How to convert QByteArray to std::istream or std::ifstream?

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:

1. 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.

2. 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.'

$