'Localdevices' with deduced type 'auto &' requires an initializer

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I am still not comfortable with using the NEW C++ " FOR " syntax.
My test code is to retrieve "name". I just want the "for" loop to function, I will add more retrieval info later.

I should not need the "index". Added for test purposes.
It did not work without index anyway.

Here is my latest attempt and the error .
I do not understand the error.

C++

<pre>    QBluetoothLocalDevice localDevices;
 //   for(auto &localDeviceName:localDevice)
    int index;
    for(auto &localDevices.allDevices(index):localDevices)
    {
        text = " TEST ";
        ui->chat_16->append(text);
    }

    QString localDeviceName = localDevices.allDevices().at(0).name();

..and the error

<pre>/mnt/07b7c3f8-0efb-45ab-8df8-2a468771de1f/PROJECTS/JAN21/BT_JAN24_BT_SPP_/CCC_SOURCE/Bluetoooth_SPP_Connect/bt_spp_connect_mainwindow.cpp:29: error: declaration of variable 'localDevices' with deduced type 'auto &' requires an initializer
bt_spp_connect_mainwindow.cpp:29:15: error: declaration of variable 'localDevices' with deduced type 'auto &' requires an initializer
    for(auto &localDevices.allDevices(index):localDevices)
              ^

What I have tried:

QString localDeviceName

Added "normal" retrieval of "name"

QString localDeviceName = localDevices.allDevices().at(0).name();

Posted 24-Jan-23 7:45am

Member 14968771

Updated 24-Jan-23 11:05am

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3 solutions

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k5054 · Answer 1 · 2023-01-24T08:01:00

Solution 1

C++ ranged for loops look like

C++

for( <variable decl> : <container> )

Where <variable decl=""> is a normal variable declaration. So for example

C++

std:vector  v<int>:
// things get added to vector here

for(auto i : v) {
   // i will have type int here
}

so probably what you want is

C++

for( auto& deviceName : localDevices ) {
    // work with device name 
}

This assumes that whatever type localDevices is it has the required member functions so that the ranged for loop can be executed, and that the type of an individual item can be deduced.

Posted 24-Jan-23 8:01am

k5054

Updated 24-Jan-23 8:02am

v2

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Member 14968771 24-Jan-23 16:04pm

SOrry, but I still get same error.
OK , I underrated the "container"m but how can I "convert" this

localDevices.allDevices().at(0).name();

to correct variable ?

k5054 24-Jan-23 16:32pm

It looks like localDevices.allDevices() returns a container of some sort. So, correcting myself, maybe you want

auto allDevices = localDevices.allDevices(); // gets a list of devices
for( auto& device : allDevices ) {
    // device has the same type as the contained object of allDevices
    auto& name = device.name();
    std::cout << name << '\n';
}

If you don't need to modify the device object within the for loop, then you should declare device as const auto& device

CPallini 24-Jan-23 16:28pm

5.

CPallini · Answer 2 · 2023-01-24T10:40:00

Solution 2

The range for loop needs a range-expression (e.g. a container).
The localDevices object is an instance of the BluetoothLocalDevice class. The latter is NOT a container, however it does provide the allDevices method, returning a list.
Hence, the proper way to write a range for loop in your scenario is:

C++

for (auto & device : localDevices.allDevices() )
{
  // access device fields here, e.g. device.name()
}

Posted 24-Jan-23 10:40am

CPallini

Comments

k5054 24-Jan-23 17:21pm

My 5. Your solution was more correct than mine.

Member 14968771 24-Jan-23 21:56pm

I do not know " who is on first" , but I think YOU wrote that solution.
Thanks.

Member 14968771 · Answer 3 · 2023-01-24T11:05:00

I am sorry , this has been answered elsewhere.

But this discussion has been very helpful too.

So the problem was how the "container" was defined...

C++

auto localAdapters = QBluetoothLocalDevice::allDevices();
 ui->chat_10->clear();
 qDebug()<< " localAdapters  count " << QString::number(localAdapters.count());
 for (const auto & device : localAdapters)
 {
   qDebug()<< " local adapter address" << device.address().toString();
   //ui->chat_10->append( device.address().toString());
   qDebug() << " local adapater name " << device.name();
   //ui->chat_10->append( device.name());
 }

'Localdevices' with deduced type 'auto &' requires an initializer

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'Localdevices' with deduced type 'auto &' requires an initializer

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