Qt Signal And Slots

  • Differences between String-Based and Functor-Based Connections (Official documentation)
  • Introduction (Woboq blog)
  • Implementation Details (Woboq blog)
  • 1Connecting in Qt 5
  • 2Disconnecting in Qt 5
  • 4Error reporting
  • 5Open questions

Connecting in Qt 5

Old syntax

New: connecting to QObject member


  • Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
  • Argument can be by typedefs or with different namespace specifier, and it works.
  • Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
  • It is possible to connect to any member function of QObject, not only slots.


  • More complicated syntax? (you need to specify the type of your object)
  • Very complicated syntax in cases of overloads? (see below)
  • Default arguments in slot is not supported anymore.

New: connecting to simple function


  • Can be used with std::bind:
  • Can be used with C++11 lambda expressions:


  • There is no automatic disconnection when the ‘receiver’ is destroyed because it’s a functor with no QObject. However, since 5.2 there is an overload which adds a ‘context object’. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).

Disconnecting in Qt 5

Qt signal and slots

Old way

  • You connected using the old way, or
  • If you want to disconnect all the slots from a given signal using wild card character

Symetric to the function pointer one

New way using QMetaObject::Connection

Asynchronous made easier

Error reporting

Missing Q_OBJECT in class definition

Qt Connect Slot

Type mismatch

Open questions

Default arguments in slot


All games i have played

Qt Signals And Slots Between Classes


Signal And Slots In Qt


Qt Signal And Slots




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