- Qt 6.8
- Qt Network
- C++ Classes
- QHostInfo
- Qt 6.8.3 Reference Documentation

# QHostInfo Class

The QHostInfo class provides static functions for host name lookups. More...

Header: | `#include <QHostInfo>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Network)` `target_link_libraries(mytarget PRIVATE Qt6::Network)`
qmake: | `QT += network`

- List of all members, including inherited members
- QHostInfo is part of Network Programming API.

Note: All functions in this class are reentrant.

## Public Types

enum | HostInfoError { NoError, HostNotFound, UnknownError }

## Public Functions

| QHostInfo(int id = -1)
| QHostInfo(const QHostInfo &other)
| QHostInfo(QHostInfo &&other)
| ~QHostInfo()
QList<QHostAddress> | addresses() const
QHostInfo::HostInfoError | error() const
QString | errorString() const
QString | hostName() const
int | lookupId() const
void | setAddresses(const QList<QHostAddress> &addresses)
void | setError(QHostInfo::HostInfoError error)
void | setErrorString(const QString &str)
void | setHostName(const QString &hostName)
void | setLookupId(int id)
void | swap(QHostInfo &other)
QHostInfo & | operator=(QHostInfo &&other)
QHostInfo & | operator=(const QHostInfo &other)

## Static Public Members

void | abortHostLookup(int id)
QHostInfo | fromName(const QString &name)
QString | localDomainName()
QString | localHostName()
int | lookupHost(const QString &name, const QObject *receiver, const char *member)
int | lookupHost(const QString &name, Functor &&functor)
int | lookupHost(const QString &name, const QObject *context, Functor functor)

## Detailed Description

QHostInfo finds the IP address(es) associated with a host name, or the host name associated with an IP address. The class provides two static convenience functions: one that works asynchronously and emits a signal once the host is found, and one that blocks and returns a QHostInfo object.

To look up a host's IP addresses asynchronously, call lookupHost(), which takes the host name or IP address, a receiver object, and a slot signature as arguments and returns an ID. You can abort the lookup by calling abortHostLookup() with the lookup ID.

Example:

     // To find the IP address of qt-project.org
     QHostInfo::lookupHost("qt-project.org", this, &MyWidget::printResults);

     // To find the host name for 4.2.2.1
     QHostInfo::lookupHost("4.2.2.1", this, &MyWidget::printResults);

The slot is invoked when the results are ready. The results are stored in a QHostInfo object. Call addresses() to get the list of IP addresses for the host, and hostName() to get the host name that was looked up.

If the lookup failed, error() returns the type of error that occurred. errorString() gives a human-readable description of the lookup error.

If you want a blocking lookup, use the QHostInfo::fromName() function:

     QHostInfo info = QHostInfo::fromName("qt-project.org");

QHostInfo supports Internationalized Domain Names (IDNs) through the IDNA and Punycode standards.

To retrieve the name of the local host, use the static QHostInfo::localHostName() function.

QHostInfo uses the mechanisms provided by the operating system to perform the lookup. As per RFC 6724 there is no guarantee that all IP addresses registered for a domain or host will be returned.

Note: Since Qt 4.6.1 QHostInfo is using multiple threads for DNS lookup instead of one dedicated DNS thread. This improves performance, but also changes the order of signal emissions when using lookupHost() compared to previous versions of Qt.

Note: Since Qt 4.6.3 QHostInfo is using a small internal 60 second DNS cache for performance improvements.

See also QAbstractSocket, RFC 3492, and RFC 6724.

## Member Type Documentation

### enum QHostInfo::HostInfoError

This enum describes the various errors that can occur when trying to resolve a host name.

Constant | Value | Description
--- | --- | ---
`QHostInfo::NoError` | `0` | The lookup was successful.
`QHostInfo::HostNotFound` | `1` | No IP addresses were found for the host.
`QHostInfo::UnknownError` | `2` | An unknown error occurred.

See also error() and setError().

## Member Function Documentation

### `[explicit]` QHostInfo::QHostInfo(int id = -1)

Constructs an empty host info object with lookup ID id.

See also lookupId().

### QHostInfo::QHostInfo(const QHostInfo &other)

Constructs a copy of other.

### `[noexcept]` QHostInfo::QHostInfo(QHostInfo &&other)

Move-constructs a new QHostInfo from other.

Note: The moved-from object other is placed in a partially-formed state, in which the only valid operations are destruction and assignment of a new value.

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

Destroys the host info object.

### `[static]` void QHostInfo::abortHostLookup(int id)

Aborts the host lookup with the ID id, as returned by lookupHost().

See also lookupHost() and lookupId().

### QList<QHostAddress> QHostInfo::addresses() const

Returns the list of IP addresses associated with hostName(). This list may be empty.

Example:

     QHostInfo info;
     ...
     if (!info.addresses().isEmpty()) {
         QHostAddress address = info.addresses().first();
         // use the first IP address
     }

See also setAddresses(), hostName(), and error().

### QHostInfo::HostInfoError QHostInfo::error() const

Returns the type of error that occurred if the host name lookup failed; otherwise returns NoError.

See also setError() and errorString().

### QString QHostInfo::errorString() const

If the lookup failed, this function returns a human readable description of the error; otherwise "Unknown error" is returned.

See also setErrorString() and error().

### `[static]` QHostInfo QHostInfo::fromName(const QString &name)

Looks up the IP address(es) for the given host name. The function blocks during the lookup which means that execution of the program is suspended until the results of the lookup are ready. Returns the result of the lookup in a QHostInfo object.

