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.