QString str = "123456789"; unsigned long long num = str.toULongLong(); // num is now equal to 123456789
QString str = "hello"; unsigned long long num = str.toULongLong(); // num is now equal to 0In this example, we create a QString object called str with the value "hello". We then call the toULongLong method on str and assign the result to an unsigned long long variable called num. Since the string "hello" cannot be converted to a number, the value of num is set to 0. The QString class is part of the Qt library, which is a cross-platform application development framework. The library provides various modules for different functionalities, including GUI, network, multimedia, and more.