访问 class QList 指针的成员

Question

I have this project where I use a QList to store instances of a class called RespirationTest . Accessing RespirationTest members is easy when using dynamic memory, but I am having trouble since I switched my QList for a pointer

//class declaration

class RespirationTest: public QListWidgetItem
{

public:
    RespirationTest();
    ~RespirationTest();

public slots:
double GetAmplitude() { return _amplitude; }

private:
double _amplitude;
}

Problem is here, when I try to access members of my QList objects (used to work when respTest was a QList )

//MainWindow

QList<RespirationTest> * respTests;

respTests = new QList<RespirationTest>;

void MainWindow::on_load_button_clicked()
{
RespirationTest *currTest = new RespirationTest;
respTests->push_back(*currTest);
qDebug() << "ampl" << i << ": " << respTests[i].GetAmplitude(); // no member named 'GetAmplitude'
}

Answer 1

Quick fix: Use .at(int idx) instead:

qDebug() << "ampl" << i << ": " << respTests->at(i).GetAmplitude();

The problem with using operator[] is that you access the memory of the pointer instead of accessing the underlying QList :

respTests[i] // returns the QList<> instance at `i` instead
             // of a `RespirationTest` object

So if you want to keep using [] , you will need to further access the i th element by using [] or .at() :

qDebug() << "ampl" << i << ": " << respTests[0].at(i).GetAmplitude();

I highly recommend using .at() if you really have to do this. Otherwise just don't use pointers at all as it's over-complicating the problem and may leak memory. Furthermore, avoid QList<> , use QVector instead.

Answer 2

since respTests is a pointer you need to use -> instead of .

qDebug() << "object at " << i << ": " << respTests->at(i).GetAmplitude();