Add dynamic cast and friends.

CHANGES.rst

@@ -56,6 +56,8 @@ Features added
 * New directive ``use_switch`` (defaults to True) to optionally disable
   the optimization of chained if statement to C switch statements.
 
+* Defines dynamic_cast et al. in `libcpp.cast`.
+
 Bugs fixed
 ----------
 

Cython/Includes/libcpp/__init__.pxd

-# empty file
-
 cdef extern from *:
     ctypedef bint bool

Cython/Includes/libcpp/cast.pxd

+# Defines the standard C++ cast operators.
+#
+# Due to type restrictions, these are only defined for pointer parameters,
+# however that is the only case where they are significantly more interesting
+# than the standard C cast operator which can be written "<T>(expression)" in
+# Cython.
+
+cdef extern from *:
+    cdef T dynamic_cast[T](void *) except +   # nullptr may also indicate failure
+    cdef T static_cast[T](void *)
+    cdef T reinterpret_cast[T](void *)
+    cdef T const_cast[T](void *)