If you pass a literal IP address to name instead of a host name, QHostInfo will search for the domain name for the IP (i.e., QHostInfo will perform a reverse lookup). On success, the returned QHostInfo will contain both the resolved domain name and IP addresses for the host name.

See also lookupHost().

### QString QHostInfo::hostName() const

Returns the name of the host whose IP addresses were looked up.

See also setHostName() and localHostName().

### `[static]` QString QHostInfo::localDomainName()

Returns the DNS domain of this machine.

Note: DNS domains are not related to domain names found in Windows networks.

See also hostName().

### `[static]` QString QHostInfo::localHostName()

Returns this machine's host name, if one is configured. Note that hostnames are not guaranteed to be globally unique, especially if they were configured automatically.

This function does not guarantee the returned host name is a Fully Qualified Domain Name (FQDN). For that, use fromName() to resolve the returned name to an FQDN.

This function returns the same as QSysInfo::machineHostName().

See also hostName() and localDomainName().

### `[static]` int QHostInfo::lookupHost(const QString &name, const QObject *receiver, const char *member)

Looks up the IP address(es) associated with host name name, and returns an ID for the lookup. When the result of the lookup is ready, the slot or signal member in receiver is called with a QHostInfo argument. The QHostInfo object can then be inspected to get the results of the lookup.

The lookup is performed by a single function call, for example:

     QHostInfo::lookupHost("www.kde.org", this, &MyWidget::lookedUp);

The implementation of the slot prints basic information about the addresses returned by the lookup, or reports an error if it failed:

     void MyWidget::lookedUp(const QHostInfo &host)
     {
         if (host.error() != QHostInfo::NoError) {
             qDebug() << "Lookup failed:" << host.errorString();
             return;
         }

         const auto addresses = host.addresses();
         for (const QHostAddress &address : addresses)
             qDebug() << "Found address:" << address.toString();
     }

If you pass a literal IP address to name instead of a host name, QHostInfo will search for the domain name for the IP (i.e., QHostInfo will perform a reverse lookup). On success, the resulting QHostInfo will contain both the resolved domain name and IP addresses for the host name. Example:

     QHostInfo::lookupHost("4.2.2.1", this, &MyWidget::lookedUp);

Note: There is no guarantee on the order the signals will be emitted if you start multiple requests with lookupHost().

Note: In Qt versions prior to 6.7, this function took receiver as (non-const) `QObject*`.

See also abortHostLookup(), addresses(), error(), and fromName().

### `[static]` template <typename Functor> int QHostInfo::lookupHost(const QString &name, Functor &&functor)

This is an overloaded function.

Looks up the IP address(es) associated with host name name, and returns an ID for the lookup. When the result of the lookup is ready, the functor is called with a QHostInfo argument. The QHostInfo object can then be inspected to get the results of the lookup.

The functor will be run in the thread that makes the call to lookupHost; that thread must have a running Qt event loop.

Note: There is no guarantee on the order the signals will be emitted if you start multiple requests with lookupHost().

See also abortHostLookup(), addresses(), error(), and fromName().

### `[static]` template <typename Functor> int QHostInfo::lookupHost(const QString &name, const QObject *context, Functor functor)

This is an overloaded function.

Looks up the IP address(es) associated with host name name, and returns an ID for the lookup. When the result of the lookup is ready, the functor is called with a QHostInfo argument. The QHostInfo object can then be inspected to get the results of the lookup.

If context is destroyed before the lookup completes, the functor will not be called. The functor will be run in the thread of context. The context's thread must have a running Qt event loop.

Here is an alternative signature for the function:

     lookupHost(const QString &name, const QObject *receiver, PointerToMemberFunction function)

In this case, when the result of the lookup is ready, the slot or signal `function` in `receiver` is called with a QHostInfo argument. The QHostInfo object can then be inspected to get the results of the lookup.

Note: There is no guarantee on the order the signals will be emitted if you start multiple requests with lookupHost().

See also abortHostLookup(), addresses(), error(), and fromName().

### int QHostInfo::lookupId() const

Returns the ID of this lookup.

See also setLookupId(), abortHostLookup(), and hostName().

### void QHostInfo::setAddresses(const QList<QHostAddress> &addresses)

Sets the list of addresses in this QHostInfo to addresses.

See also addresses().

### void QHostInfo::setError(QHostInfo::HostInfoError error)

Sets the error type of this QHostInfo to error.

See also error() and errorString().

### void QHostInfo::setErrorString(const QString &str)

Sets the human readable description of the error that occurred to str if the lookup failed.

See also errorString() and setError().

### void QHostInfo::setHostName(const QString &hostName)

Sets the host name of this QHostInfo to hostName.

See also hostName().

### void QHostInfo::setLookupId(int id)

Sets the ID of this lookup to id.

See also lookupId() and lookupHost().

### `[noexcept]` void QHostInfo::swap(QHostInfo &other)

Swaps this host-info with other. This operation is very fast and never fails.

### `[noexcept]` QHostInfo &QHostInfo::operator=(QHostInfo &&other)

Move-assigns other to this QHostInfo instance.

Note: The moved-from object other is placed in a partially-formed state, in which the only valid operations are destruction and assignment of a new value.

### QHostInfo &QHostInfo::operator=(const QHostInfo &other)

Assigns the data of the other object to this host info object, and returns a reference to it.