Added is_row_major(). Also made sum() run over the matrix in column major

order if that's the memory layout of the argument.

dlib/matrix/matrix_utilities.h

@@ -1415,6 +1415,16 @@ namespace dlib
         return typename matrix_exp<EXP>::matrix_type (m);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    template <typename EXP>
+    constexpr bool is_row_major (
+        const matrix_exp<EXP>&
+    )
+    {
+        return is_same_type<typename EXP::layout_type,row_major_layout>::value;
+    }
+
 // ----------------------------------------------------------------------------------------
 
     template <
@@ -1427,11 +1437,24 @@ namespace dlib
         typedef typename matrix_exp<EXP>::type type;
 
         type val = 0;
-        for (long r = 0; r < m.nr(); ++r)
+        if (is_row_major(m))
+        {
+            for (long r = 0; r < m.nr(); ++r)
+            {
+                for (long c = 0; c < m.nc(); ++c)
+                {
+                    val += m(r,c);
+                }
+            }
+        }
+        else
         {
             for (long c = 0; c < m.nc(); ++c)
             {
-                val += m(r,c);
+                for (long r = 0; r < m.nr(); ++r)
+                {
+                    val += m(r,c);
+                }
             }
         }
         return val;

dlib/matrix/matrix_utilities_abstract.h

@@ -16,6 +16,17 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 //                                   Simple matrix utilities 
 // ----------------------------------------------------------------------------------------
+// ----------------------------------------------------------------------------------------
+
+    template <typename EXP>
+    constexpr bool is_row_major (
+        const matrix_exp<EXP>&
+    );
+    /*!
+        ensures
+            - returns true if and only if the given matrix expression uses the row_major_layout.
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     const matrix_exp diag (