emit
statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit
keyword will continue immediately, and the slots will be executed later..cpp
file. They can never have return types (i.e. use void
).new
or delete
operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new
or delete
, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.signals
or slots
may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef
the offending preprocessor symbol.valueChanged()
, and it has a slot which other objects can send signals to.Counter::setValue()
slot:emit
line emits the signal valueChanged()
from the object, with the new value as argument.Counter
objects and connect the first object's valueChanged()
signal to the second object's setValue()
slot using QObject::connect():a.setValue(12)
makes a
emit a valueChanged(12)
signal, which b
will receive in its setValue()
slot, i.e. b.setValue(12)
is called. Then b
emits the same valueChanged()
signal, but since no slot has been connected to b
's valueChanged()
signal, the signal is ignored.setValue()
function sets the value and emits the signal only if value != m_value
. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged()
were connected to a.setValue()
).false
.LcdNumber
inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.moc
; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber
', you have probably forgotten to run the moc or to include the moc output in the link command.public
members, we declare the class signals
. The LcdNumber
class emits a signal, overflow()
, when it is asked to show an impossible value.overflow()
signal, i.e. don't connect it to any slot.LcdNumber
uses it, as the code above indicates, to set the displayed number. Since display()
is part of the class's interface with the rest of the program, the slot is public.display()
slot, so the LCD number continuously shows the value of the scroll bar.display()
is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.SIGNAL
and SLOT
macros. The rule about whether to include arguments or not in the SIGNAL()
and SLOT()
macros, if the arguments have default values, is that the signature passed to the SIGNAL()
macro must not have fewer arguments than the signature passed to the SLOT()
macro.signals
, slots
, and emit
, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords
flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.