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| <div class="title">Localized Text Formatting </div> </div> |
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| <div class="textblock"><p>The <code>iostream</code> manipulators are very useful, but when we create a messages for the user, sometimes we need something like good old <code>printf</code> or <code>boost::format</code>.</p> |
| <p>Unfortunately <code>boost::format</code> has several limitations in context of localization:</p> |
| <ol type="1"> |
| <li>It renders all parameters using global locale rather than target <code>ostream</code> locale. For example: <br/> |
| <div class="fragment"><pre class="fragment"> std::locale::global(std::locale(<span class="stringliteral">"en_US.UTF-8"</span>)); |
| output.imbue(std::locale(<span class="stringliteral">"de_DE.UTF-8"</span>)) |
| output << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">boost::format</a>(<span class="stringliteral">"%1%"</span>) % 1234.345; |
| </pre></div> <br/> |
| This would write "1,234.235" to output, instead of the "1.234,234" that is expected for "de_DE" locale</li> |
| <li>It knows nothing about the new Boost.Locale manipulators.</li> |
| <li>The <code>printf-like</code> syntax is very limited for formatting complex localized data, not allowing the formatting of dates, times, or currencies</li> |
| </ol> |
| <p>Thus a new class, <a class="el" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">boost::locale::format</a>, was introduced. For example:</p> |
| <div class="fragment"><pre class="fragment"> wcout << <a class="code" href="group__format.html#ga610f3ae827801febc962019cf82a2227">wformat</a>(L<span class="stringliteral">"Today {1,date} I would meet {2} at home"</span>) % <a class="code" href="group__manipulators.html#gae669b101cbeaed6f6d246ebdcaa8f39c">time</a>(0) % name <<endl; |
| </pre></div><p>Each format specifier is enclosed within <code>{}</code> brackets, is separated with a comma "," and may have an additional option after an equals symbol '='. This option may be simple ASCII text or single-quoted localized text. If a single-quote should be inserted within the text, it may be represented with a pair of single-quote characters.</p> |
| <p>Here is an example of a format string:</p> |
| <div class="fragment"><pre class="fragment"> |
| "Ms. {1} had arrived at {2,ftime='%I o''clock'} at home. The exact time is {2,time=full}" |
| </pre></div><p>The syntax is described by following grammar:</p> |
| <div class="fragment"><pre class="fragment"> |
| format : '{' parameters '}' |
| parameters: parameter | parameter ',' parameters; |
| parameter : key ["=" value] ; |
| key : [0-9a-zA-Z<>]+ ; |
| value : ascii-string-excluding-"}"-and="," | local-string ; |
| local-string : quoted-text | quoted-text local-string; |
| quoted-text : '[^']*' ; |
| </pre></div><p>You can include literal '{' and '}' by inserting double "{{" or "}}" to the text.</p> |
| <div class="fragment"><pre class="fragment">cout << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<a class="code" href="group__message.html#ga58e9599005608845d2b022d499dc97f6" title="Translate a message, msg is not copied.">translate</a>(<span class="stringliteral">"Unexpected `{{' in line {1} in file {2}"</span>)) % pos % file; |
| </pre></div><p>Would display something like</p> |
| <div class="fragment"><pre class="fragment"> |
| Unexpected `{' in line 5 in file source.cpp |
| </pre></div><p>The following format key-value pairs are supported:</p> |
| <ul> |
| <li><code>[0-9]+</code> -- digits, the index of the formatted parameter -- required.</li> |
| <li><code>num</code> or <code>number</code> -- format a number. Options are: <br/> |
| <ul> |
| <li><code>hex</code> -- display in hexadecimal format</li> |
| <li><code>oct</code> -- display in octal format</li> |
| <li><code>sci</code> or <code>scientific</code> -- display in scientific format</li> |
| <li><code>fix</code> or <code>fixed</code> -- display in fixed format <br/> |
| For example, <code>number=sci</code> </li> |
| </ul> |
| </li> |
| <li><code>cur</code> or <code>currency</code> -- format currency. Options are: <br/> |
| <ul> |
| <li><code>iso</code> -- display using ISO currency symbol.</li> |
| <li><code>nat</code> or <code>national</code> -- display using national currency symbol. <br/> |
| </li> |
| </ul> |
| </li> |
| <li><code>per</code> or <code>percent</code> -- format a percentage value.</li> |
| <li><code>date</code>, <code>time</code>, <code>datetime</code> or <code>dt</code> -- format a date, a time, or a date and time. Options are: <br/> |
| <ul> |
| <li><code>s</code> or <code>short</code> -- display in short format.</li> |
| <li><code>m</code> or <code>medium</code> -- display in medium format.</li> |
| <li><code>l</code> or <code>long</code> -- display in long format.</li> |
| <li><code>f</code> or <code>full</code> -- display in full format.</li> |
| </ul> |
| </li> |
| <li><code>ftime</code> with string (quoted) parameter -- display as with <code>strftime</code>. See <code>as::ftime</code> manipulator.</li> |
| <li><code>spell</code> or <code>spellout</code> -- spell the number.</li> |
