docs/checks/README-qstring-arg.md

0001 # qstring-arg
0002
0003 Implements three warnings:
0004
0005 1. Detects when you're using chained `QString::arg()` calls and should instead use the multi-arg overload to save memory allocations
0006
0007         QString("%1 %2").arg(a).arg(b);
0008         QString("%1 %2").arg(a, b); // one less temporary heap allocation
0009
0010 2. Detects when you're passing an integer to QLatin1String::arg() as that gets implicitly cast to QChar.
0011 It's preferable to state your intention and cast to QChar explicitly.
0012
0013 3. Detects when you're using misleading `QString::arg()` overloads
0014
0015         QString arg(qlonglong a, int fieldwidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0016         QString arg(qulonglong a, int fieldwidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0017         QString arg(long a, int fieldwidth = 0, int base=10, QChar fillChar = QLatin1Char(' ')) const
0018         QString arg(ulong a, int fieldwidth = 0, int base=10, QChar fillChar = QLatin1Char(' ')) const
0019         QString arg(int a, int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0020         QString arg(uint a, int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0021         QString arg(short a, int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0022         QString arg(ushort a, int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const
0023         QString arg(double a, int fieldWidth = 0, char fmt = 'g', int prec = -1, QChar fillChar = QLatin1Char(' ')) const
0024         QString arg(char a, int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const
0025         QString arg(QChar a, int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const
0026         QString arg(const QString &a, int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const
0027
0028 because they are commonly misused, for example:
0029
0030         int hours = ...;
0031         int minutes = ...;
0032         // This won't do what you think it would at first glance.
0033         QString s("The time is %1:%2").arg(hours, minutes);
0034
0035 To reduce false positives, some cases won't be warned about:
0036
0037         str.arg(hours, 2); // User explicitly used a integer literal, it's probably fine
0038         str.arg(foo); // We're only after cases where the second argument (or further) is specified, so this is safe
0039         str.arg(foo, width); // Second argument is named width, or contains the name "width", it's safe. Same for third argument and "base".
0040
0041 Using these misleading overloads is perfectly valid, so only warning (1) is enabled by default.
0042 To enable warning (2), `export CLAZY_EXTRA_OPTIONS="qstring-arg-fillChar-overloads"`