QValidator Class

The QValidator class provides validation of input text. More...

Header: #include <QValidator>
qmake: QT += gui
Inherits: QObject
Inherited By:

QDoubleValidator, QIntValidator, QRegExpValidator, and QRegularExpressionValidator

Public Types

enum State { Invalid, Intermediate, Acceptable }

Public Functions

QValidator(QObject *parent = nullptr)
virtual ~QValidator()
virtual void fixup(QString &input) const
QLocale locale() const
void setLocale(const QLocale &locale)
virtual QValidator::State validate(QString &input, int &pos) const = 0


void changed()

Detailed Description

The class itself is abstract. Two subclasses, QIntValidator and QDoubleValidator, provide basic numeric-range checking, and QRegExpValidator provides general checking using a custom regular expression.

If the built-in validators aren't sufficient, you can subclass QValidator. The class has two virtual functions: validate() and fixup().

validate() must be implemented by every subclass. It returns Invalid, Intermediate or Acceptable depending on whether its argument is valid (for the subclass's definition of valid).

These three states require some explanation. An Invalid string is clearly invalid. Intermediate is less obvious: the concept of validity is difficult to apply when the string is incomplete (still being edited). QValidator defines Intermediate as the property of a string that is neither clearly invalid nor acceptable as a final result. Acceptable means that the string is acceptable as a final result. One might say that any string that is a plausible intermediate state during entry of an Acceptable string is Intermediate.

Here are some examples:

  • For a line edit that accepts integers from 10 to 1000 inclusive, 42 and 123 are Acceptable, the empty string, 5, or 1234 are Intermediate, and "asdf" and 10114 is Invalid.
  • For an editable combobox that accepts URLs, any well-formed URL is Acceptable, "http://example.com/," is Intermediate (it might be a cut and paste action that accidentally took in a comma at the end), the empty string is Intermediate (the user might select and delete all of the text in preparation for entering a new URL) and "http:///./" is Invalid.
  • For a spin box that accepts lengths, "11cm" and "1in" are Acceptable, "11" and the empty string are Intermediate, and "http://example.com" and "hour" are Invalid.

fixup() is provided for validators that can repair some user errors. The default implementation does nothing. QLineEdit, for example, will call fixup() if the user presses Enter (or Return) and the content is not currently valid. This allows the fixup() function the opportunity of performing some magic to make an Invalid string Acceptable.

A validator has a locale, set with setLocale(). It is typically used to parse localized data. For example, QIntValidator and QDoubleValidator use it to parse localized representations of integers and doubles.

QValidator is typically used with QLineEdit, QSpinBox and QComboBox.

See also QIntValidator, QDoubleValidator, QRegExpValidator, and Line Edits Example.

Member Type Documentation

enum QValidator::State

This enum type defines the states in which a validated string can exist.

QValidator::Invalid0The string is clearly invalid.
QValidator::Intermediate1The string is a plausible intermediate value.
QValidator::Acceptable2The string is acceptable as a final result; i.e. it is valid.

Member Function Documentation

QValidator::QValidator(QObject *parent = nullptr)

Sets up the validator. The parent parameter is passed on to the QObject constructor.

[signal] void QValidator::changed()

This signal is emitted when any property that may affect the validity of a string has changed.

[virtual] QValidator::~QValidator()

Destroys the validator, freeing any storage and other resources used.

[virtual] void QValidator::fixup(QString &input) const

This function attempts to change input to be valid according to this validator's rules. It need not result in a valid string: callers of this function must re-test afterwards; the default does nothing.

Reimplementations of this function can change input even if they do not produce a valid string. For example, an ISBN validator might want to delete every character except digits and "-", even if the result is still not a valid ISBN; a surname validator might want to remove whitespace from the start and end of the string, even if the resulting string is not in the list of accepted surnames.

QLocale QValidator::locale() const

Returns the locale for the validator. The locale is by default initialized to the same as QLocale().

See also setLocale() and QLocale::QLocale().

void QValidator::setLocale(const QLocale &locale)

Sets the locale that will be used for the validator. Unless setLocale has been called, the validator will use the default locale set with QLocale::setDefault(). If a default locale has not been set, it is the operating system's locale.

See also locale() and QLocale::setDefault().

[pure virtual] QValidator::State QValidator::validate(QString &input, int &pos) const

This virtual function returns Invalid if input is invalid according to this validator's rules, Intermediate if it is likely that a little more editing will make the input acceptable (e.g. the user types "4" into a widget which accepts integers between 10 and 99), and Acceptable if the input is valid.

The function can change both input and pos (the cursor position) if required.

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