Exception Safety

Containers

Qt's container classes are generally exception neutral. They pass any exception that happens within their contained type T to the user while keeping their internal state valid.

Example:

 QList<QString> list;
 ...
 try {
     list.append("hello");
 } catch (...) {
 }
 // list is safe to use - the exception did not affect it.

Exceptions to that rule are containers for types that can throw during assignment or copy constructions. For those types, functions that modify the container as well as returning a value, are unsafe to use:

 MyType s = list.takeAt(2);

If an exception occurs during the assignment of s, the value at index 2 is already removed from the container, but hasn't been assigned to s yet. It is lost without chance of recovery.

The correct way to write it:

 MyType s = list.at(2);
 list.removeAt(2);

If the assignment throws, the container still contains the value, no data loss occured.

Note that implicitly shared Qt classes will not throw in their assignment operators or copy constructors, so the limitation above does not apply.

 QApplication app(argc, argv);
 ...
 try {
     app.exec();
 } catch (const std::bad_alloc &) {
     // clean up here, e.g. save the session
     // and close all config files.

     return 0; // exit the application
 }