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 {
 InputForm(int ibot, int itop, double dbot, double dtop);
 void setupForm();
 public slots:
 void computeResult();
 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() {
 [ . . . . ]
 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.

Part I: Introduction to C++ and Qt 4

C++ Introduction


Introduction to Qt



Inheritance and Polymorphism

Part II: Higher-Level Programming


Introduction to Design Patterns


Generics and Containers

Qt GUI Widgets


Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments


MP3 Jukebox Assignments

show all menu

An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268
Similar book on Amazon © 2008-2017.
If you may any questions please contact us: