QCommandLineParser Class | Qt Core 6.8.3
Description: The QCommandLineParser class provides a means for handling the command line options.

- Qt 6.8
- Qt Core
- C++ Classes
- QCommandLineParser
- Qt 6.8.3 Reference Documentation

# QCommandLineParser Class

The QCommandLineParser class provides a means for handling the command line options. More...

Header: | `#include <QCommandLineParser>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members

## Public Types

enum | OptionsAfterPositionalArgumentsMode { ParseAsOptions, ParseAsPositionalArguments }
enum | SingleDashWordOptionMode { ParseAsCompactedShortOptions, ParseAsLongOptions }

## Public Functions

| QCommandLineParser()
| ~QCommandLineParser()
QCommandLineOption | addHelpOption()
bool | addOption(const QCommandLineOption &option)
bool | addOptions(const QList<QCommandLineOption> &options)
void | addPositionalArgument(const QString &name, const QString &description, const QString &syntax = QString())
QCommandLineOption | addVersionOption()
QString | applicationDescription() const
void | clearPositionalArguments()
QString | errorText() const
QString | helpText() const
bool | isSet(const QString &name) const
bool | isSet(const QCommandLineOption &option) const
QStringList | optionNames() const
bool | parse(const QStringList &arguments)
QStringList | positionalArguments() const
void | process(const QStringList &arguments)
void | process(const QCoreApplication &app)
void | setApplicationDescription(const QString &description)
void | setOptionsAfterPositionalArgumentsMode(QCommandLineParser::OptionsAfterPositionalArgumentsMode parsingMode)
void | setSingleDashWordOptionMode(QCommandLineParser::SingleDashWordOptionMode singleDashWordOptionMode)
void | showHelp(int exitCode = 0)
void | showVersion()
QStringList | unknownOptionNames() const
QString | value(const QString &optionName) const
QString | value(const QCommandLineOption &option) const
QStringList | values(const QString &optionName) const
QStringList | values(const QCommandLineOption &option) const

## Detailed Description

QCoreApplication provides the command-line arguments as a simple list of strings. QCommandLineParser provides the ability to define a set of options, parse the command-line arguments, and store which options have actually been used, as well as option values.

Any argument that isn't an option (i.e. doesn't start with a `-`) is stored as a "positional argument".

The parser handles short names, long names, more than one name for the same option, and option values.

Options on the command line are recognized as starting with one or two `-` characters, followed by the option name. The option `-` (single dash alone) is a special case, often meaning standard input, and is not treated as an option. The parser will treat everything after the option `--` (double dash) as positional arguments.

Short options are single letters. The option `v` would be specified by passing `-v` on the command line. In the default parsing mode, short options can be written in a compact form, for instance `-abc` is equivalent to `-a -b -c`. The parsing mode can be changed to ParseAsLongOptions, in which case `-abc` will be parsed as the long option `abc`.

Long options are more than one letter long and cannot be compacted together. The long option `verbose` would be passed as `--verbose` or `-verbose`.

Passing values to options can be done by using the assignment operator (`-v=value`, `--verbose=value`), or with a space (`-v value`, `--verbose value`). This works even if the the value starts with a `-`.

The parser does not support optional values - if an option is set to require a value, one must be present. If such an option is placed last and has no value, the option will be treated as if it had not been specified.

The parser does not automatically support negating or disabling long options by using the format `--disable-option` or `--no-option`. However, it is possible to handle this case explicitly by making an option with `no-option` as one of its names, and handling the option explicitly.

Example:

     int main(int argc, char *argv[])
     {
         QCoreApplication app(argc, argv);
         QCoreApplication::setApplicationName("my-copy-program");
         QCoreApplication::setApplicationVersion("1.0");

         QCommandLineParser parser;
         parser.setApplicationDescription("Test helper");
         parser.addHelpOption();
         parser.addVersionOption();
         parser.addPositionalArgument("source", QCoreApplication::translate("main", "Source file to copy."));
         parser.addPositionalArgument("destination", QCoreApplication::translate("main", "Destination directory."));

         // A boolean option with a single name (-p)
         QCommandLineOption showProgressOption("p", QCoreApplication::translate("main", "Show progress during copy"));
         parser.addOption(showProgressOption);

         // A boolean option with multiple names (-f, --force)
         QCommandLineOption forceOption(QStringList() << "f" << "force",
                 QCoreApplication::translate("main", "Overwrite existing files."));
         parser.addOption(forceOption);

         // An option with a value
         QCommandLineOption targetDirectoryOption(QStringList() << "t" << "target-directory",
                 QCoreApplication::translate("main", "Copy all source files into <directory>."),
                 QCoreApplication::translate("main", "directory"));
         parser.addOption(targetDirectoryOption);

         // Process the actual command line arguments given by the user
         parser.process(app);

         const QStringList args = parser.positionalArguments();
         // source is args.at(0), destination is args.at(1)

         bool showProgress = parser.isSet(showProgressOption);
         bool force = parser.isSet(forceOption);
         QString targetDir = parser.value(targetDirectoryOption);
         // ...
     }

The three addOption() calls in the above example can be made more compact by using addOptions():

         parser.addOptions({
             // A boolean option with a single name (-p)
             {"p",
                 QCoreApplication::translate("main", "Show progress during copy")},
             // A boolean option with multiple names (-f, --force)
             {{"f", "force"},
                 QCoreApplication::translate("main", "Overwrite existing files.")},
             // An option with a value
             {{"t", "target-directory"},
                 QCoreApplication::translate("main", "Copy all source files into <directory>."),
                 QCoreApplication::translate("main", "directory")},
         });

Known limitation: the parsing of Qt options inside QCoreApplication and subclasses happens before QCommandLineParser exists, so it can't take it into account. This means any option value that looks like a builtin Qt option will be treated by QCoreApplication as a builtin Qt option. Example: `--profile -reverse` will lead to QGuiApplication seeing the -reverse option set, and removing it from QCoreApplication::arguments() before QCommandLineParser defines the `profile` option and parses the command line.

#### How to Use QCommandLineParser in Complex Applications

In practice, additional error checking needs to be performed on the positional arguments and option values. For example, ranges of numbers should be checked.

It is then advisable to introduce a function to do the command line parsing which takes a struct or class receiving the option values returning an object representing the result. The dnslookup example of the QtNetwork module illustrates this:

     struct DnsQuery
     {
         DnsQuery() : type(QDnsLookup::A) {}

         QDnsLookup::Type type;
         QHostAddress nameServer;
         QString name;
     };

     struct CommandLineParseResult
     {
         enum class Status {
             Ok,
             Error,
             VersionRequested,
             HelpRequested
         };
         Status statusCode = Status::Ok;
         std::optional<QString> errorString = std::nullopt;
     };

     CommandLineParseResult parseCommandLine(QCommandLineParser &parser, DnsQuery *query)
     {
         using Status = CommandLineParseResult::Status;

         parser.setSingleDashWordOptionMode(QCommandLineParser::ParseAsLongOptions);
         const QCommandLineOption nameServerOption("n", "The name server to use.", "nameserver");
         parser.addOption(nameServerOption);
         const QCommandLineOption typeOption("t", "The lookup type.", "type");
         parser.addOption(typeOption);
         parser.addPositionalArgument("name", "The name to look up.");
         const QCommandLineOption helpOption = parser.addHelpOption();
         const QCommandLineOption versionOption = parser.addVersionOption();

         if (!parser.parse(QCoreApplication::arguments()))
             return { Status::Error, parser.errorText() };

         if (parser.isSet(versionOption))
             return { Status::VersionRequested };

         if (parser.isSet(helpOption))
             return { Status::HelpRequested };

         if (parser.isSet(nameServerOption)) {
             const QString nameserver = parser.value(nameServerOption);
             query->nameServer = QHostAddress(nameserver);
             if (query->nameServer.isNull()
                 || query->nameServer.protocol() == QAbstractSocket::UnknownNetworkLayerProtocol) {
                 return { Status::Error,
                          u"Bad nameserver address: %1"_s.arg(nameserver) };
             }
         }

         if (parser.isSet(typeOption)) {
             const QString typeParameter = parser.value(typeOption);
             if (std::optional<QDnsLookup::Type> type = typeFromParameter(typeParameter))
                 query->type = *type;
             else
                 return { Status::Error, u"Bad record type: %1"_s.arg(typeParameter) };
         }

         const QStringList positionalArguments = parser.positionalArguments();
         if (positionalArguments.isEmpty())
             return { Status::Error, u"Argument 'name' missing."_s };
         if (positionalArguments.size() > 1)
             return { Status::Error, u"Several 'name' arguments specified."_s };
         query->name = positionalArguments.first();

         return { Status::Ok };
     }

In the main function, help should be printed to the standard output if the help option was passed and the application should return the exit code 0.

If an error was detected, the error message should be printed to the standard error output and the application should return an exit code other than 0.

         QCoreApplication::setApplicationVersion(QT_VERSION_STR);
         QCoreApplication::setApplicationName(QCoreApplication::translate("QDnsLookupExample",
                                                                          "DNS Lookup Example"));
         QCommandLineParser parser;
         parser.setApplicationDescription(QCoreApplication::translate("QDnsLookupExample",
                                                                      "An example demonstrating the "
                                                                      "class QDnsLookup."));
         DnsQuery query;
         using Status = CommandLineParseResult::Status;
         CommandLineParseResult parseResult = parseCommandLine(parser, &query);
         switch (parseResult.statusCode) {
         case Status::Ok:
             break;
         case Status::Error:
             std::fputs(qPrintable(parseResult.errorString.value_or(u"Unknown error occurred"_s)),
                        stderr);
             std::fputs("\n\n", stderr);
             std::fputs(qPrintable(parser.helpText()), stderr);
             return 1;
         case Status::VersionRequested:
             parser.showVersion();
             Q_UNREACHABLE_RETURN(0);
         case Status::HelpRequested:
             parser.showHelp();
             Q_UNREACHABLE_RETURN(0);
         }

A special case to consider here are GUI applications on Windows and mobile platforms. These applications may not use the standard output or error channels since the output is either discarded or not accessible.

On Windows, QCommandLineParser uses message boxes to display usage information and errors if no console window can be obtained. These message boxes can be omitted by setting the `QT_COMMAND_LINE_PARSER_NO_GUI_MESSAGE_BOXES` environment variable.

For other platforms, it is recommended to display help texts and error messages using a QMessageBox. To preserve the formatting of the help text, rich text with `<pre>` elements should be used:

     switch (parseResult.statusCode) {
     case Status::Ok:
         break;
     case Status::Error: {
         QString errorMessage = parseResult.errorString.value_or(u"Unknown error occurred"_qs);
         QMessageBox::warning(0, QGuiApplication::applicationDisplayName(),
                              "<html><head/><body><h2>" + errorMessage + "</h2><pre>"
                              + parser.helpText() + "</pre></body></html>");
         return 1;
     }
     case Status::VersionRequested:
         QMessageBox::information(0, QGuiApplication::applicationDisplayName(),
                                  QGuiApplication::applicationDisplayName() + ' '
                                  + QCoreApplication::applicationVersion());
         return 0;
     case Status::HelpRequested:
         QMessageBox::warning(0, QGuiApplication::applicationDisplayName(),
                              "<html><head/><body><pre>"
                              + parser.helpText() + "</pre></body></html>");
         return 0;
     }

However, this does not apply to the dnslookup example, because it is a console application.

See also QCommandLineOption and QCoreApplication.

## Member Type Documentation

### enum QCommandLineParser::OptionsAfterPositionalArgumentsMode

This enum describes the way the parser interprets options that occur after positional arguments.

Constant | Value | Description
--- | --- | ---
`QCommandLineParser::ParseAsOptions` | `0` | `application argument --opt -t` is interpreted as setting the options `opt` and `t`, just like `application --opt -t argument` would do. This is the default parsing mode. In order to specify that `--opt` and `-t` are positional arguments instead, the user can use `--`, as in `application argument -- --opt -t`.
`QCommandLineParser::ParseAsPositionalArguments` | `1` | `application argument --opt` is interpreted as having two positional arguments, `argument` and `--opt`. This mode is useful for executables that aim to launch other executables (e.g. wrappers, debugging tools, etc.) or that support internal commands followed by options for the command. `argument` is the name of the command, and all options occurring after it can be collected and parsed by another command line parser, possibly in another executable.

See also setOptionsAfterPositionalArgumentsMode().

### enum QCommandLineParser::SingleDashWordOptionMode

This enum describes the way the parser interprets command-line options that use a single dash followed by multiple letters, as `-abc`.

Constant | Value | Description
--- | --- | ---
`QCommandLineParser::ParseAsCompactedShortOptions` | `0` | `-abc` is interpreted as `-a -b -c`, i.e. as three short options that have been compacted on the command-line, if none of the options take a value. If `a` takes a value, then it is interpreted as `-a bc`, i.e. the short option `a` followed by the value `bc`. This is typically used in tools that behave like compilers, in order to handle options such as `-DDEFINE=VALUE` or `-I/include/path`. This is the default parsing mode. New applications are recommended to use this mode.
`QCommandLineParser::ParseAsLongOptions` | `1` | `-abc` is interpreted as `--abc`, i.e. as the long option named `abc`. This is how Qt's own tools (uic, rcc...) have always been parsing arguments. This mode should be used for preserving compatibility in applications that were parsing arguments in such a way. There is an exception if the `a` option has the QCommandLineOption::ShortOptionStyle flag set, in which case it is still interpreted as `-a bc`.

See also setSingleDashWordOptionMode().

## Member Function Documentation

### QCommandLineParser::QCommandLineParser()

Constructs a command line parser object.

### `[noexcept]` QCommandLineParser::~QCommandLineParser()

Destroys the command line parser object.

### QCommandLineOption QCommandLineParser::addHelpOption()

Adds help options to the command-line parser.

The options specified for this command-line are described by `-h` or `--help`. On Windows, the alternative `-?` is also supported. The option `--help-all` extends that to include generic Qt options, not defined by this command, in the output.

These options are handled automatically by QCommandLineParser.

Remember to use setApplicationDescription() to set the application description, which will be displayed when this option is used.

Example:

     int main(int argc, char *argv[])
     {
         QCoreApplication app(argc, argv);
         QCoreApplication::setApplicationName("my-copy-program");
         QCoreApplication::setApplicationVersion("1.0");

         QCommandLineParser parser;
         parser.setApplicationDescription("Test helper");
         parser.addHelpOption();
         parser.addVersionOption();
         parser.addPositionalArgument("source", QCoreApplication::translate("main", "Source file to copy."));
         parser.addPositionalArgument("destination", QCoreApplication::translate("main", "Destination directory."));

         // A boolean option with a single name (-p)
         QCommandLineOption showProgressOption("p", QCoreApplication::translate("main", "Show progress during copy"));
         parser.addOption(showProgressOption);

         // A boolean option with multiple names (-f, --force)
         QCommandLineOption forceOption(QStringList() << "f" << "force",
                 QCoreApplication::translate("main", "Overwrite existing files."));
         parser.addOption(forceOption);

         // An option with a value
         QCommandLineOption targetDirectoryOption(QStringList() << "t" << "target-directory",
                 QCoreApplication::translate("main", "Copy all source files into <directory>."),
                 QCoreApplication::translate("main", "directory"));
         parser.addOption(targetDirectoryOption);

         // Process the actual command line arguments given by the user
         parser.process(app);

         const QStringList args = parser.positionalArguments();
         // source is args.at(0), destination is args.at(1)

         bool showProgress = parser.isSet(showProgressOption);
         bool force = parser.isSet(forceOption);
         QString targetDir = parser.value(targetDirectoryOption);
         // ...
     }

Returns the option instance, which can be used to call isSet().

### bool QCommandLineParser::addOption(const QCommandLineOption &option)

Adds the option option to look for while parsing.

Returns `true` if adding the option was successful; otherwise returns `false`.

Adding the option fails if there is no name attached to the option, or the option has a name that clashes with an option name added before.

### bool QCommandLineParser::addOptions(const QList<QCommandLineOption> &options)

Adds the options to look for while parsing. The options are specified by the parameter options.

Returns `true` if adding all of the options was successful; otherwise returns `false`.

See the documentation for addOption() for when this function may fail.

### void QCommandLineParser::addPositionalArgument(const QString &name, const QString &description, const QString &syntax = QString())

Defines an additional argument to the application, for the benefit of the help text.

The argument name and description will appear under the `Arguments:` section of the help. If syntax is specified, it will be appended to the Usage line, otherwise the name will be appended.

Example:

     // Usage: image-editor file
     //
     // Arguments:
     //   file                  The file to open.
     parser.addPositionalArgument("file", QCoreApplication::translate("main", "The file to open."));

     // Usage: web-browser [urls...]
     //
     // Arguments:
     //   urls                URLs to open, optionally.
     parser.addPositionalArgument("urls", QCoreApplication::translate("main", "URLs to open, optionally."), "[urls...]");

     // Usage: cp source destination
     //
     // Arguments:
     //   source                Source file to copy.
     //   destination           Destination directory.
     parser.addPositionalArgument("source", QCoreApplication::translate("main", "Source file to copy."));
     parser.addPositionalArgument("destination", QCoreApplication::translate("main", "Destination directory."));

See also addHelpOption() and helpText().

### QCommandLineOption QCommandLineParser::addVersionOption()

Adds the `-v` / `--version` option, which displays the version string of the application.

This option is handled automatically by QCommandLineParser.

You can set the actual version string by using QCoreApplication::setApplicationVersion().

Returns the option instance, which can be used to call isSet().

### QString QCommandLineParser::applicationDescription() const

Returns the application description set in setApplicationDescription().

See also setApplicationDescription().

### void QCommandLineParser::clearPositionalArguments()

Clears the definitions of additional arguments from the help text.

This is only needed for the special case of tools which support multiple commands with different options. Once the actual command has been identified, the options for this command can be defined, and the help text for the command can be adjusted accordingly.

Example:

     QCoreApplication app(argc, argv);
     QCommandLineParser parser;

     parser.addPositionalArgument("command", "The command to execute.");

     // Call parse() to find out the positional arguments.
     parser.parse(QCoreApplication::arguments());

     const QStringList args = parser.positionalArguments();
     const QString command = args.isEmpty() ? QString() : args.first();
     if (command == "resize") {
         parser.clearPositionalArguments();
         parser.addPositionalArgument("resize", "Resize the object to a new size.", "resize [resize_options]");
         parser.addOption(QCommandLineOption("size", "New size.", "new_size"));
         parser.process(app);
         // ...
     }

     /*
     This code results in context-dependent help:

     $ tool --help
     Usage: tool command

     Arguments:
       command  The command to execute.

     $ tool resize --help
     Usage: tool resize [resize_options]

     Options:
       --size <size>  New size.

     Arguments:
       resize         Resize the object to a new size.
     */

### QString QCommandLineParser::errorText() const

Returns a translated error text for the user. This should only be called when parse() returns `false`.

### QString QCommandLineParser::helpText() const

Returns a string containing the complete help information.

See also showHelp().

### bool QCommandLineParser::isSet(const QString &name) const

Checks whether the option name was passed to the application.

Returns `true` if the option name was set, false otherwise.

The name provided can be any long or short name of any option that was added with addOption(). All the options names are treated as being equivalent. If the name is not recognized or that option was not present, false is returned.

Example:

     bool verbose = parser.isSet("verbose");

### bool QCommandLineParser::isSet(const QCommandLineOption &option) const

This is an overloaded function.

Checks whether the option was passed to the application.

Returns `true` if the option was set, false otherwise.

This is the recommended way to check for options with no values.

Example:

     QCoreApplication app(argc, argv);
     QCommandLineParser parser;
     QCommandLineOption verboseOption("verbose");
     parser.addOption(verboseOption);
     parser.process(app);
     bool verbose = parser.isSet(verboseOption);

### QStringList QCommandLineParser::optionNames() const

Returns a list of option names that were found.

This returns a list of all the recognized option names found by the parser, in the order in which they were found. For any long options that were in the form {–option=value}, the value part will have been dropped.

The names in this list do not include the preceding dash characters. Names may appear more than once in this list if they were encountered more than once by the parser.

Any entry in the list can be used with value() or with values() to get any relevant option values.

### bool QCommandLineParser::parse(const QStringList &arguments)

Parses the command line arguments.

Most programs don't need to call this, a simple call to process() is enough.

parse() is more low-level, and only does the parsing. The application will have to take care of the error handling, using errorText() if parse() returns `false`. This can be useful for instance to show a graphical error message in graphical programs.

Calling parse() instead of process() can also be useful in order to ignore unknown options temporarily, because more option definitions will be provided later on (depending on one of the arguments), before calling process().

Don't forget that arguments must start with the name of the executable (ignored, though).

