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- <title>The Boost Format library</title>
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- <body bgcolor="white" text="black">
- <h1><img align="middle" alt="boost.png (6897 bytes)" height="86" src=
- "../../../boost.png" width="277">The Boost Format library</h1>
- <p>The <code><a href=
- "../../../boost/format.hpp"><boost/format.hpp></a></code> format
- class provides printf-like formatting, in a type-safe manner which allows
- output of user-defined types.<br></p>
- <ul>
- <li><a href="#synopsis">Synopsis</a></li>
- <li><a href="#how_it_works">How it works</a></li>
- <li><a href="#examples">Examples</a></li>
- <li>
- <a href="#syntax">Syntax</a>
- <ul>
- <li><a href="#printf_directives">printf format-specification
- syntax</a></li>
- <li><a href="#printf_differences">Incompatibilities with
- printf</a></li>
- </ul>
- </li>
- <li><a href="#manipulators">Manipulators and the internal stream
- state</a></li>
- <li><a href="#user-defined">User-defined types</a></li>
- <li><a href="#alternatives">Alternatives</a></li>
- <li><a href="#exceptions">Exceptions</a></li>
- <li><a href="#performance">Performance</a></li>
- <li><a href="#extract">Class Interface Extract</a></li>
- <li><a href="#rationale">Rationale</a></li>
- </ul><a name="synopsis" id="synopsis"></a>
- <hr>
- <h2>Synopsis</h2>
- <p>A format object is constructed from a format-string, and is then given
- arguments through repeated calls to <i>operator%</i>.<br>
- Each of those arguments are then converted to strings, who are in turn
- combined into one string, according to the format-string.</p>
- <blockquote>
- <pre>
- cout << boost::format("writing %1%, x=%2% : %3%-th try") % "toto" % 40.23 % 50;
- // prints "writing toto, x=40.230 : 50-th try"
- </pre>
- </blockquote><a name="how_it_works" id="how_it_works"></a>
- <hr>
- <h2>How it works</h2>
- <ol>
- <li>When you call <i>format(s)</i>, where s is the format-string, it
- constructs an object, which parses the format string and look for all
- directives in it and prepares internal structures for the next step.</li>
- <li>Then, either immediately, as in
- <blockquote>
- <pre>
- cout << format("%2% %1%") % 36 % 77;
- </pre>
- </blockquote>or later on, as in
- <blockquote>
- <pre>
- format fmter("%2% %1%");
- fmter % 36; fmter % 77;
- </pre>
- </blockquote>you <i>feed</i> variables into the formatter.<br>
- those variables are dumped into an internal stream, which state is set
- according to the given formatting options in the format-string -if
- there are any-, and the format object stores the string results for the
- last step.
- </li>
- <li>Once all arguments have been fed you can dump the format object to a
- stream, or get its string value by using the <i>str()</i> member
- function, or the free function <i>str(const format& )</i> in
- namespace <i>boost</i>. The result string stays accessible in the format
- object until another argument is passed, at which time it is
- reinitialised.
- <blockquote>
- <pre>
- // fmter was previously created and fed arguments, it can print the result :
- cout << fmter ;
- // You can take the string result :
- string s = fmter.str();
- // possibly several times :
- s = fmter.str( );
- // You can also do all steps at once :
- cout << boost::format("%2% %1%") % 36 % 77;
- // using the str free function :
- string s2 = str( format("%2% %1%") % 36 % 77 );
- </pre>
- </blockquote>
- </li>
- <li>Optionnally, after step 3, you can re-use a format object and restart
- at step2 : <i>fmter % 18 % 39;</i><br>
- to format new variables with the same format-string, saving the expensive
- processing involved at step 1.</li>
- </ol>All in all, the format class translates a format-string (with
- eventually printf-like directives) into operations on an internal stream,
- and finally returns the result of the formatting, as a string, or directly
- into an output stream. <a name="examples" id="examples"></a>
- <hr>
- <h2>Examples</h2>
- <blockquote>
- <pre>
- using namespace std;
- using boost::format;
- using boost::io::group;
- </pre>
- </blockquote>
- <ul>
- <li>Simple output, with reordering :
- <blockquote>
- <pre>
- cout << format("%1% %2% %3% %2% %1% \n") % "11" % "22" % "333"; // 'simple' style.
