Help me code and understand "foreach"

Anonymous_Banned275

Trying to iterate thru "localAdapters" using "foreach".
I do not fully understand the "foreach" parameters.

    localAdapters = QBluetoothLocalDevice::allDevices();
        int index;
        text = localAdapters.at(index).address().toString();
        // run foreach
        foreach(text,localAdapters)
        {
            qDebug()<< text;
        }

Of course my code is wrong ....
I got way too many errors to analyze ....

/mnt/07b7c3f8-0efb-45ab-8df8-2a468771de1f/PROJECTS/BT_JAN16_/CCC_SOURCE/btchat/chat.cpp:222: error: no viable overloaded '='
chat.cpp:222:9: error: no viable overloaded '='
        foreach(text,localAdapters)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qglobal.h:1126:21: note: expanded from macro 'foreach'
#    define foreach Q_FOREACH
                    ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qglobal.h:1118:19: note: expanded from macro 'Q_FOREACH'
    for (variable = *_container_.i; _container_.control; _container_.control = 0)
         ~~~~~~~~ ^ ~~~~~~~~~~~~~~
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:270:14: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'QChar' for 1st argument
    QString &operator=(QChar c);
             ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:271:14: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'const QString' for 1st argument
    QString &operator=(const QString &) noexcept;
             ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:272:14: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'QLatin1String' for 1st argument
    QString &operator=(QLatin1String latin1);
             ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:274:21: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'QString' for 1st argument
    inline QString &operator=(QString &&other) noexcept
                    ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:841:40: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'const char *' for 1st argument
    inline QT_ASCII_CAST_WARN QString &operator=(const char *ch)
                                       ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:843:40: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'const QByteArray' for 1st argument
    inline QT_ASCII_CAST_WARN QString &operator=(const QByteArray &a)
                                       ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:845:40: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'char' for 1st argument
    inline QT_ASCII_CAST_WARN QString &operator=(char c)
                                       ^
/home/nov25-1/Qt/Nov26/5.15.2/gcc_64/include/QtCore/qstring.h:956:21: note: candidate function not viable: no known conversion from 'const QBluetoothHostInfo' to 'const QString::Null' for 1st argument
    inline QString &operator=(const Null &) { *this = QString(); return *this; }
                    ^

Christian Ehrlicher

See your question 23 days ago: https://forum.qt.io/topic/141587/how-to-implement-for-foreach-iteration and the documentation: https://doc.qt.io/qt-6/foreach-keyword.html

JoeCFD

@AnneRanch said in Help me code and understand "foreach":

QBluetoothLocalDevice

Try

for ( auto const & host_info : QBluetoothLocalDevice::allDevices() ) {
      qDebug()<< host_info.address().toString();
}

Kent-Dorfman

Refer back to the source:

https://en.cppreference.com/w/cpp/language/range-for

Oh... "foreach" not "range based for" so the closest "standard" thing would be std::for_each, and as used above foreach is purely a Qt thing I believe.

TomZ

Qts foreach is very old. Please consider using the new C++ language support for a very similar thing;

for (auto item : container) {
}

You can find more information about that and it will likely be easier to understand when the compiler complains about problems.

Anonymous_Banned275

@TomZ OK, I got the message... pretty much in same line as "new style connect"...
or it is not C++ "standard" ?
I really do not see why QT is inventing any of these macros anyway, obsolete or not. .

I am little curious about the usage of type "auto".
Was it / is in not original "C" and I have seldom see it used.
Or am I mixing it up with "no type" is automatically "int"?

Christian Ehrlicher

@AnneRanch said in Help me code and understand "foreach":

I really do not see why QT is inventing any of these macros anyway, obsolete or not. .

They were invented to use new stuff despite having old compilers / no standard yet supported it but it's useful.

Kent-Dorfman

auto in c++ is automatic type determination based on the right side of the expression

it is handy as a shorthand for long convoluted iterator types, or things that are deeply embedded and where the return type is not obvious upon casual inspection.

auto x="my string"; // yields const char*
auto v=1; // yields int
auto b=56U; yields unsigned
auto i=std::map::begin(); // yields std::map::iterator

get it now?

JonB

auto existed in C from the start too! It just didn't mean the same as now in C++, and was pointless :)

void func()
{
    auto int i;
}