Made dlib::file implicitly convertible to std::string.

dlib/dir_nav/dir_nav_kernel_1.h

@@ -101,6 +101,9 @@ namespace dlib
         inline  const std::string& full_name (
         ) const { return state.full_name; }
 
+        operator std::string (
+        ) const { return full_name(); }
+
         inline uint64 size (
         ) const { return state.file_size; }
 

dlib/dir_nav/dir_nav_kernel_2.h

@@ -109,6 +109,9 @@ namespace dlib
         inline uint64 size (
         ) const { return state.file_size; }
 
+        operator std::string (
+        ) const { return full_name(); }
+
         bool operator == (
             const file& rhs
         ) const;

dlib/dir_nav/dir_nav_kernel_abstract.h

@@ -139,6 +139,14 @@ namespace dlib
                 - returns the size of this file in bytes.
         !*/
 
+        operator std::string (
+        ) const; 
+        /*!
+            ensures
+                - returns full_name()
+                  (i.e. provides an implicit conversion to string from dlib::file)
+        !*/
+
         file& operator= (
             const file& rhs
         );