Question

I have a variable in the argument of a function that is defined like this:

QVector< QVector<qreal> > *var;

In a certain point of the code I want to do this:

*var[i][j] = some_value.

However this does not compile because

error: no match for ‘operator*’ in ‘*(var + ((long unsigned int)(((long unsigned int)row) * 8ul)))->QVector::operator[] with T = QVector’

How do I properly reference the value so that it is modified? (The idea is that the parameter passed in the function is really modified.)

Answer 1

You need to put *var in parentheses:

(*var)[i][j] = some_value;

Though why are you using a pointer to a QVector? Since QVector is part of Qt’s generic containers family, it uses implicit sharing. This means that if you pass-by-value instead, you’ll only perform a shallow copy, which is to say that you’ll effectively only be passing the thin book-keeping part of the data structure – a pointer to a larger block of data which exists elsewhere.