- </pre>
- </blockquote>It prints : "11 22 333 22 11 \n"
- </li>
- <li>More precise formatting, with Posix-printf positional directives :
- <blockquote>
- <pre>
- cout << format("(x,y) = (%1$+5d,%2$+5d) \n") % -23 % 35; // Posix-Printf style
- </pre>
- </blockquote>It prints : "(x,y) = ( -23, +35) \n"
- </li>
- <li>classical printf directive, no reordering :
- <blockquote>
- <pre>
- cout << format("writing %s, x=%s : %d-th step \n") % "toto" % 40.23 % 50;
- </pre>
- </blockquote>It prints : "writing toto, x=40.23 : 50-th step \n"
- </li>
- <li>Several ways to express the same thing :
- <blockquote>
- <pre>
- cout << format("(x,y) = (%+5d,%+5d) \n") % -23 % 35;
- cout << format("(x,y) = (%|+5|,%|+5|) \n") % -23 % 35;
- cout << format("(x,y) = (%1$+5d,%2$+5d) \n") % -23 % 35;
- cout << format("(x,y) = (%|1$+5|,%|2$+5|) \n") % -23 % 35;
- </pre>
- </blockquote>all those print : "(x,y) = ( -23, +35) \n"
- </li>
- <li>Using manipulators to modify the format-string :
- <blockquote>
- <pre>
- format fmter("_%1$+5d_ %1$d \n");
- format fmter2("_%1%_ %1% \n");
- fmter2.modify_item(1, group(showpos, setw(5)) );
- cout << fmter % 101 ;
- cout << fmter2 % 101 ;
- </pre>
- </blockquote>Both print the same : "_ +101_ 101 \n"
- </li>
- <li>Using manipulators with arguments :
- <blockquote>
- <pre>
- cout << format("_%1%_ %1% \n") % group(showpos, setw(5), 101);
- </pre>
- </blockquote>The manipulators are applied at each occurrence of %1%, and
- thus it prints : "_ +101_ +101 \n"
- </li>
- <li>New formatting feature : 'absolute tabulations', useful inside loops,
- to insure a field is printed at the same position from one line to the
- next, even if the widthes of the previous arguments can vary a lot.
- <blockquote>
- <pre>
- for(unsigned int i=0; i < names.size(); ++i)
- cout << format("%1%, %2%, %|40t|%3%\n") % names[i] % surname[i] % tel[i];
- </pre>
- </blockquote>For some std::vector <i>names</i>, <i>surnames</i>, and
- <i>tel</i> (see sample_new_features.cpp) it prints :
- <blockquote>
- <pre>
- Marc-François Michel, Durand, +33 (0) 123 456 789
- Jean, de Lattre de Tassigny, +33 (0) 987 654 321
- </pre>
- </blockquote>
- </li>
- </ul>
- <hr>
- <h2>Sample Files</h2>
- <p>The program <a href=
- "../example/sample_formats.cpp">sample_formats.cpp</a> demonstrates simple
- uses of <b>format</b>.<br></p>
- <p><a href="../example/sample_new_features.cpp">sample_new_features.cpp</a>
- illustrates the few formatting features that were added to printf's syntax
- such as simple positional directives, centered alignment, and
- 'tabulations'.<br></p>
- <p><a href="../example/sample_advanced.cpp">sample_advanced.cpp</a>
- demonstrates uses of advanced features, like reusing, and modifying, format
- objects, etc..<br></p>
- <p>And <a href="../example/sample_userType.cpp">sample_userType.cpp</a>
- shows the behaviour of the <b>format</b> library on user-defined
- types.</p><a name="syntax" id="syntax"></a>
- <hr>
- <h2>Syntax</h2>
- <p><b>boost::format(</b> format-string <b>) %</b> arg1 <b>%</b> arg2
- <b>%</b> ... <b>%</b> argN</p>
- <p>The <i>format-string</i> contains text in which special directives will
- be replaced by strings resulting from the formatting of the given
- arguments.<br>
- The legacy syntax in the C and C++ worlds is the one used by printf, and
- thus format can use directly printf format-strings, and produce the same
- result (in almost all cases. see <a href=
