Defining a two dimensional QVector class (Q2DDoubleVector)

[ufechner]

Hello,

I am trying to build a class, that can hold a resizable two
dimensional array of double values.

I need this, to hold a matrix of 1e6 rows and 5 or more colums of real-time data.
When the data aquisition is running I want to avoid to allocate memory, because that can be slow.

But I want to be able to change the exact number of rows and columns in a configuration file (and
in a menu option, when the aquisition is not running), and therefore I need a dynamic array.

The following code crashes:

class Q2DDoubleVector : public QVector< QVector<double> > // Overloading Class QVector to allow for 2 dimensional vectors.
{
public:
    // constructor
    Q2DDoubleVector() : QVector< QVector<double> >(){}
    // methods
    // matrix resize
    void mresize(int rows, int columns);
    // destructor
    virtual ~Q2DDoubleVector() {}
};
 
void Q2DDoubleVector::mresize(int rows, int columns)
{
    this->resize(rows);
    for(int r=0; r<rows; r++) {
        this[r].resize(columns);
    }
}
 
Q2DDoubleVector matrix;
matrix.mresize(ROWS, COLS);

class Q2DDoubleVector : public QVector< QVector<double> > // Overloading Class QVector to allow for 2 dimensional vectors.
{
public:
    // constructor
    Q2DDoubleVector() : QVector< QVector<double> >(){}
    // methods
    // matrix resize
    void mresize(int rows, int columns);
    // destructor
    virtual ~Q2DDoubleVector() {}
};

void Q2DDoubleVector::mresize(int rows, int columns)
{
    this->resize(rows);
    for(int r=0; r<rows; r++) {
        this[r].resize(columns);
    }
}

Q2DDoubleVector matrix;
matrix.mresize(ROWS, COLS);

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If I do it in a very similar way, it doesn't crash:

QVector<QVector <double> >  matrix;
 
matrix.resize(ROWS);
for(int r=0; r < ROWS; r++) {
     matrix[r].resize(COLS);
}

QVector<QVector <double> >  matrix;

matrix.resize(ROWS);
for(int r=0; r < ROWS; r++) {
     matrix[r].resize(COLS);
}

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Why does the first approach not work?

I don't want to implement the resize loop for each 2D matrix, that I use again.

You can download both projects (the example that works (ArrayTest) and
the example that doesn't work (ArrayTest2) from the following URL:
http://db.tt/IUWpBCnY

Any hints welcome.

Actually, I was using Delphi in the past, and with Delphi this was very easy:

var Matrix: array of array of Double;
 begin
 SetLength(Matrix, 10, 20)
 end;

var Matrix: array of array of Double;
 begin
 SetLength(Matrix, 10, 20)
 end;

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Best regards:

Uwe Fechner

I am using Ubuntu 11.04 32 bit and QT 4.7.2 .