| <li><code>ord</code> or <code>ordinal</code> -- format an ordinal number (1st, 2nd... etc)</li> |
| <li><code>left</code> or <code><</code> -- align-left.</li> |
| <li><code>right</code> or <code>></code> -- align-right.</li> |
| <li><code>width</code> or <code>w</code> -- set field width (requires parameter).</li> |
| <li><code>precision</code> or <code>p</code> -- set precision (requires parameter).</li> |
| <li><code>locale</code> -- with parameter -- switch locales for the current operation. This command generates a locale with formatting facets, giving more fine grained control of formatting. For example: <br/> |
| <div class="fragment"><pre class="fragment"> cout << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<span class="stringliteral">"This article was published at {1,date=l} (Gregorian) {1,locale=he_IL@calendar=hebrew,date=l} (Hebrew)"</span>) % <a class="code" href="group__manipulators.html#gae05b82e6658dc573521518fed5f5c77f">date</a>; |
| </pre></div></li> |
| <li><code>timezone</code> or <code>tz</code> -- the name of the timezone to display the time in. For example:<br/> |
| <div class="fragment"><pre class="fragment"> cout << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<span class="stringliteral">"Time is: Local {1,time}, ({1,time,tz=EET} Eastern European Time)"</span>) % <a class="code" href="group__manipulators.html#gae05b82e6658dc573521518fed5f5c77f">date</a>; |
| </pre></div></li> |
| <li><code>local</code> - display the time in local time</li> |
| <li><code>gmt</code> - display the time in UTC time scale <div class="fragment"><pre class="fragment"> cout << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<span class="stringliteral">"Local time is: {1,time,local}, universal time is {1,time,gmt}"</span>) % <a class="code" href="group__manipulators.html#gae669b101cbeaed6f6d246ebdcaa8f39c">time</a>; |
| </pre></div></li> |
| </ul> |
| <p>The constructor for the <a class="el" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a> class can take an object of type <a class="el" href="group__message.html#ga556e3e7696302902b2242a7a94516dee">message</a>, simplifying integration with message translation code.</p> |
| <p>For example:</p> |
| <div class="fragment"><pre class="fragment"> cout<< <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<a class="code" href="group__message.html#ga58e9599005608845d2b022d499dc97f6" title="Translate a message, msg is not copied.">translate</a>(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>)) % a % b % (a+b) << endl; |
| </pre></div><p>A formatted string can be fetched directly by using the <a class="el" href="classboost_1_1locale_1_1basic__format.html#a6bc65d7993e3ab6ad51809ef8fb65400">str(std::locale const &loc=std::locale())</a> member function. For example:</p> |
| <div class="fragment"><pre class="fragment"> std::wstring de = (<a class="code" href="group__format.html#ga610f3ae827801febc962019cf82a2227">wformat</a>(<a class="code" href="group__message.html#ga58e9599005608845d2b022d499dc97f6" title="Translate a message, msg is not copied.">translate</a>(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>)) % a % b % (a+b)).str(de_locale); |
| std::wstring fr = (<a class="code" href="group__format.html#ga610f3ae827801febc962019cf82a2227">wformat</a>(<a class="code" href="group__message.html#ga58e9599005608845d2b022d499dc97f6" title="Translate a message, msg is not copied.">translate</a>(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>)) % a % b % (a+b)).str(fr_locale); |
| </pre></div><dl class="note"><dt><b>Note:</b></dt><dd>There is one significant difference between <code>boost::format</code> and <code><a class="el" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">boost::locale::format</a></code>: Boost.Locale's format converts its parameters only when written to an <code>ostream</code> or when the `str()` member function is called. It only saves references to the objects that can be written to a stream.</dd></dl> |
| <p>This is generally not a problem when all operations are done in one statement, such as:</p> |
| <div class="fragment"><pre class="fragment"> cout << <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a>(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>) % a % b % (a+b); |
| </pre></div><p>Because the temporary value of <code></code>(a+b) exists until the formatted data is actually written to the stream. But following code is wrong:</p> |
| <div class="fragment"><pre class="fragment"> <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a> fmt(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>); |
| fmt % a; |
| fmt % b; |
| fmt % (a+b); |
| cout << fmt; |
| </pre></div><p>Because the temporary value of <code></code>(a+b) no longer exists when <code>fmt</code> is written to the stream. A correct solution would be:</p> |
| <div class="fragment"><pre class="fragment"> <a class="code" href="group__format.html#gad7914df7b54382c1ad7f5360676fe2e8">format</a> fmt(<span class="stringliteral">"Adding {1} to {2}, we get {3}"</span>); |
| fmt % a; |
| fmt % b; |
| <span class="keywordtype">int</span> a_plus_b = a+b; |
| fmt % a_plus_b; |
| cout << fmt; |
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