stream device "tell" method absent!

[fsmoke]

I am trying write custom io extension - and i need get size to the end of device(stream or file no matter) I already wrote all classes and tags for it, but it seems i found bug

i wrote following code to calc size

reader(Device const & device, image_read_settings<my_tag> const & settings) :
            _io_dev(device), _settings(settings)
        {
            //read image 
            auto pos = _io_dev.tell();
            _io_dev.seek(0, SEEK_END);
            auto size = _io_dev.tell() - pos;
            _io_dev.seek(pos, SEEK_SET);
        }

following code compiles just fine

using texture_res_image = boost::gil::any_image<
    gil::gray8_image_t,
    gil::rgb8_image_t,
    gil::rgba8_image_t
>;
int main()
{
    texture_res_image img;
    gil::read_image("anton.myext", img, gil::my_tag());
}

but i'ev got error when use istream

using texture_res_image = boost::gil::any_image<
    gil::gray8_image_t,
    gil::rgb8_image_t,
    gil::rgba8_image_t
>;
int main()
{
    std::ifstream is("anton.myext", std::ios::in | std::ios::binary);
    texture_res_image img;
    gil::read_image(is, img, boost::gil::my_tag());
}

error C2039: 'tell': is not a member of 'boost::gil::detail::istream_device'

i investigated boost_1_80_0\boost\gil\io\device.hpp - and it's true there is no tell method in istream_device class :( but in file_stream_device it's present.

• Compiler version: MSVC 17.4.4
• Build settings: default
• Boost 1.80

[fsmoke]

For now i wrote very ugly workaround - it's looks like very very bad!

        namespace detail
    {
                //FIXME see BUG https://github.com/boostorg/gil/issues/724
        template<>
        class istream_device<jpeg_tag> : public istream_device<void>
        {
        public:
            istream_device(std::istream & in) : istream_device<void>(in), _in(in) {}

            long int tell()
            {
                auto pos = _in.tellg();

                io_error_if(pos == -1L, "istream_device: file position error");

                return (long int)pos;
            }
        private:

            std::istream & _in;
        };
      }

so i'll waiting for your realization

as addition - specialization of devices very unclear - cos it's inside boost::gil::detail - i think it must be as public api

[striezel]

Yes, the tell() method is indeed missing in boost::gil::detail::istream_device. And in boost::gil::detail::ostream_device. And so is the error() method, too. I see that this can be problematic in cases like yours.

To fix this, I opened a PR that implements both of those methods for both of the mentioned classes, #747.