QLatin1String与QString

The QLatin1String class provides a thin wrapper around an US-ASCII/Latin-1 encoded string literal.

Many of QString's member functions are overloaded to accept const char * instead of QString. This includes the copy constructor, the assignment operator, the comparison operators, and various other functions such as insert(), replace(), and indexOf(). These functions are usually optimized to avoid constructing a QStringobject for the const char * data. For example, assuming str is a QString,

 if (str == "auto" || str == "extern"
         || str == "static" || str == "register") {
     ...
 }

is much faster than

 if (str == QString("auto") || str == QString("extern")
         || str == QString("static") || str == QString("register")) {
     ...
 }

because it doesn't construct four temporary QString objects and make a deep copy of the character data.

Applications that define QT_NO_CAST_FROM_ASCII (as explained in the QString documentation) don't have access to QString's const char * API. To provide an efficient way of specifying constant Latin-1 strings, Qt provides the QLatin1String, which is just a very thin wrapper around a const char *. Using QLatin1String, the example code above becomes

 if (str == QLatin1String("auto")
         || str == QLatin1String("extern")
         || str == QLatin1String("static")
         || str == QLatin1String("register") {
     ...
 }

This is a bit longer to type, but it provides exactly the same benefits as the first version of the code, and is faster than converting the Latin-1 strings usingQString::fromLatin1().

Thanks to the QString(const QLatin1String &) constructor, QLatin1String can be used everywhere a QString is expected. For example:

 QLabel *label = new QLabel(QLatin1String("MOD"), this);

From:Qt Reference Document