interesting problem with QVector

mzimmers

I'm trying to populate a 2D QVector in my app. I'd assumed that QVectors would behave as do STL vectors, but I discovered something odd.

This builds and runs fine:

    vector <vector <int> > v;
    v.resize(3);
    v.at(0).push_back(55);

But this does not:

    QVector <QVector <int> > qv;
    qv.resize(3);
    qv.at(0).push_back(55);

I get a compile-time error:
passing 'const QVector<int>' as 'this' argument discards qualifiers [-fpermissive]

Am I doing something wrong, or is this a limitation of QVectors? Interestingly enough, if I use brackets instead of the ".at()" it also works.

JonB

@mzimmers

http://doc.qt.io/qt-5/qvector.html :

const T &QVector::at(int i) const

T &QVector::operator[](int i)

Correct me if I'm wrong, but doesn't at() deliver read-only?

mzimmers

@JNBarchan hmm, yes, you're absolutely right. That explains the compiler error.

So, I guess I have to use the brackets? Not a problem, though I think that syntax is discouraged by the C++ purists (when using the STL).

T &QVector::operator[](int i)
Returns the item at index position i as a modifiable reference. (emphasis mine)

JonB

@mzimmers

I'm guessing push_back() wants to modify the thing?

And what's the declaration of the STL vector::at()?

http://doc.qt.io/qt-5/qvector.html#details :

QVector uses 0-based indexes, just like C++ arrays. To access the item at a particular index position, you can use operator. On non-const vectors, operator returns a reference to the item that can be used on the left side of an assignment:

if (vector[0] == "Liz")
    vector[0] = "Elizabeth";

For read-only access, an alternative syntax is to use at():

for (int i = 0; i < vector.size(); ++i) {
    if (vector.at(i) == "Alfonso")
        cout << "Found Alfonso at position " << i << endl;
}

at() can be faster than operator, because it never causes a deep copy to occur.