- Qt 6.9
- Qt D-Bus
- C++ Classes
- QDBusPendingReply
- Qt 6.9.3 Reference Documentation

# QDBusPendingReply Class

template <typename... Types> class QDBusPendingReply

The QDBusPendingReply class contains the reply to an asynchronous method call. More...

Header: | `#include <QDBusPendingReply>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS DBus)` `target_link_libraries(mytarget PRIVATE Qt6::DBus)`
qmake: | `QT += dbus`
Inherits: | QDBusPendingReplyBase

- List of all members, including inherited members

## Public Types

enum | anonymous { Count }

## Public Functions

| QDBusPendingReply()
| QDBusPendingReply(const QDBusMessage &message)
| QDBusPendingReply(const QDBusPendingCall &call)
| QDBusPendingReply(const QDBusPendingReply<Types...> &other)
QVariant | argumentAt(int index) const
int | count() const
QDBusError | error() const
bool | isError() const
bool | isFinished() const
bool | isValid() const
QDBusMessage | reply() const
typename Select<0>::Type | value() const
void | waitForFinished()
typename Select<0>::Type | operator typename Select<0>::Type() const
QDBusPendingReply<Types...> & | operator=(const QDBusMessage &message)
QDBusPendingReply<Types...> & | operator=(const QDBusPendingCall &call)
QDBusPendingReply<Types...> & | operator=(const QDBusPendingReply<Types...> &other)

## Detailed Description

The QDBusPendingReply is a variadic template class. The template parameters are the types that will be used to extract the contents of the reply's data.

This class is similar in functionality to QDBusReply, but with two important differences:

- QDBusReply accepts exactly one return type, whereas QDBusPendingReply can have any number of types
- QDBusReply only works on already completed replies, whereas QDBusPendingReply allows one to wait for replies from pending calls

Where with QDBusReply you would write:

     QDBusReply<QString> reply = interface->call("RemoteMethod");
     if (reply.isValid())
         // use the returned value
         useValue(reply.value());
     else
         // call failed. Show an error condition.
         showError(reply.error());

with QDBusPendingReply, the equivalent code (including the blocking wait for the reply) would be:

         QDBusPendingReply<QString> reply = iface->asyncCall("RemoteMethod");
         reply.waitForFinished();
         if (reply.isError())
             // call failed. Show an error condition.
             showErrorD(reply.error());
         else
             // use the returned value
             useValue(reply.value());

For method calls that have more than one output argument, with QDBusReply, you would write:

     reply = interface->call("RemoteMethod");

whereas with QDBusPendingReply, all of the output arguments should be template parameters:

         QDBusPendingReply<bool, QString> reply = iface->asyncCall("RemoteMethod");
         reply.waitForFinished();
         if (!reply.isError()) {
             if (reply.argumentAt<0>())
                 showSuccess(reply.argumentAt<1>());
             else
                 showFailure(reply.argumentAt<1>());
         }

QDBusPendingReply objects can be associated with QDBusPendingCallWatcher objects, which emit signals when the reply arrives.

See also QDBusPendingCallWatcher and QDBusReply.

## Member Type Documentation

### enum QDBusPendingReply::anonymous

Constant | Value | Description
--- | --- | ---
`QDBusPendingReply::Count` | `std::is_same_v<typename Select<0>::Type, void> ? 0 : sizeof...(Types)` | The number of arguments the reply is expected to have

## Member Function Documentation

### QDBusPendingReply::QDBusPendingReply()

Creates an empty QDBusPendingReply object. Without assigning a QDBusPendingCall object to this reply, QDBusPendingReply cannot do anything. All functions return their failure values.

### QDBusPendingReply::QDBusPendingReply(const QDBusMessage &message)

Creates a QDBusPendingReply object that will take its contents from the message message. In this case, this object will be already in its finished state and the reply's contents will be accessible.

See also isFinished().

### QDBusPendingReply::QDBusPendingReply(const QDBusPendingCall &call)

Creates a QDBusPendingReply object that will take its contents from the call pending asynchronous call. This QDBusPendingReply object will share the same pending call reference as call.

### QDBusPendingReply::QDBusPendingReply(const QDBusPendingReply<Types...> &other)

Creates a copy of the other QDBusPendingReply object. Just like QDBusPendingCall and QDBusPendingCallWatcher, this QDBusPendingReply object will share the same pending call reference. All copies share the same return values.