Returns `false` in case of a parse error (unknown option or missing value); returns `true` otherwise.

See also process().

### QStringList QCommandLineParser::positionalArguments() const

Returns a list of positional arguments.

These are all of the arguments that were not recognized as part of an option.

### void QCommandLineParser::process(const QStringList &arguments)

Processes the command line arguments.

In addition to parsing the options (like parse()), this function also handles the builtin options and handles errors.

The builtin options are `--version` if addVersionOption was called and `--help` / `--help-all` if addHelpOption was called.

When invoking one of these options, or when an error happens (for instance an unknown option was passed), the current process will then stop, using the exit() function.

See also QCoreApplication::arguments() and parse().

### void QCommandLineParser::process(const QCoreApplication &app)

This is an overloaded function.

The command line is obtained from the QCoreApplication instance app.

### void QCommandLineParser::setApplicationDescription(const QString &description)

Sets the application description shown by helpText().

See also applicationDescription().

### void QCommandLineParser::setOptionsAfterPositionalArgumentsMode(QCommandLineParser::OptionsAfterPositionalArgumentsMode parsingMode)

Sets the parsing mode to parsingMode. This must be called before process() or parse().

### void QCommandLineParser::setSingleDashWordOptionMode(QCommandLineParser::SingleDashWordOptionMode singleDashWordOptionMode)

Sets the parsing mode to singleDashWordOptionMode. This must be called before process() or parse().

### void QCommandLineParser::showHelp(int exitCode = 0)

Displays the help information, and exits the application. This is automatically triggered by the –help option, but can also be used to display the help when the user is not invoking the application correctly. The exit code is set to exitCode. It should be set to 0 if the user requested to see the help, and to any other value in case of an error.

See also helpText().

### void QCommandLineParser::showVersion()

Displays the version information from QCoreApplication::applicationVersion(), and exits the application. This is automatically triggered by the –version option, but can also be used to display the version when not using process(). The exit code is set to EXIT_SUCCESS (0).

See also addVersionOption().

### QStringList QCommandLineParser::unknownOptionNames() const

Returns a list of unknown option names.

This list will include both long an short name options that were not recognized. For any long options that were in the form {–option=value}, the value part will have been dropped and only the long name is added.

The names in this list do not include the preceding dash characters. Names may appear more than once in this list if they were encountered more than once by the parser.

See also optionNames().

### QString QCommandLineParser::value(const QString &optionName) const

Returns the option value found for the given option name optionName, or an empty string if not found.

The name provided can be any long or short name of any option that was added with addOption(). All the option names are treated as being equivalent. If the name is not recognized or that option was not present, an empty string is returned.

For options found by the parser, the last value found for that option is returned. If the option wasn't specified on the command line, the default value is returned.

If the option does not take a value, a warning is printed, and an empty string is returned.

See also values(), QCommandLineOption::setDefaultValue(), and QCommandLineOption::setDefaultValues().

### QString QCommandLineParser::value(const QCommandLineOption &option) const

This is an overloaded function.

Returns the option value found for the given option, or an empty string if not found.

For options found by the parser, the last value found for that option is returned. If the option wasn't specified on the command line, the default value is returned.

An empty string is returned if the option does not take a value.

See also values(), QCommandLineOption::setDefaultValue(), and QCommandLineOption::setDefaultValues().

### QStringList QCommandLineParser::values(const QString &optionName) const

Returns a list of option values found for the given option name optionName, or an empty list if not found.

The name provided can be any long or short name of any option that was added with addOption(). All the options names are treated as being equivalent. If the name is not recognized or that option was not present, an empty list is returned.

For options found by the parser, the list will contain an entry for each time the option was encountered by the parser. If the option wasn't specified on the command line, the default values are returned.

An empty list is returned if the option does not take a value.

See also value(), QCommandLineOption::setDefaultValue(), and QCommandLineOption::setDefaultValues().

### QStringList QCommandLineParser::values(const QCommandLineOption &option) const

This is an overloaded function.

Returns a list of option values found for the given option, or an empty list if not found.

For options found by the parser, the list will contain an entry for each time the option was encountered by the parser. If the option wasn't specified on the command line, the default values are returned.

An empty list is returned if the option does not take a value.

See also value(), QCommandLineOption::setDefaultValue(), and QCommandLineOption::setDefaultValues().