- "#printf_differences">Incompatibilities with printf</a> for details)<br>
- This core syntax was extended, to allow new features, but also to adapt to
- the C++ streams context. Thus, format accepts several forms of directives
- in format-strings :</p>
- <ul>
- <li>Legacy printf format strings : <b>%</b><i>spec</i> where <i>spec</i>
- is a <a href="#printf_directives">printf format specification</a><br>
- <i>spec</i> passes formatting options, like width, alignment, numerical
- base used for formatting numbers, as well as other specific flags. But
- the classical <i>type-specification</i> flag of printf has a weaker
- meaning in format. It merely sets the appropriate flags on the internal
- stream, and/or formatting parameters, but does not require the
- corresponding argument to be of a specific type.<br>
- e.g. : the specification <i>2$x</i>, meaning "print argument number 2,
- which is an integral number, in hexa" for printf, merely means "print
- argument 2 with stream basefield flags set to <i>hex</i>" for
- format.</li>
- <li><b>%|</b><i>spec</i><b>|</b> where <i>spec</i> is a printf format
- specification.<br>
- This pipe-delimited syntax is introduced, to improve the readability of the
- format-string, but primarily, to make the <i>type-conversion
- character</i> optional in <i>spec</i>. This information is not necessary
- with C++ variables, but with direct printf syntax, it is necessary to
- always give a type-conversion character, merely because this character is
- crucial to determine the end of a format-specification.<br>
- e.g. : "%|-5|" will format the next variable with width set to 5, and
- left-alignment just like the following printf directives : "%-5g",
- "%-5f", "%-5s" ..</li>
- <li><b>%</b><i>N</i><b>%</b><br>
- This simple positional notation requests the formatting of the
- <i>N</i>-th argument - wihout any formatting option.<br>
- (It's merely a shortcut to Printf's positional directives (like
- "%<i>N</i>$s"), but a major benefit is that it's much more readable, and
- does not use a "type-conversion" character)</li>
- </ul>On top of the standard printf format specifications, new features were
- implemented, like centered alignment. See <a href="#new_directives">new
- format specification</a> for details. <a name="printf_directives" id=
- "printf_directives"></a>
- <h3>printf format specifications</h3>
- <p>The printf format specifications supported by Boost.format follows the
- Unix98 <a href=
- "http://www.opengroup.org/onlinepubs/7908799/xsh/fprintf.html">Open-group
- printf</a> precise syntax, rather than the standard C printf, which does
- not support positional arguments. (Common flags have the same meaning in
- both, so it should not be a headache for anybody)<br>
- <i>Note that it is an error to use positional format specifications</i>
- (e.g. <i>%3$+d</i>) <i>mixed with non-positional ones</i> (e.g. <i>%+d</i>)
- <i>in the same format string.</i><br>
- In the Open-group specification, referring to the same argument several
- times (e.g. <i>"%1$d %1$d"</i>) has undefined behaviour. Boost.format's
- behaviour in such cases is to allow each argument to be reffered to any
- number of times. The only constraint is that it expects exactly <i>P</i>
- arguments, <i>P</i> being the maximum argument number used in the format
- string. (e.g., for "%1$d %10$d", <i>P</i> == 10 ).<br>
- Supplying more, or less, than <i>P</i> arguments raises an exception.
