feat: add tell() and error() functions to istream + ostream devices (#…

…747)

The class file_stream_device already has such functions, and now
both istream_device and ostream_device have them, too, making the
device classes more consistent.

Closes #724

include/boost/gil/io/device.hpp

@@ -1,5 +1,6 @@
 //
 // Copyright 2007-2012 Christian Henning, Andreas Pokorny
+// Copyright 2024 Dirk Stolle
 //
 // Distributed under the Boost Software License, Version 1.0
 // See accompanying file LICENSE_1_0.txt or copy at
@@ -409,13 +410,29 @@ class istream_device
                  );
     }
 
+    long int tell()
+    {
+        auto pos = _in.tellg();
+
+        io_error_if( pos == std::istream::pos_type(-1)
+                   , "istream_device: file position error"
+                   );
+
+        return static_cast<long int>(pos);
+    }
+
     void write(const byte_t*, std::size_t)
     {
         io_error( "istream_device: Bad io error." );
     }
 
     void flush() {}
 
+    int error()
+    {
+        return _in.fail();
+    }
+
 private:
 
     std::istream& _in;
@@ -450,6 +467,17 @@ class ostream_device
                   );
     }
 
+    long int tell()
+    {
+        auto pos = _out.tellp();
+
+        io_error_if( pos == std::ostream::pos_type(-1)
+                   , "ostream_device: file position error"
+                   );
+
+        return static_cast<long int>(pos);
+    }
+
     void write( const byte_t* data
               , std::size_t   count )
     {
@@ -509,7 +537,10 @@ class ostream_device
         _out << line;
     }
 
-
+    int error()
+    {
+        return _out.fail();
+    }
 
 private: