A more extended explanation for Q_UNUSED macro ?

[tonnot]

I dont understand the benefits of use it.
Can anybody explain it ?
Thanks

[high_flyer]

As the docs say:

Q_UNUSED ( name )

Indicates to the compiler that the parameter with the specified name is not used in the body of a function. This can be used to suppress compiler warnings while allowing functions to be defined with meaningful parameter names in their signatures.

[tonnot]

I already read the docs.
I dont understand what means :

.... with the specified name is not used in the body of a function ...

Thanks

[squidge]

If you have a method which takes two variables and you don't use one of them, then one of them is unused, yes? So you can use Q_UNUSED macro to tell the compiler that you didn't forget about it and you know it's unused, otherwise you could receive warning message.

int foo::bar(int a1, int a2, int a3)
{
Q_UNUSED(a3);
int r = a1 + a2;
return r;
}

int foo::bar(int a1, int a2, int a3)
{
Q_UNUSED(a3);
int r = a1 + a2;
return r;
}

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[Octal]

for example :

bool Foo::removeRow(const QModelIndex &index, const QModelIndex &parent)
{
    Q_UNUSED(parent);
    list.removeAt(index.row());
    return true;
}

bool Foo::removeRow(const QModelIndex &index, const QModelIndex &parent)
{
    Q_UNUSED(parent);
    list.removeAt(index.row());
    return true;
}

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The parent parameter is not used inside the body of the function, which could result in a warning. Q_UNUSED here helps avoiding this warning, explicitly telling the compiler you're not using the concerned parameter.

I didn't really check in the Qt sources, but I guess the Q_UNUSED macro is defined like this :

#define Q_UNUSED(arg) (void)arg;

#define Q_UNUSED(arg) (void)arg;

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[tonnot]

Ok, I understand !!!
( And I have delete what I had written.)
Thanks

[Quasar]

But i can't understand WHY to pass unusued parameter?

[ChrisW67]

You might get unused parameters when you implement a slot to match a signal, or implement a virtual function, but do not need everything that is passed in order to complete your processing. For example, you might want to know when the "textChanged(QString)" signal is emitted from a QLineEdit but not care what the text changed to:

void handleTextChanged(const QString &text) 
{
  Q_UNUSED(text);
  // make obnoxious noise
}

void handleTextChanged(const QString &text) 
{
  Q_UNUSED(text);
  // make obnoxious noise
}

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You can also suppress the warnings like this:

void handleTextChanged(const QString & /*text*/ ) 
{
  // make obnoxious noise
}

void handleTextChanged(const QString & /*text*/ ) 
{
  // make obnoxious noise
}

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