push_back 到循环列表

Question

I have a problem with pushing elements to a QList when iterating over it. Let's see example code below:

typedef struct
{
    int a[2];
} myType;

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;

    QList<myType> list;

    // Create list
    for( int i = 0; i < 1000; i++)
    {
        list << (myType){ i, i };
    }

    int iteration = 0;

    for ( auto &i : list )
    {
        i.a[1] = 5;

        if ( ! (i.a[0] % 10) )
        {
            list.push_back( (myType){ 7, 7 } );
        }
        iteration++;
    }

    w.show();
    return a.exec();
}

When I debug the code I have a segmentation fault ( i got value 0xfeeefeeefeeefeee ):

What is the reason?

Answer 1

The reason for the segmentation fault is that altering the size of the container that you are looping over in a ranged-based for loop is undefined behavior. Thus the loop needs to be rewritten.

Looking at your code, you could still use the ranged-based for , but not alter the list. The following code seems to be equivalent (but not tested):

int numExtra = 0;
for ( auto &i : list )
{
    i.a[1] = 5;

    if ( ! (i.a[0] % 10) )
        ++numExtra;
    iteration++;
}

for (int i = 0; i < numExtra; ++i)
    list.push_back( (myType){ 7, 7 } );

Since the value being added to the list is the same value, the code just counts up the number of eventual push_back calls that will be invoked. After the initial loop is completed, we just call push_back a total of numExtra times.

Answer 2

If you use iterator based for loop, this will still work if you are only appending to the list:

for ( auto it = list.begin(); it != list.end(); ++it )
{
    it->a[1] = 5;

    if ( ! (it->a[0] % 10) )
    {
        list.push_back( myType({ 7, 7 }) );
    }
    iteration++;
}

With a range-based for , list.end() is evaluated only once at the start and so will become invalid when a new item is inserted.