Using QString effectively
June 22nd, 2010
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QLatin1String : Avoid mallocs in operator ”==”
Creating a QString from a C-string involves a malloc. For example, there is a hidden-malloc cost in the following code.
if (fruit== "apple") { ... } // hidden malloc
In the code above, “apple” gets converted into a QString using QString::fromAscii(). This means that QString will allocate memory for the string “apple” and creates a deep copy of the C-style string. This deep copy is not really necessary and can be avoided by not creating a QString to start with and if the encoding of the C-string was somehow known. Since Latin-1 string comparisons are most common, Qt provides a special class called QLatin1String that just holds a pointer to the C-string provided in it’s constructor. In addition, QString provides a QString::operator==(const QLatin1String &) overload that has special code to compare without malloc+deep copy. We can make the above code fast by writing it instead as,
if (fruit== QLatin1String("apple")) { ... } // fast and mentions encoding
QStringRef : String manipulation without the malloc
QString has various methods for string manipulations like mid(), left(), right(). All of them create a new QString and hence a malloc/deep copy of data in an existing QString. Instead, one can use QString::midRef(), QString::leftRef() and QString::rightRef() to obtain a QStringRef. A QStringRef is a reference of a portion of a QString. QString also provides many overloads like QString::operator==(const QStringRef &) for optimizations with QStringRef.
QString::reserve and QString::squeeze
If you expect a QString to grow, it’s better to call QString::reserve to allocate extra memory in advance so that every call to QString::append() does not result in a malloc. Extra memory can be reclaimed using QString::squeeze.
QStringBuilder : Fast QString concatenation
The code below requires atleast 2 mallocs.
if (foo.startsWith("(" + type + ") 0x"))
Qt 4.6 introduces an internal class called QStringBuilder that “reserves” memory for a concatenation chain in a single shot. It does so by having each of the + operations above return a different class (not QString). This class keeps track of the string’s that are being appended and the required memory at each step. At the final step, where the concatenation operation gets converted into a QString it allocates memory in a single shot and copies all the strings in the chain one after another. This features can be enabled by using QT_USE_FAST_CONCATENATION. With this defined, one can use the operator ’%’ instead of ’+’. One would now write,
if (foo.startsWith("(" % type % ") 0x"))
If we want to use ’+’ itself instead of ’%’, one can also define QT_USE_FAST_OPERATOR_PLUS. See Fast concatenation for more details
Use QStringMatcher to match a string repetitively
If you are looking for a string repetitively in many strings or in the same string many times, you should use QStringMatcher. It uses Boyer-Moore string search algorithm to perform fast searches.
