[SOLVED] Sending a argument to a slot

Timmoth

I understand that i am not able to pass a argument to a slot that was not a argument of the signal.
However this is exactly what i want to be able to achieve. and im sure there must be a work around if anyone can help?

basically when a button is pressed i want to send some text through to my slot function:
e.g
@QObject::connect(button, SIGNAL(clicked()),this, SLOT(updateL("C:/")));@

regards, Tim.

kevina67

You can use C++11 lambdas :

@
QObject::connect(button, &QPushButton::clicked, [this] () {
updateL("C:/")
});
@

andre

Or you use a [[doc:QSignalMapper]] object, especially if you're still on Qt4 or using a compiler that doesn't understand C++/11

dbzhang800

Simply create a slot function without argument, connect your signal to this slot, in which your original slot get called as a normal function.

If you do not want to create a named function and your are using Qt5, you can connect your signal to the lambda function, in which you can call your original slot. [as kevlna65 suggested]

[quote author="Timmoth" date="1375433317"]I understand that i am not able to pass a argument to a slot that was not a argument of the signal.
However this is exactly what i want to be able to achieve. and im sure there must be a work around if anyone can help?

basically when a button is pressed i want to send some text through to my slot function:
e.g
@QObject::connect(button, SIGNAL(clicked()),this, SLOT(updateL("C:/")));@

regards, Tim.[/quote]

Timmoth

Thanks for both replys i think im going to go with Kevlna's method though since i am using Qt5.
@ QObject::connect(button, &QPushButton::clicked, this{Hi("C:/");});@
worked perfectly!

Timmoth

However i have an issue now, when i try to pass a Qstring through i get an error:

error: C3493: 'path' cannot be implicitly captured because no default capture mode has been specified
@
QString path = d.absoluteFilePath();
QObject::connect(button, &QPushButton::clicked, this{Hi(path);});
@
regards, Tim

Edit: please use @ tags around code sections; Andre

kevina67

Use this instead :

@
QObject::connect(button, &QPushButton::clicked, [this] () {

// You're inside the lambda, you can write almoste everything here.    
QString path = d.absoluteFilePath();    
Hi(path);

});@

Timmoth

i understand it a bit better now thanks Kevlna but im still getting the error message:

error: C3493: ‘path’ cannot be implicitly captured because no default capture mode has been specified

PeerS

As d is a local variable you have tell the lambda how to capture it:

@
(1) auto l = &d { // only d is captured by reference };
(2) auto l = d { // only d is captured by value };
(3) auto l = & { // all local variables are captured by reference. };
(4) auto l = = { // all local variables are captured by value. (A copy will be made!) };
@

There are several ways to combine different capture methods, refer to a c++ book of your choice for more details. (E.g. The C++ Programming Language 4, 11.4.3, page 293).

Timmoth

ok thanks alot PeerS! ill look into capture methods.

Timmoth

I cannot seem to get this to work! ive looked up a few examples to no success.

@
auto func1 = & { return d.absoluteFilePath(); };
QObject::connect(button, &QPushButton::clicked, this {
Hi(func1);
});
@
am i using this wrongly?
regards, Tim.

Timmoth

Ok - finally i have gotten it to work.

@
auto func1 = = { return d.absoluteFilePath(); };
QObject::connect(button, &QPushButton::clicked, = {
Hi(func1());
});
@