Include <cstddef> for size_t
diff --git a/gen/write.rs b/gen/write.rs
index f0c9077..a4e2eb2 100644
--- a/gen/write.rs
+++ b/gen/write.rs
@@ -91,10 +91,11 @@
     for ty in types {
         match ty {
             Type::Ident(ident) => match Atom::from(ident) {
-                Some(U8) | Some(U16) | Some(U32) | Some(U64) | Some(Usize) | Some(I8)
-                | Some(I16) | Some(I32) | Some(I64) | Some(Isize) => out.include.cstdint = true,
+                Some(U8) | Some(U16) | Some(U32) | Some(U64) | Some(I8) | Some(I16) | Some(I32)
+                | Some(I64) => out.include.cstdint = true,
+                Some(Usize) => out.include.cstddef = true,
                 Some(CxxString) => out.include.string = true,
-                Some(Bool) | Some(F32) | Some(F64) | Some(RustString) | None => {}
+                Some(Bool) | Some(Isize) | Some(F32) | Some(F64) | Some(RustString) | None => {}
             },
             Type::RustBox(_) => out.include.type_traits = true,
             Type::UniquePtr(_) => out.include.memory = true,