Added seekg() and made vectorstream more robust.

dlib/vectorstream/vectorstream.h

@@ -29,6 +29,12 @@ namespace dlib
                 buffer(buffer_) 
             {}
 
+
+            void seekg(size_type pos)
+            {
+                read_pos = pos;
+            }
+
             // ------------------------ OUTPUT FUNCTIONS ------------------------
 
             int_type overflow ( int_type c)
@@ -85,15 +91,15 @@ namespace dlib
                 std::streamsize n
             )
             { 
-                const size_type num = std::min<size_type>(n, buffer.size()-read_pos);
-                if (num > 0)
+                if (read_pos < buffer.size())
                 {
+                    const size_type num = std::min<size_type>(n, buffer.size()-read_pos);
                     std::memcpy(s, &buffer[read_pos], num);
                     read_pos += num;
-                }
-
                     return num;
                 }
+                return 0;
+            }
 
         };
 
@@ -106,6 +112,14 @@ namespace dlib
             buf(buffer)
         {}
 
+        std::istream& seekg (
+            std::streampos pos
+        )
+        {
+            buf.seekg(pos);
+            return *this;
+        }
+
     private:
         vector_streambuf buf;
     };

dlib/vectorstream/vectorstream_abstract.h

@@ -43,6 +43,17 @@ namespace dlib
                   immediately show up in the buffer.
         !*/
 
+        std::istream& seekg (
+            std::streampos pos
+        );
+        /*!
+            ensures
+                - The next read from this object will read from the position buffer[pos],
+                  where buffer is the std::vector given to this object's constructor.  Note
+                  that if pos >= buffer.size() then the next read will simply return EOF.
+                - returns *this
+        !*/
+
     };
 }