diff --git a/src/index/id_map.rs b/src/index/id_map.rs
index 8831efe..a55fe76 100644
--- a/src/index/id_map.rs
+++ b/src/index/id_map.rs
@@ -67,6 +67,8 @@ use std::mem;
 use std::os::raw::c_int;
 use std::ptr;
 
+use super::IndexImpl;
+
 /// Wrapper for implementing arbitrary ID mapping to an index.
 ///
 /// See the [module level documentation] for more information.
@@ -364,6 +366,24 @@ where
     }
 }
 
+impl IndexImpl {
+    /// Attempt a dynamic cast of the index to one that is [ID-mapped][1].
+    /// 
+    /// [1]: crate::IdMap
+    pub fn into_id_map(self) -> Result<IdMap<IndexImpl>> {
+        unsafe {
+            let new_inner = faiss_IndexIDMap_cast(self.inner_ptr());
+            if new_inner.is_null() {
+                Err(Error::BadCast)
+            } else {
+                mem::forget(self);
+                let index_inner = faiss_IndexIDMap_sub_index(new_inner);
+                Ok(IdMap { inner: new_inner, index_inner, phantom: PhantomData })
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::IdMap;
@@ -471,4 +491,12 @@ mod tests {
         let flat_index: FlatIndexImpl = id_index.try_into_inner().unwrap();
         assert_eq!(flat_index.d(), 4);
     }
+
+    #[test]
+    fn index_impl_to_id_map() {
+        let index = index_factory(4, "IDMap,Flat", MetricType::L2).unwrap();
+        let id_map = index.into_id_map().unwrap();
+
+        assert_eq!(id_map.d(), 4);
+    }
 }