Qt - modify slot's parameter of previous connected signal using lambda function

1
votes

I'm trying to reconnect a signal to the same slot but with a different parameter. This would work if I just wanted to reconnect the same signal to a diferent slot without using parameters or lambda functions but I need to do it this way.

At first I tried something like this:

connect(remove_btn,&QPushButton::clicked,[this, id] {function1(varA);});
connect(remove_btn,&QPushButton::clicked,[this, id] {function1(varB);});

function1 receives varA and never varB.

I found a way to do this by keeping a list of the connections and when I wanted to modify the parameter I would just disconnect the old one and make a new connection.

What I'd like to know is if is there a way to make this modification without the need of keeping track of all the connections and go through the discconnect/connect process.

2
c++qtlambdaqt-signalsqt-connection
@AlexanderVX forget the "id" var. I forgot to remove it from my example, it's irrelevant. About your example I'm not sure what you tried to show me but both connects have the same parameter and that's not my issue. Thanks though. – RubenC

2 Answers

0
votes

You might be able to use std::bind, since the QPushButton::clicked signal has 1 parameter (bool checked), it should be sufficient to use:

auto f = std::bind(function1, varA); // checked argument ignored (std::placeholders::_1)
connect(remove_btn,&QPushButton::clicked,f);
0
votes

Two solutions come to my mind:

  1. Capture var by reference and change it instead of reconnecting:

    class Foo : public QWidget {
  Q_OBJECT
  int remove_btn_var;
  int varA = 10, varB = 20;
  QPushButton remove_btn;
  void function1(int);
public:
  Foo() {
    remove_btn_var = varA;
    connect(&remove_btn, &QPushButton::clicked, [&] { function1(remove_btn_var); });
  }
  void switch() {
    remove_btn_var = varB;
  }
};

  2. Add a property to the button that you can modify:

    static const char k_function1_var[] = "function1_var";
class Foo : public QWidget {
  Q_OBJECT
  int varA, varB;
  QPushButton remove_btn;
  void function1(int);
public:
  Foo() {
    remove_btn.setProperty(k_function1_var, varA);
    connect(&remove_btn, &QPushButton::clicked, [this] { 
      function1(remove_btn.property(k_function1_var).toInt())
    });
  }
  void switch() {
    remove_btn.setProperty(k_function1_var, varB);
  }
};