Connect a Qt signal to a slot using std::bind

Question

I have many QActions and I would like to connect their triggered(bool) signal to a specific slot which gets an integere number as input, let's say setX(int x). I need to specify x in connect callback. For example:

connect(actionV, &QAction::triggered,
        this, &TheClass::setX /* somehow x=10 */);

I tried using std::bind and it does not work:

connect(actionV, &QAction::triggered,
        std::bind(this, &TheClass::setX, 10));

To begin with, you use [`std::bind`](http://en.cppreference.com/w/cpp/utility/functional/bind) wrong. The first argument is the "function" that should be called. — Some programmer dude, Jul 13 '18 at 08:20
For this, I prefer lambdas and it's working fine (especially if receiver is not a `QObject`). e.g. `QObject::connect(actionV, &QAction::triggered, [&](bool) { setX(10); });` — Scheff's Cat, Jul 13 '18 at 08:21
Related: https://stackoverflow.com/questions/17363003/why-use-stdbind-over-lambdas-in-c14/17545183 If you use C++14, you should avoid `std::bind` altogether. — Jaa-c, Jul 13 '18 at 08:22
@Someprogrammerdude Oh... Yes it is wrong indeed (: Passing this as second argument, now works. Thank you — sorush-r, Jul 13 '18 at 08:42

Mahmoud Badri · Answer 1 · 2020-04-15T13:15:52.493

4

You can solve this easily using a lambda:

connect(actionV, &QAction::triggered, [&] { 
   m_theClass.setX(10);
});

edited Apr 15 '20 at 13:15

answered Feb 07 '19 at 19:49

Mahmoud Badri

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1

Missing `);` at the end – Stefan Apr 13 '20 at 15:03
added the missing ) :) – Mahmoud Badri Apr 15 '20 at 13:16

score 2 · Answer 2 · answered Mar 05 '22 at 07:36

The syntax you used for std::bind was incorrect, that's why it didn't compile.

While using a lambda is faster, I am going to post some examples using std::bind for completeness shake in the hope of being helpful to other programmers as well.

Example A:

// Declaration somewhere in a header file.
void setX(int x);

/*
 * In a cpp file:
 * Notice the order of the arguments. 
 * a) The pointer to the class member
 * b) the instance of the class (in this case is "this")
 * c) The arguments to the function are the rest.
 */
connect(actionV, &QAction::triggered, this, std::bind(&MyClass::setX, this, 10));

Now, let's pretend that you must capture the parameter of a signal. In this case it's a bool. I have to remind you, that the signature is void QAction::triggered(bool checked = false).

Example B:

// Again, declaration in a header file.
void setX(bool c, int x);

/*
 * In a cpp file:
 * Notice how the emmited bool value is captured with
 * std::placeholders::_1 and then you pass 10 as well.
 */
connect(actionV,
        &QAction::triggered,
        this,
        std::bind(&MyClass::setX, this, std::placeholders::_1, 10));

To make this even more extreme, let's pretend once again that you have two overloads of setX.

Example C:

// Declaration in a header file
void setX(bool x, int x);
void setX(int x);

/*
 * In your .cpp file:
 * a) Choose the first signature
 * b) Choose the second signature
 */
connect(actionV,
        &QAction::triggered,
        this,
        std::bind(qOverload<bool, int>(&MyClass::setX),
                  this, std::placeholders::_1, 10));
// OR

connect(actionV,
        &QAction::triggered,
        this,
        std::bind(qOverload<int>(&MyClass::setX), this, 10));

But the qOverload macro needs at least a C++14 capable compiler though. You don't want to go to:

connect(actionV,
        &QAction::triggered,
        this,
        std::bind(static_cast<void(MyClass::*)(bool, int)>(&MyClass::setX),
                  this,
                  std::placeholders::_1,
                  10));

I hope I helped a bit to clarify the syntax. I can't stress enough though that using a lambda is more efficient and simple and that this was just a fun "exercise".

Connect a Qt signal to a slot using std::bind

2 Answers2