- (unless it was set otherwise, see <a href="#exceptions">exceptions</a>)</p>
- <p>A specification <i>spec</i> has the form:
- <pre> [ <i>N</i><b>$</b> ] [ <i>flags</i> ] [ <i>width</i> ] [ <b>.</b> <i>precision</i> ] [ <i>argument-type</i> ] <i>conversion-specifier</i></pre>
- Fields inside square brackets are optional. Each of those fields are
- explained one by one in the following list:</p>
- <ul>
- <li><i>N</i> <b>$</b> (optional field) specifies that the format
- specification applies to the <i>N</i>-th argument (it is called a
- <i>positional format specification</i>).<br>
- If this is not present, arguments are taken one by one. (and it is then
- an error to later supply an argument number)</li><br />
- <li>
- <i>flags</i> is a sequence of any of these:
- <blockquote>
- <table border="1" cellpadding="5" summary="">
- <tr>
- <td><b>Flag</b></td>
- <td><b>Meaning</b></td>
- <td><b>effect on internal stream</b></td>
- </tr>
- <tr>
- <td><b>'-'</b></td>
- <td>left alignment</td>
- <td>N/A (applied later on the string)</td>
- </tr>
- <tr>
- <td><b>'='</b></td>
- <td>centered alignment</td>
- <td>N/A (applied later on the string)<br>
- <i>- note : added feature, not in printf -</i></td>
- </tr>
- <tr>
- <td><b>'_'</b></td>
- <td>internal alignment</td>
- <td>sets internal alignment<br>
- <i>- note : added feature, not in printf -</i></td>
- </tr>
- <tr>
- <td><b>'+'</b></td>
- <td>show sign even for positive numbers</td>
- <td>sets <i>showpos</i></td>
- </tr>
- <tr>
- <td><b>'#'</b></td>
- <td>show numerical base, and decimal point</td>
- <td>sets <i>showbase</i> and <i>showpoint</i></td>
- </tr>
- <tr>
- <td><b>'0'</b></td>
- <td>pad with 0's (inserted after sign or base indicator)</td>
- <td>if not left-aligned, calls <i>setfill('0')</i> and sets
- <i>internal</i><br>
- Extra actions are taken after stream conversion to handle
- <a href="#user-defined">user-defined output</a>.</td>
- </tr>
- <tr>
- <td><b>' '</b></td>
- <td>if the string does not begin with <i>+</i> or <i>-</i>,
- insert a <i>space</i> before the converted string</td>
- <td>N/A (applied later on the string)<br>
- Different to printf's behaviour : it is not affected by internal
- alignment</td>
- </tr>
- </table>
- </blockquote><br />
- </li>
- <li><i>width</i> specifies a minimal width for the string resulting form
- the conversion. If necessary, the string will be padded with alignment
- and fill characters either set on the stream via manipulators, or
- specified by the format-string (e.g. flags '0', '-', ..)<br>
- Note that width is not just set on the conversion stream. To support
- output of <a href="#user-defined">user-defined types</a> (that might call
- <i>operator<<</i> many times on several members), the width is
- handled after stream conversion of the whole argument object, in the
- format class code.</li><br />
- <li>
- <i>precision</i> (preceded by a point), sets the stream's
- <i>precision</i>
- <ul>
- <li>When outputting a floatting type number, it sets the maximum
- number of digits
- <ul>
- <li>after decimal point when in fixed or scientific mode</li>
- <li>in total when in default mode ('<i>general mode</i>', like
- <i>%g</i>)</li>
- </ul>
- </li>
- <li>When used with type-char <b>s</b> or <b>S</b> it takes another
- meaning : the conversion string is truncated to the <i>precision</i>
- first chars. (Note that the eventual padding to <i>width</i> is done
- after truncation.)</li>
- </ul><br />
- </li>
- <li>
- <i>argument-type</i> is used by the printf family to properly process
- the arguments being passed in through varargs. With <code>boost::format</code>
- the arguments are fed into format through <code>operator %</code> which
- allows the template to carry the argument type. Therefore the classical
- printf style argument type is consumed and ignored.
- Argument-types <code>hh</code>, <code>h</code>, <code>l</code>, <code>ll</code>, <code>j</code>,
- <code>z</code>, and <code>L</code> are recognized, as are the Microsoft extensions <code>w</code>,
- <code>I</code> (capital i), <code>I32</code>, and <code>I64</code>. Argument-type <code>t</code>
- 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);
- void clear(); // reset buffers
- 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);
- 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>
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