# Invalid object size Detects invalid pointer casts due to differences in the sizes of types. ## Overview Use this check to detect pointer casts when the size of the source type is less than the size of the destination type. Using the result of such a cast to access out-of-bounds data has undefined behavior. Available in Xcode 9 and later. ### Downcast from type with insufficient space in C++ In the following example, the cast from `Base *` to `Derived *` is suspect because `Base` isn’t large enough to contain an instance of `Derived`: ```occ struct Base { int pad1; }; struct Derived : Base { int pad2; }; Derived *getDerived() { return static_cast(new Base); // Error: invalid downcast } ``` The optimizer may remove an expression, such as `getDerived()->pad2`, because `getDerived()` returns a pointer to an object that isn’t large enough to contain a `pad2` field. > Note: This UBSan check may not trigger at low optimization levels. #### Solution One way to fix this issue is to avoid the downcast, such as by using instances of the `Derived` object wherever you need them. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)