Made the rectangle have an operator< rather than overloading std::less since

the former works with a wider range of tools in the STL.

dlib/geometry/rectangle.h

@@ -260,6 +260,19 @@ namespace dlib
             return !(*this == rect);
         }
 
+        inline bool operator< (const dlib::rectangle& b) const
+        { 
+            if      (left() < b.left()) return true;
+            else if (left() > b.left()) return false;
+            else if (top() < b.top()) return true;
+            else if (top() > b.top()) return false;
+            else if (right() < b.right()) return true;
+            else if (right() > b.right()) return false;
+            else if (bottom() < b.bottom()) return true;
+            else if (bottom() > b.bottom()) return false;
+            else                    return false;
+        }
+
     private:
         long l;
         long t;
@@ -753,29 +766,6 @@ namespace dlib
 
 }
 
-namespace std
-{
-    /*!
-        Define std::less<rectangle> so that you can use rectangles in the associative containers.
-    !*/
-    template<>
-    struct less<dlib::rectangle> : public binary_function<dlib::rectangle ,dlib::rectangle,bool>
-    {
-        inline bool operator() (const dlib::rectangle& a, const dlib::rectangle& b) const
-        { 
-            if      (a.left() < b.left()) return true;
-            else if (a.left() > b.left()) return false;
-            else if (a.top() < b.top()) return true;
-            else if (a.top() > b.top()) return false;
-            else if (a.right() < b.right()) return true;
-            else if (a.right() > b.right()) return false;
-            else if (a.bottom() < b.bottom()) return true;
-            else if (a.bottom() > b.bottom()) return false;
-            else                    return false;
-        }
-    };
-}
-
 #endif // DLIB_RECTANGLe_
 
 

dlib/geometry/rectangle_abstract.h

@@ -363,6 +363,16 @@ namespace dlib
             ensures
                 - returns !(*this == rect)
         !*/
+
+        bool operator< (
+            const dlib::rectangle& a,
+            const dlib::rectangle& b
+        ) const;
+        /*!
+            ensures
+                - Defines a total ordering over rectangles so they can be used in
+                  associative containers.
+        !*/
     };
 
 // ----------------------------------------------------------------------------------------
@@ -789,28 +799,5 @@ namespace dlib
 
 }
 
-namespace std
-{
-    /*!
-        Define std::less<rectangle> so that you can use rectangles in the associative containers.
-    !*/
-    template<>
-    struct less<dlib::rectangle> : public binary_function<dlib::rectangle,dlib::rectangle,bool>
-    {
-        inline bool operator() (const dlib::rectangle& a, const dlib::rectangle& b) const
-        { 
-            if      (a.left() < b.left()) return true;
-            else if (a.left() > b.left()) return false;
-            else if (a.top() < b.top()) return true;
-            else if (a.top() > b.top()) return false;
-            else if (a.right() < b.right()) return true;
-            else if (a.right() > b.right()) return false;
-            else if (a.bottom() < b.bottom()) return true;
-            else if (a.bottom() > b.bottom()) return false;
-            else                    return false;
-        }
-    };
-}
-
 #endif // DLIB_RECTANGLe_ABSTRACT_