C++ Question

[Ashkan_s]

Below is the implementation of QInputDialog::getText from Qt 4.7

QString QInputDialog::getText(QWidget *parent, const QString &title, const QString &label,
                              QLineEdit::EchoMode mode, const QString &text, bool *ok,
                              Qt::WindowFlags flags)
{
    QInputDialog dialog(parent, flags);
    dialog.setWindowTitle(title);
    dialog.setLabelText(label);
    dialog.setTextValue(text);
    dialog.setTextEchoMode(mode);
 
    int ret = dialog.exec();
    if (ok)
        *ok = !!ret;
    if (ret) {
        return dialog.textValue();
    } else {
        return QString();
    }
}

QString QInputDialog::getText(QWidget *parent, const QString &title, const QString &label,
                              QLineEdit::EchoMode mode, const QString &text, bool *ok,
                              Qt::WindowFlags flags)
{
    QInputDialog dialog(parent, flags);
    dialog.setWindowTitle(title);
    dialog.setLabelText(label);
    dialog.setTextValue(text);
    dialog.setTextEchoMode(mode);

    int ret = dialog.exec();
    if (ok)
        *ok = !!ret;
    if (ret) {
        return dialog.textValue();
    } else {
        return QString();
    }
}

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My question is about line 13. First I thought !! is used to convert int to bool, but soon I realized the conversion takes place automatically. Is it something special about !! ?

[lanz]

I don't know for shure, but I guess it is here to catch reader's attention, that a conversion is taking place.
AFAIK !! doesn't have any special meaning in C++.

EDIT: OK, I did additional research, it seems to be C idiom of clamping result to the range 0,1. So if it was not zero it becomes 1, if it was zero it stays zero, so that variable will be always either 1 or 0.
I think it have no meaning here, just a C leftover and a point of attention.

[Lykurg]

I think it have no meaning here, just a C leftover and a point of attention.

I hope it is not just a leftover! I makes perfect sense to restrict the value to 0 or 1 here, because it is passed to a bool pointer. Since you do not know how a bool is implemented on the different system, it could be a serious issue if ret would be any other number than 0,1.
But you are right, right now it is useless because QDialog::DialogCode has only 1 an 0. But if one decide to add an other dialog code in future, this code is still valid and safe (and more important: there is no danger that it could be overseen to change!)

[wysota]

It's not being passed to a bool pointer but to a bool variable. bool in C++ doesn't take "1" or "0" as a value, it only accepts "true" and "false" (lowercase). If you give it an integer, the compiler converts it to true or false with false being "0" and true being everything else. I admit it is the first time I see a "!!" construct, it looks like a double negation operator that indeed might be preventing some badly written compilers to wreck havok in your program (maybe assuming that either the compiler thinks !0 != 1 (x)or !1 != 0).