How to convert QByteArray to std::istream or std::ifstream? - c++

Question

I want to create istream from QByteArray at runtime, without saving a physical file in memory of QByteArray.
I found that there are many ways to do the opposite conversion, i.e. istream to QByteArray, but not this one.
How to accomplish that?

Answer 1

To read via std::istringstream from QByteArray seems quite easy:
testQByteArray-istream.cc:
#include <iostream>
#include <sstream>
#include <QtCore>
int main()
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
// make a QByteArray
QByteArray data("Hello Qt World.");
// convert to std::string
std::istringstream in(data.toStdString());
// read from istringstream
for (;;) {
std::string buffer;
if (!std::getline(in, buffer)) break;
std::cout << "Got: '" << buffer << "'\n";
}
// done
return 0;
}
testQByteArray-istream.pro:
SOURCES = testQByteArray-istream.cc
QT = core
Compiled and tested on cygwin64:
$ qmake-qt5 testQByteArray-istream.pro
$ make
$ ./testQByteArray-istream
Qt Version: 5.9.4
Got: 'Hello Qt World.'
$
Done. Stop, wait!
without saving a physical file in memory
I'm not quite sure how to read this. Probably, it means
without copying data saved in QByteArray
I see only two solutions:
Use a QDataStream instead of std::stream. According to doc. QDataStream::QDataStream(const QByteArray &a)
Constructs a read-only data stream that operates on byte array a.
This sounds very promising that data is not copied.
DIY. Make a class derived from std::stream which may read from a QByteArray without copying.
Concerning 2. option, I found Dietmar Kühl's answer to SO: Creating an input stream from constant memory. Applying this to the above sample, it would look like this:
#include <iostream>
#include <QtCore>
// borrowed from https://stackoverflow.com/a/13059195/7478597
struct membuf: std::streambuf {
membuf(char const* base, size_t size) {
char* p(const_cast<char*>(base));
this->setg(p, p, p + size);
}
};
struct imemstream: virtual membuf, std::istream {
imemstream(char const *base, size_t size):
membuf(base, size),
std::istream(static_cast<std::streambuf*>(this)) {
}
};
int main()
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
// make a QByteArray
QByteArray data("Hello Qt World.");
imemstream in(data.data(), (size_t)data.size());
// read from istringstream
for (;;) {
std::string buffer;
if (!std::getline(in, buffer)) break;
std::cout << "Got: '" << buffer << "'\n";
}
// done
return 0;
}
Compiled and tested again on cygwin64:
$ qmake-qt5 testQByteArray-istream.pro
$ make
$ ./testQByteArray-istream
Qt Version: 5.9.4
Got: 'Hello Qt World.'
$