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docs(coding): fix string usage guidelines

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Update docs to recommend using `%` operator with `QLatin1String` even for joining two strings (not only *more than two* strings).

Mentioned optimisation of "lightweight" QLatin1String copying-and-converting-to-utf16 when appending only works with QStringBuilder (operator %). Otherwise, QLatin1String will be converted to QString, and only then copied (appended), which is slower.
This commit is contained in:
Vincas Dargis 2017-08-15 14:08:11 +00:00
parent 5d159e0203
commit eb9b897797
1 changed files with 13 additions and 12 deletions
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25
doc/coding_standards.md
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@ -489,23 +489,23 @@ temporary QString and thus avoids malloc:
handleToxSave(firstParam.toUtf8());
```
* Use `QLatin1String` when appending to string, joining two strings.
* Use `QStringBuilder` and `QLatin1String` when joining strings (and chars)
together.
QLatin1String is literal type and knows string length at compile time
`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.
copying here is unavoidable (and thus `QStringLiteral` loses it's purpose).
Include `<QStringBuilder>` and use `%` operator for optimized single-pass
concatination with help of expression template's lazy evaluation:
concatenation with help of expression template's lazy evaluation:
```
if (!dir.rename(logFileDir % QLatin1String("qtox.log"),
logFileDir % QLatin1String("qtox.log.1")))
qCritical() << "Unable to move logs";
```
```
QCommandLineParser parser;
parser.setApplicationDescription(QLatin1String("qTox, version: ")
@ -513,7 +513,8 @@ concatination with help of expression template's lazy evaluation:
% QLatin1String(__TIME__) % QLatin1Char(' ') % QLatin1String(__DATE__));
```
* Use `QLatin1Char` to avoid UTF-16-char handling (same as in previous example):
* Use `QLatin1Char` to avoid UTF-16-char handling (same as in previous
example):
```
QString path = QString(__FILE__);
path = path.left(path.lastIndexOf(QLatin1Char('/')) + 1);