-specification in the format-string which would set
|
|
decimal output :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
cout << format("%1$d %2% %1%\n") % group(hex, showbase, 40) % 50;
|
|
// prints "0x28 50 0x28\n"
|
|
</pre>
|
|
</blockquote><a name="alternatives" id="alternatives"></a>
|
|
|
|
<h2>Alternatives</h2>
|
|
|
|
<ul>
|
|
<li><b>printf</b> is the classical alternative, that is not type safe and
|
|
not extendable to user-defined types.</li>
|
|
|
|
<li>ofrstream.cc by Karl Nelson's design was a big source of inspiration
|
|
to this format class.</li>
|
|
|
|
<li>James Kanze's library has a format class (in
|
|
<i>srcode/Extended/format</i> ) which looks very well polished. Its
|
|
design has in common with this class the use of internal stream for the
|
|
actual conversions, as well as using operators to pass arguments. (but
|
|
his class, as ofrstream, uses <i>operator<<</i> rather <i>than
|
|
operator%</i> )</li>
|
|
|
|
<li><a href="http://groups.yahoo.com/group/boost/files/format3/">Karl
|
|
Nelson's library</a> was intented as demonstration of alternative
|
|
solutions in discussions on Boost's list for the design of
|
|
Boost.format.</li>
|
|
|
|
<li><a href="http://fmtlib.net/latest/index.html">{fmt}</a> by Victor Zverovich.</li>
|
|
</ul><a name="exceptions" id="exceptions"></a>
|
|
<hr>
|
|
|
|
<h2>Exceptions</h2>
|
|
|
|
<p>Boost.format enforces a number of rules on the usage of format objects.
|
|
The format-string must obeys the syntax described above, the user must
|
|
supply exactly the right number of arguments before outputting to the final
|
|
destination, and if using modify_item or bind_arg, items and arguments
|
|
index must not be out of range.<br>
|
|
When format detects that one of these rules is not satisfied, it raises a
|
|
corresponding exception, so that the mistakes don't go unnoticed and
|
|
unhandled.<br>
|
|
But the user can change this behaviour to fit his needs, and select which
|
|
types of errors may raise exceptions using the following functions :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
|
|
unsigned char exceptions(unsigned char newexcept); // query and set
|
|
unsigned char exceptions() const; // just query
|
|
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>The user can compute the argument <i>newexcept</i> by combining the
|
|
following atoms using binary arithmetic :</p>
|
|
|
|
<ul>
|
|
<li><b>boost::io::bad_format_string_bit</b> selects errors due to
|
|
ill-formed format-strings.</li>
|
|
|
|
<li><b>boost::io::too_few_args_bit</b> selects errors due to asking for
|
|
the srting result before all arguments are passed.</li>
|
|
|
|
<li><b>boost::io::too_many_args_bit</b> selects errors due to passing too
|
|
many arguments.</li>
|
|
|
|
<li><b>boost::io::out_of_range_bit</b> select errors due to out of range
|
|
index supplied by the user when calling <i>modify_item</i> or other
|
|
functions taking an item index (or an argument index)</li>
|
|
|
|
<li><b>boost::io::all_error_bits</b> selects all errors</li>
|
|
|
|
<li><b>boost::io::no_error_bits</b> selects no error.</li>
|
|
</ul>
|
|
|
|
<p>For instance, if you don't want Boost.format to detect bad number of
|
|
arguments, you can define a specific wrapper function for building format
|
|
objects with the right exceptions settings :</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
|
|
boost::format my_fmt(const std::string & f_string) {
|
|
using namespace boost::io;
|
|
format fmter(f_string);
|
|
fmter.exceptions( all_error_bits ^ ( too_many_args_bit | too_few_args_bit ) );
|
|
return fmter;
|
|
}
|
|
|
|
</pre>
|
|
</blockquote>It is then allowed to give more arguments than needed (they
|
|
are simply ignored) :
|
|
|
|
<blockquote>
|
|
<pre>
|
|
|
|
cout << my_fmt(" %1% %2% \n") % 1 % 2 % 3 % 4 % 5;
|
|
|
|
</pre>
|
|
</blockquote>And if we ask for the result before all arguments are
|
|
supplied, the corresponding part of the result is simply empty
|
|
|
|
<blockquote>
|
|
<pre>
|
|
|
|
cout << my_fmt(" _%2%_ _%1%_ \n") % 1 ;
|
|
// prints " __ _1_ \n"
|
|
|
|
</pre>
|
|
</blockquote><a name="performance" id="performance"></a>
|
|
<hr>
|
|
|
|
<h2>A Note about performance</h2>
|
|
|
|
<p>The performance of boost::format for formatting a few builtin type
|
|
arguments with reordering can be compared to that of Posix-printf, and of
|
|
the equivalent stream manual operations to give a measure of the overhead
|
|
incurred. The result may greatly depend on the compiler, standard library
|
|
implementation, and the precise choice of format-string and arguments.</p>
|
|
|
|
<p>Common stream implementations eventually call functions of the
|
|
printf family for the actual formatting of numbers. In general, printf
|
|
will be noticeably faster than the direct stream operations due to the
|
|
reordering overhead (allocations to store the pieces of string, stream
|
|
initialisation at each item formatting, ..). The direct stream operations
|
|
would be faster than boost::format - one can expect a ratio ranging from 2
|
|
to 5 or more.</p>
|
|
|
|
<p>When iterated formattings are a performance bottleneck, performance can
|
|
be slightly increased by parsing the format string into a format object,
|
|
and copying it at each formatting, in the following way:</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
