Using QVector<> as std::vector<> without duplicating the data

Yosemite

Howdy!

I'm working with a C++ library that takes a pointer to std::vector<double> and iterates over it, potentially reading millions of values. However, I also plot that data using QCustomPlot, which handles huge data sets quite nicely.

The lib wants a std::vector<double>, QCustomPlot wants a QVector<double>. Is there a way to keep them both happy without duplicating the data into two vectors? The copy is fast, it just feels like a waste of memory.

dheerendra

Nothing exist like that. If contents are address variable it would have saved something

mrjj

Hi
I assume you are using
http://doc.qt.io/qt-5/qvector.html#fromStdVector
( its still copy. just as in contrast to using a loop)

SGaist

Hi,

fromStdVector copies the data.

From a quick look at QCustomPlot (and really just a look), one solution that doesn't seem unreasonable is to modify the sources from QCustomPlot to use std::vector in place of QVector to handle your data.

Yosemite

Got it, thanks for these answers. Modifying one library or the other seems like a good solution.

kshegunov

Well, there's also an alternative to modifying the QCustomPlot sources, but it comes with a disclaimer. Consider this specialization:

class MyType; //< Whatever your type is

template <>
static inline QVector<MyType> QVector<MyType>::fromStdVector(const std::vector<MyType> & vector)
{
    QVector<MyType> tmp;
    tmp.d = QTypedArrayData<MyType>::fromRawData(vector.data(), vector.size(), Unsharable);
    return tmp;
}

Here goes the disclaimer:

I haven't tested that, it's hacking into Qt's internals, so use at your own discretion.
You have to ensure the std::vector instance will not go out of scope while any one QVector instance holds reference to the data