Bad multithreading

[Mikee]

Hi.
It works well

QVector<double> Perebor; 
        for (double i=5;i<100;i=i+3)
        {
            Perebor.append(i);
        }
        qDebug()<<Perebor;
        QFuture<void> Perebor2 = QtConcurrent::map(Perebor,[&w](const double& d){ w.StrategyCod(d); });

It works badly

w.MultiThreading=true; 
  //-----------------------------------------------------------------------
   if (w.MultiThreading==true)
   {
       QVector<double> Perebor; 
       for (double i=5;i<100;i=i+3)
       {
           Perebor.append(i);
       }
       //qDebug()<<Perebor;
       QFuture<void> Perebor2 = QtConcurrent::map(Perebor,[&w](const double& d){ w.StrategyCod(d); }); 
   }


void MainWindow::StrategyCod(double Parametr)
{
   double i=Parametr;
   qDebug()<<i;
}

return: 2.122e-314,0,0,0,0,0,0,0,0,1.16149e+260,0,1.15973e+260,4.94066e-324,0,0,0,0,0,0,0,0,0,0,3.31024e-322,0,0,0,0,0,0,0,0
Why does not this code?

[mrjj]

Hi
Where is the "works well" located ?
in main.cpp ?

i think in "works badly" , you variable runs out of scope and get deleted

if (w.MultiThreading==true) {
QFuture<void> Perebor2 = QtConcurrent::map(Perebor,[&w](const double& d){
w.StrategyCod(d); });
} ///Perebor dies here.
....

[koahnig]

@Mikee said in Bad multithreading:

if (w.MultiThreading==true)
{
QVector<double> Perebor;
for (double i=5;i<100;i=i+3)
{
Perebor.append(i);
}
//qDebug()<<Perebor;
QFuture<void> Perebor2 = QtConcurrent::map(Perebor,[&w](const double& d){ w.StrategyCod(d); });
}

QVector<double> Perebor; will be destroyed after directly after you start QtConcurrent::map(Perebor, ...

You do not show what happens with the vector. Possibly it stays, because you do not loose focus.

[edit: koahnig] Already mentioned by @mrjj

[Mikee]

Why destroyed QVector<double> Perebor; ?
Code in main.cpp

[jsulm]

@Mikee said in Bad multithreading:

Why destroyed QVector<double> Perebor; ?

Because it is in a scope:

if (w.MultiThreading==true)
{
QVector<double> Perebor; // Perebor does only exist in this if block
for (double i=5;i<100;i=i+3)
{

You should read about C/C++ scopes, it is not related to Qt.

[Mikee]

thank you

[koahnig]

@Mikee

To add to @jsulm it is not a Qt related problem. It is a multi-threading issue here.

void foo() 
{
     if ( true) 
    {
          int i = 10;
    }
    int j = i;
}

This will give you a compiling error and it is very obvious with scopes.

void assign_sometimes_later ( int & i )
{
     int j = i;
}
void foo() 
{
     if ( true) 
    {
          int i = 10;
          assign_sometimes_later (i);
    }
}

This will compile, but the compiler cannot see when the routine is being executed. The problem is the reference in the calling list of the routine.

[JonB]

@koahnig
For the record, I don't see what you are saying would go wrong with your 2nd example code --- seems to me it would work OK?

[koahnig]

@JNBarchan

The second is basically what the compiler sees. It is completely valid, but when you assume the routine is executed sometimes later through an imaginary multi-threading call, it would fail in execution.

Therefore, this part is given to highlight the misery happened in the original example and why it was not detected.