HasValue

HasValue is a conventional flag used in programming to indicate whether an optional, nullable, or otherwise optional value currently holds a value. It is typically exposed as a boolean property or method and is used to guard access to the underlying value.

In languages with nullable value types, HasValue signals the presence of a value. For example, in C#, Nullable<T> defines a HasValue property and a Value property. HasValue returns true when the variable contains a value, and false when it represents null. If HasValue is true, the Value property yields the stored value; if false, accessing Value typically throws an exception. Many languages provide convenient ways to supply a default when a value is not present, such as a GetValueOrDefault method or null-coalescing patterns.

Other language implementations adopt similar concepts under different names. C++ standard library’s std::optional<T> exposes a has_value() function to check presence, after which the contained value can be accessed via operator* or value(). Some other languages and libraries use is_present, isDefined, or isPresent to indicate optional value presence. Across implementations, the common theme is the separation between the existence of a value and the value itself, enabling safer handling of missing data.

Usage guidance typically emphasizes checking HasValue before accessing the underlying value and handling the “no value” case explicitly, either by providing a default, performing alternative logic, or throwing a well-defined error in exceptional circumstances.

See also: Nullable types, Optional, nullability, value retrieval, default values.