### QVariant QDBusPendingReply::argumentAt(int index) const

Returns the argument at position index in the reply's contents. If the reply doesn't have that many elements, this function's return value is undefined (will probably cause an assertion failure), so it is important to verify that the processing is finished and the reply is valid.

If the reply does not contain an argument at position index or if the reply was an error, this function returns an invalid QVariant. Since D-Bus messages can never contain invalid QVariants, this return can be used to detect an error condition.

### `[constexpr]` int QDBusPendingReply::count() const

Return the number of arguments the reply is supposed to have. This number matches the number of non-void template parameters in this class.

If the reply arrives with a different number of arguments (or with different types), it will be transformed into an error reply indicating a bad signature.

### QDBusError QDBusPendingReply::error() const

Retrieves the error content of the reply message, if it has finished processing. If the reply message has not finished processing or if it contains a normal reply message (non-error), this function returns an invalid QDBusError.

### bool QDBusPendingReply::isError() const

Returns `true` if the reply contains an error message, false if it contains a normal method reply.

If the pending call has not finished processing, this function also returns `true`.

### bool QDBusPendingReply::isFinished() const

Returns `true` if the pending call has finished processing and the reply has been received. If this function returns `true`, the isError(), error() and reply() methods should return valid information.

Note that this function only changes state if you call waitForFinished() or if an external D-Bus event happens, which in general only happens if you return to the event loop execution.

See also QDBusPendingCallWatcher::isFinished().

### bool QDBusPendingReply::isValid() const

Returns `true` if the reply contains a normal reply message, false if it contains anything else.

If the pending call has not finished processing, this function return false.

### QDBusMessage QDBusPendingReply::reply() const

Retrieves the reply message received for the asynchronous call that was sent, if it has finished processing. If the pending call is not finished, this function returns a QDBusMessage of type QDBusMessage::InvalidMessage.

After it has finished processing, the message type will either be an error message or a normal method reply message.

### typename Select<0>::Type QDBusPendingReply::value() const

Returns the first argument in this reply, cast to type `Types[0]` (the first template parameter of this class). This is equivalent to calling argumentAt<0>().

This function is provided as a convenience, matching the QDBusReply::value() function.

Note that, if the reply hasn't arrived, this function causes the calling thread to block until the reply is processed.

If the reply is an error reply, this function returns a default-constructed `Types[0]` object, which may be indistinguishable from a valid value. To reliably determine whether the message was an error, use isError().

### void QDBusPendingReply::waitForFinished()

Suspends the execution of the calling thread until the reply is received and processed. After this function returns, isFinished() should return true, indicating the reply's contents are ready to be processed.

See also QDBusPendingCallWatcher::waitForFinished().

### typename Select<0>::Type QDBusPendingReply::operator typename Select<0>::Type() const

Returns the first argument in this reply, cast to type `Types[0]` (the first template parameter of this class). This is equivalent to calling argumentAt<0>().

This function is provided as a convenience, matching the QDBusReply::value() function.

Note that, if the reply hasn't arrived, this function causes the calling thread to block until the reply is processed.

If the reply is an error reply, this function returns a default-constructed `Types[0]` object, which may be indistinguishable from a valid value. To reliably determine whether the message was an error, use isError().

### QDBusPendingReply<Types...> &QDBusPendingReply::operator=(const QDBusMessage &message)

Makes this object take its contents from the message message and drops the reference to the current pending call. If the current reference is to an unfinished pending call and this is the last reference, the pending call will be canceled and there will be no way of retrieving the reply's contents, when they arrive.

After this function is finished, the QDBusPendingReply object will be in its "finished" state and the message contents will be accessible.

See also isFinished().

### QDBusPendingReply<Types...> &QDBusPendingReply::operator=(const QDBusPendingCall &call)

Makes this object take its contents from the call pending call and drops the reference to the current pending call. If the current reference is to an unfinished pending call and this is the last reference, the pending call will be canceled and there will be no way of retrieving the reply's contents, when they arrive.

### QDBusPendingReply<Types...> &QDBusPendingReply::operator=(const QDBusPendingReply<Types...> &other)

Makes a copy of other and drops the reference to the current pending call. If the current reference is to an unfinished pending call and this is the last reference, the pending call will be canceled and there will be no way of retrieving the reply's contents, when they arrive.