const boost::format fmter(fstring);
|
|
dest << boost::format(fmter) % arg1 % arg2 % arg3 ;
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>As an example of performance results, the author measured the time of
|
|
execution of iterated formattings with 4 different methods:</p>
|
|
|
|
<ol>
|
|
<li>posix printf</li>
|
|
|
|
<li>manual stream output (to a dummy <i>nullStream</i> stream sending the
|
|
bytes into oblivion)</li>
|
|
|
|
<li>boost::format copied from a const object as shown above</li>
|
|
|
|
<li>the straight boost::format usage</li>
|
|
</ol>
|
|
|
|
<p>the test was compiled with g++-3.3.3 and the following timings were
|
|
measured (in seconds, and ratios):</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
string fstring="%3$0#6x %1$20.10E %2$g %3$0+5d \n";
|
|
double arg1=45.23;
|
|
double arg2=12.34;
|
|
int arg3=23;
|
|
|
|
- release mode :
|
|
printf : 2.13
|
|
nullStream : 3.43, = 1.61033 * printf
|
|
boost::format copied : 6.77, = 3.1784 * printf , = 1.97376 * nullStream
|
|
boost::format straight :10.67, = 5.00939 * printf , = 3.11079 * nullStream
|
|
|
|
- debug mode :
|
|
printf : 2.12
|
|
nullStream : 3.69, = 1.74057 * printf
|
|
boost::format copied :10.02, = 4.72642 * printf , = 2.71545 * nullStream
|
|
boost::format straight :17.03, = 8.03302 * printf , = 4.61518 * nullStream
|
|
</pre>
|
|
</blockquote><a name="extract" id="extract"></a>
|
|
<hr>
|
|
|
|
<h2>Class Interface Extract</h2>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
namespace boost {
|
|
|
|
template<class charT, class Traits=std::char_traits<charT> >
|
|
class basic_format
|
|
{
|
|
public:
|
|
typedef std::basic_string<charT, Traits> string_type;
|
|
typedef typename string_type::size_type size_type;
|
|
basic_format(const charT* str);
|
|
basic_format(const charT* str, const std::locale & loc);
|
|
basic_format(const string_type& s);
|
|
basic_format(const string_type& s, const std::locale & loc);
|
|
basic_format& operator= (const basic_format& x);
|
|
|
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void clear(); // reset buffers
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basic_format& parse(const string_type&); // clears and parse a new format string
|
|
|
|
string_type str() const;
|
|
size_type size() const;
|
|
|
|
// pass arguments through those operators :
|
|
template<class T> basic_format& operator%(T& x);
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template<class T> basic_format& operator%(const T& x);
|
|
|
|
// dump buffers to ostream :
|
|
friend std::basic_ostream<charT, Traits>&
|
|
operator<< <> ( std::basic_ostream<charT, Traits>& , basic_format& );
|
|
|
|
// Choosing which errors will throw exceptions :
|
|
unsigned char exceptions() const;
|
|
unsigned char exceptions(unsigned char newexcept);
|
|
|
|
// ............ this is just an extract .......
|
|
}; // basic_format
|
|
|
|
typedef basic_format<char > format;
|
|
typedef basic_format<wchar_t > wformat;
|
|
|
|
|
|
// free function for ease of use :
|
|
template<class charT, class Traits>
|
|
std::basic_string<charT,Traits> str(const basic_format<charT,Traits>& f) {
|
|
return f.str();
|
|
}
|
|
|
|
|
|
} // namespace boost
|
|
</pre>
|
|
</blockquote>
|
|
<hr>
|
|
<a name="rationale" id="rationale"></a>
|
|
|
|
<h2>Rationale</h2>
|
|
|
|
<p>This class's goal is to bring a better, C++, type-safe and
|
|
type-extendable <i>printf</i> equivalent to be used with
|
|
streams.</p>Precisely, <b>format</b> was designed to provide the following
|
|
features :
|
|
|
|
<ul>
|
|
<li>support positional arguments (required for internationalisation)</li>
|
|
|
|
<li>accept an unlimited number of arguments.</li>
|
|
|
|
<li>make formatting commands visually natural.</li>
|
|
|
|
<li>support the use of manipulators to modify the display of an argument.
|
|
in addition to the format-string syntax.</li>
|
|
|
|
<li>accept any types of variables, by relying on streams for the actual
|
|
conversion to string. This specifically concerns user-defined types, for
|
|
which the formatting options effects should be intuitively natural.</li>
|
|
|
|
<li>provide printf-compatibility, as much as it makes sense in a
|
|
type-safe and type-extendable context.</li>
|
|
</ul>
|
|
|
|
<p>In the process of the design, many issues were faced, and some choices
|
|
were made, that might not be intuitively right. But in each case they were
|
|
taken for <a href="choices.html">some reasons</a>.</p>
|
|
<hr>
|
|
|
|
<h2>Credits</h2>
|
|
|
|
<p>The author of Boost format is Samuel Krempp. He used ideas from
|
|
Rüdiger Loos' format.hpp and Karl Nelson's formatting classes.</p>
|
|
<hr>
|
|
|
|
<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
|
|
"../../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
|
|
height="31" width="88"></a></p>
|
|
|
|
<p>Revised
|
|
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->23 October, 2017<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
|
|
|
|
<p><i>Copyright © 2002 Samuel Krempp</i></p>
|
|
|
|
<p><i>Distributed under the Boost Software License, Version 1.0. (See
|
|
accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
|
|
copy at <a href=
|
|
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
|
|
</body>
|
|
</html>
|