Fixed compiler warnings in gcc 8

dlib/array/array_kernel.h

@@ -250,7 +250,7 @@ namespace dlib
             for (size_t i = 0; i < item.size(); ++i)
                 serialize(item[i],out);
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array"); 
         }
@@ -275,7 +275,7 @@ namespace dlib
             for (size_t i = 0; i < size; ++i)
                 deserialize(item[i],in);
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array"); 

dlib/array2d/array2d_kernel.h

@@ -394,7 +394,7 @@ namespace dlib
                 serialize(item.element(),out);
             item.reset();
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -432,7 +432,7 @@ namespace dlib
                 deserialize(item.element(),in); 
             item.reset();
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 

dlib/array2d/serialize_pixel_overloads.h

@@ -43,7 +43,7 @@ namespace dlib
             if (item.size() != 0)
                 out.write((char*)&item[0][0], sizeof(rgb_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -81,7 +81,7 @@ namespace dlib
             if (item.size() != 0)
                 in.read((char*)&item[0][0], sizeof(rgb_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 
@@ -112,7 +112,7 @@ namespace dlib
             if (item.size() != 0)
                 out.write((char*)&item[0][0], sizeof(bgr_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -151,7 +151,7 @@ namespace dlib
             if (item.size() != 0)
                 in.read((char*)&item[0][0], sizeof(bgr_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 
@@ -182,7 +182,7 @@ namespace dlib
             if (item.size() != 0)
                 out.write((char*)&item[0][0], sizeof(hsi_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -221,7 +221,7 @@ namespace dlib
             if (item.size() != 0)
                 in.read((char*)&item[0][0], sizeof(hsi_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 
@@ -252,7 +252,7 @@ namespace dlib
             if (item.size() != 0)
                 out.write((char*)&item[0][0], sizeof(rgb_alpha_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -291,7 +291,7 @@ namespace dlib
             if (item.size() != 0)
                 in.read((char*)&item[0][0], sizeof(rgb_alpha_pixel)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 
@@ -320,7 +320,7 @@ namespace dlib
             if (item.size() != 0)
                 out.write((char*)&item[0][0], sizeof(unsigned char)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type array2d"); 
         }
@@ -356,7 +356,7 @@ namespace dlib
             if (item.size() != 0)
                 in.read((char*)&item[0][0], sizeof(unsigned char)*item.size());
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type array2d"); 

dlib/binary_search_tree/binary_search_tree_kernel_1.h

@@ -534,7 +534,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type binary_search_tree_kernel_1"); 

dlib/binary_search_tree/binary_search_tree_kernel_2.h

@@ -482,7 +482,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type binary_search_tree_kernel_2"); 

dlib/gui_widgets/fonts.cpp

@@ -148,7 +148,7 @@ namespace dlib
                 serialize(item.points[i].y,out);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             throw serialization_error(e.info + "\n   while serializing object of type letter"); 
         }
@@ -178,7 +178,7 @@ namespace dlib
                 deserialize(item.points[i].y,in);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.w = 0;
             item.count = 0;

dlib/hash_map/hash_map_kernel_1.h

@@ -169,7 +169,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type hash_map_kernel_1"); 

dlib/hash_set/hash_set_kernel_1.h

@@ -151,7 +151,7 @@ namespace dlib
                 item.add(temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type hash_set_kernel_1"); 

dlib/hash_table/hash_table_kernel_1.h

@@ -226,7 +226,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type hash_table_kernel_1"); 

dlib/hash_table/hash_table_kernel_2.h

@@ -194,7 +194,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type hash_table_kernel_2"); 

dlib/map/map_kernel_1.h

@@ -160,7 +160,7 @@ namespace dlib
                 item.add(d,r);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type map_kernel_1"); 

dlib/optimization/optimization_bobyqa.h

@@ -289,7 +289,7 @@ namespace dlib
             doublereal ratio = 0, dnorm = 0, vquad = 0, pqold = 0, tenth = 0;
             integer itest;
             doublereal sumpq, scaden;
-            doublereal errbig, cauchy, fracsq, biglsq, densav;
+            doublereal errbig, cauchy=0, fracsq, biglsq, densav;
             doublereal bdtest;
             doublereal crvmin, frhosq;
             doublereal distsq;

dlib/queue/queue_kernel_1.h

@@ -184,7 +184,7 @@ namespace dlib
                 item.enqueue(temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type queue_kernel_1"); 

dlib/queue/queue_kernel_2.h

@@ -202,7 +202,7 @@ namespace dlib
                 item.enqueue(temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type queue_kernel_2"); 

dlib/sequence/sequence_kernel_1.h

@@ -379,7 +379,7 @@ namespace dlib
                 item.add(i,temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type sequence_kernel_1"); 

dlib/sequence/sequence_kernel_2.h

@@ -199,7 +199,7 @@ namespace dlib
                 item.add(i,temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type sequence_kernel_2"); 

dlib/set/set_kernel_1.h

@@ -146,7 +146,7 @@ namespace dlib
                 item.add(temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type set_kernel_1"); 

dlib/sliding_buffer/sliding_buffer_kernel_1.h

@@ -215,7 +215,7 @@ namespace dlib
                     deserialize(item[i],in);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type sliding_buffer_kernel_1"); 

dlib/stack/stack_kernel_1.h

@@ -183,7 +183,7 @@ namespace dlib
                 item.push(temp);
             }
         }
-        catch (serialization_error e)
+        catch (serialization_error& e)
         { 
             item.clear();
             throw serialization_error(e.info + "\n   while deserializing object of type stack_kernel_1"); 

dlib/static_set/static_set_kernel_1.h

@@ -61,7 +61,7 @@ namespace dlib
                     deserialize(item.d[i],in);
                 }
             }
-            catch (serialization_error e)
+            catch (serialization_error& e)
             { 
                 item.set_size = 0;
                 if (item.d)