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Merge pull request #4471

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Vincas Dargis (1):
      docs(coding): add guides about string usage
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sudden6 2017-06-21 18:10:30 +02:00
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@ -467,8 +467,76 @@ someWidget->setTooltip(QStringLiteral("<html><!-- some HTML text -->") + tr("Tra
+ QStringLiteral("</html>"));
```
## Strings
* Use `QStringLiteral` macro when creating new string.
In this example, string is not intended to be modified or copied (like
appending) into other string:
```
QApplication a(argc, argv);
a.setApplicationName(QStringLiteral("qTox"));
```
* Use `QLatin1String` when specialized overload exists.
Overload such as `QString::operator==(QLatin1String)` helps to avoid creating
temporary QString and thus avoids malloc:
```
if (eventType == QLatin1String("uri"))
handleToxURI(firstParam.toUtf8());
else if (eventType == QLatin1String("save"))
handleToxSave(firstParam.toUtf8());
```
* Use `QLatin1String` when appending to string, joining two strings.
QLatin1String is literal type and knows string length at compile time
(compared to `QString(const char*)` run-time cost with plain C++
string literal). Also, copying 8-bit latin string requires less memory
bandwith compared to 16-bit `QStringLiteral` mentioned earlier, and
copying here is unavoidable (and thus `QStringLiteral` loses it's purpose):
```
if (!dir.rename(logFileDir + QLatin1String("qtox.log"),
logFileDir + QLatin1String("qtox.log.1")))
qCritical() << "Unable to move logs";
```
* Use `QStringBuilder` when joining more than two strings (and chars) together.
Include `<QStringBuilder>` and use `%` operator for optimized single-pass
concatination with help of expression template's lazy evaluation:
```
QCommandLineParser parser;
parser.setApplicationDescription(QLatin1String("qTox, version: ")
% QLatin1String(GIT_VERSION) % QLatin1String("\nBuilt: ")
% QLatin1String(__TIME__) % QLatin1Char(' ') % QLatin1String(__DATE__));
```
* Use `QLatin1Char` to avoid UTF-16-char handling (same as in previous example):
```
QString path = QString(__FILE__);
path = path.left(path.lastIndexOf(QLatin1Char('/')) + 1);
```
* Use `QLatin1String` and `QLatin1Char` _only_ for Latin-1 strings and chars.
[Latin-1][Latin-1] is ASCII-based standard character encoding, use
`QStringLiteral` for Unicode instead.
For more info, see:
* [Using QString Effectively]
* [QStringLiteral explained]
* [String concatenation with QStringBuilder]
<!-- Markdown links -->
[ISO/IEC/C++11]: http://www.iso.org/iso/catalogue_detail.htm?csnumber=50372
[Exceptions]: https://en.wikipedia.org/wiki/C%2B%2B#Exception_handling
[RTTI]: https://en.wikipedia.org/wiki/Run-time_type_information
[`tools/format-code.sh`]: /tools/format-code.sh
[Using QString Effectively]: https://wiki.qt.io/Using_QString_Effectively
[QStringLiteral explained]: https://woboq.com/blog/qstringliteral.html
[String concatenation with QStringBuilder]: https://blog.qt.io/blog/2011/06/13/string-concatenation-with-qstringbuilder/
[Latin-1]: https://en.wikipedia.org/wiki/ISO/IEC_8859-1