C++ - Is it possible to edit the data inside an ifstream?

I'm trying to use Boost-GIL to load an image. The read_image function takes ifstream as a parameter, but I will already have the image as a base64 encoded string. Is it possible for me to add the decoded image into an ifstream manually, so that I wont have to write and read from disk to get the image loaded into GIL? Another possibility could be to somehow use a string stream to add the data and cast that to an ifstream, though I haven't had luck trying that.

Solution

Boost.GIL's read_image function you mentioned seems to support istream interface. If you have an array, you can make use of boost.iostreams to represent the array as a stream.

Here is a made-up example since you do not provide a code snippet.

#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/gil.hpp>
#include <boost/gil/io/read_image.hpp>
#include <boost/gil/extension/io/tiff.hpp>

int main(int argc, char* argv[]) {
  const char* data = "decoded-data";
  boost::iostreams::stream<boost::iostreams::array_source> in{data, std::strlen(data)};
  boost::gil::rgb8_image_t img;
  read_image(in, img, boost::gil::tiff_tag());

  return 0;
}

Alternatively, you could use std::stringstream to store your decoded image and give that to the read_image function. Something along the lines of:

#include <boost/archive/iterators/binary_from_base64.hpp>
#include <boost/archive/iterators/insert_linebreaks.hpp>
#include <boost/archive/iterators/transform_width.hpp>
#include <boost/archive/iterators/ostream_iterator.hpp>
#include <boost/gil.hpp>
#include <boost/gil/io/read_image.hpp>
#include <boost/gil/extension/io/tiff.hpp>

#include <sstream>

using base64_decode_iterator = transform_width<binary_from_base64<const char*>, 8, 6>;

int main(int argc, char* argv[]) {
  const char* data = "base64-data";
  std::stringstream ss;

  std::copy(
    base64_decode_iterator{data},
    base64_decode_iterator{data + std::strlen(data)},
    std::ostream_iterator<char>{ss}
  );
  boost::gil::rgb8_image_t img;
  read_image(ss, img, boost::gil::tiff_tag());

  return 0;
}