Validators

Validators are nonvisual objects that are attached to input widgets (such as QLineEdit, QSpinBox, and QComboBox) to provide a general framework for checking user input. Qt has an abstract class named QValidator that establishes the interface for all built-in and custom validators.

There are two concrete subclasses that can be used for numeric range checking: QIntValidator and QDoubleValidator. There is also a concrete subclass that can be used for validating a string with a specified regular expression. We will discuss regular expressions in the next section.

QValidator::validate() is a pure virtual method that returns an enumerated value, as follows:

• Invalid The expression does not satisfy the required conditions and further input will not help.
• Intermediate The expression does not satisfy the required conditions, but further input might produce an acceptable result.
• Acceptable The expression satisfies the required conditions.

Other member functions enable the setting of the conditions that validate() uses.

In Example 13.1 we have a short app that uses the two numerical validators. It takes an int and a double from the user to display the product. Each input is given a range check when the user presses return.

Example 13.1. src/validate/inputform.h

[ . . . . ]
class InputForm : public QMainWindow {
 Q_OBJECT
 public:
 InputForm(int ibot, int itop, double dbot, double dtop);
 void setupForm();
 public slots:
 void computeResult();
 private:
 int m_BotI, m_TopI;
 double m_BotD, m_TopD;
 QIntValidator* m_IValid;
 QDoubleValidator* m_DValid;
 QGridLayout* m_Layout;
 QLineEdit *m_IntEntry, *m_DoubleEntry;
 QLabel* m_Result;
 QWidget* m_Center;
};
[ . . . . ]

In Example 13.2, validators are initialized with range values in the constructor and are assigned to their respective input widgets in the setupForm() function.

Example 13.2. src/validate/inputform.cpp

[ . . . . ]
InputForm::InputForm(int ibot, int itop, double dbot, double dtop):
 m_BotI(ibot), m_TopI(itop), m_BotD(dbot), m_TopD(dtop),
 m_IValid(new QIntValidator(ibot, itop, this)),
 m_DValid(new QDoubleValidator(dbot, dtop, 2, this)),
 [ . . . . ]
void InputForm::setupForm() {
 [ . . . . ]
m_IntEntry->setValidator(m_IValid);
 m_DoubleEntry->setValidator(m_DValid);
 connect(m_IntEntry, SIGNAL(returnPressed()),
 this, SLOT(computeResult()));
 connect(m_DoubleEntry, SIGNAL(returnPressed()),
 this, SLOT(computeResult()));
}
[ . . . . ]

The running program looks like the screenshot that follows.

If you run it, try to enter a non-int textfield, or a non-float in the second. You should notice that it does not recognize invalid characters as input.