Made deserialize print more informative error messages.

dlib/serialize.h

@@ -1498,22 +1498,87 @@ namespace dlib
     public:
         explicit proxy_deserialize (
             const std::string& filename
-        ) 
+        )  : filename(filename)
         {
             fin.reset(new std::ifstream(filename.c_str(), std::ios::binary));
             if (!(*fin))
                 throw serialization_error("Unable to open " + filename + " for reading.");
+
+            // read the file header into a buffer and then seek back to the start of the
+            // file.
+            fin->read(file_header,4);
+            fin->clear();
+            fin->seekg(0);
         }
 
         template <typename T>
         inline proxy_deserialize& operator>>(T& item)
         {
+            try
+            {
+                if (fin->peek() == EOF)
+                    throw serialization_error("No more objects were in the file!");
                 deserialize(item, *fin);
+            }
+            catch (serialization_error& e)
+            {
+                std::string suffix;
+                if (looks_like_a_compressed_file())
+                    suffix = "\n *** THIS LOOKS LIKE A COMPRESSED FILE.  DID YOU FORGET TO DECOMPRESS IT? *** \n";
+
+                if (objects_read == 0)
+                {
+                    throw serialization_error("An error occurred while trying to read the first" 
+                        " object from the file " + filename + ".\nERROR: " + e.info + "\n" + suffix);
+                }
+                else if (objects_read == 1)
+                {
+                    throw serialization_error("An error occurred while trying to read the second" 
+                        " object from the file " + filename +
+                        ".\nERROR: " + e.info + "\n" + suffix);
+                }
+                else if (objects_read == 2)
+                {
+                    throw serialization_error("An error occurred while trying to read the third" 
+                        " object from the file " + filename +
+                        ".\nERROR: " + e.info + "\n" + suffix);
+                }
+                else 
+                {
+                    throw serialization_error("An error occurred while trying to read the " +
+                        std::to_string(objects_read+1) + "th object from the file " + filename +
+                        ".\nERROR: " + e.info + "\n" + suffix);
+                }
+            }
+            ++objects_read;
             return *this;
         }
 
     private:
+        int objects_read = 0;
+        std::string filename;
         std::shared_ptr<std::ifstream> fin;
+
+        // We don't need to look at the file header.  However, it's here because people
+        // keep posting questions to the dlib forums asking why they get file load errors.
+        // Then it turns out that the problem is they have a compressed file that NEEDS TO
+        // BE DECOMPRESSED by bzip2 or whatever and the reason they are getting
+        // deserialization errors is because they didn't decompress the file.  So we are
+        // going to check if this file looks like a compressed file and if so then emit an
+        // error message telling them to unzip the file. :(
+        char file_header[4] = {0,0,0,0};
+
+        bool looks_like_a_compressed_file(
+        ) const 
+        {
+            if (file_header[0] == 'B' && file_header[1] == 'Z' && file_header[2] == 'h' &&
+                ('0' <= file_header[3] && file_header[3] <= '9') )
+            {
+                return true;
+            }
+
+            return false;
+        }
     };
 
     inline proxy_serialize serialize(const std::string& filename)