nonnull

Nonnull refers to a nullability annotation used in some programming languages, notably Objective-C and C, to indicate that a pointer or return value must not be null. It is part of a broader set that also includes nullable and null_unspecified. These annotations help static analysis tools detect potential null dereferences and assist with interoperability with Swift, where nullability affects whether a value is imported as an optional.

In Objective-C, nullability annotations are applied to pointer types and can be combined with macros that control default assumptions. Common forms include _Nonnull and _Nullable type qualifiers, as well as __nonnull and __nullable attributes. Developers often wrap header files with NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END to assume nonnull by default, then use nullable annotations to override as needed. For example, a method parameter of a nonnull string would be declared as NSString * _Nonnull name.

Beyond Objective-C, the term also appears in C/C++ contexts via the Clang/GCC attribute __attribute__((nonnull)) that can be attached to function parameters or the return value (via returns_nonnull) to enforce non-null usage at compile time. These attributes can specify which parameters must be non-null by index, enabling more precise static analysis and optimization.

Nullability annotations are primarily a development-time aid rather than runtime checks. They do not automatically enforce non-null values during execution, but they guide developers, enable safer API design, and improve correctness when bridging to Swift. Misuse can still lead to null dereferences if runtime checks are not performed.

See also: nullability, nullable, nonnull in various languages, Swift interoperability, static analysis, ARC.