allocatable

Allocatable is a term used in computer science to describe an object whose storage is not fixed at program start but can be allocated and released at runtime. The concept is most closely associated with the Fortran programming language, where allocatable objects are data items declared with a specific attribute and whose memory is managed dynamically.

In Fortran, an allocatable object is declared with the ALLOCATABLE attribute, for example: integer, allocatable :: a(:). Such objects can be allocated with the ALLOCATE statement, as in allocate(a(100)). Allocation reserves memory for the requested shape, and the contents of uninitialized allocatable storage are undefined until explicitly set. The allocation status can be checked with the allocated intrinsic function, and the allocate statement can specify a stat variable to capture errors. Deallocate(a) frees the allocated memory and marks the object as deallocated. Fortran also provides move_alloc to transfer an allocation from one allocatable to another without a separate allocation step, and optional source or initializers to initialize newly allocated storage.

Lifecycle and semantics: An allocatable variable may begin unallocated; after a successful allocation, it has storage of the requested shape and may require explicit initialization of its elements. Referencing an unallocated allocatable typically results in a runtime error unless the code ensures allocation first. Allocatable objects own their storage, in contrast to pointers, which may alias or reference other data.

Compared with other dynamic structures, allocatable objects differ from fixed-shape automatic arrays and from pointers in ownership and deallocation semantics. In modern Fortran, allocatable arrays are a common mechanism for dynamic memory management and are compatible with features such as array operations and status checking. See also: dynamic memory allocation, pointers, Fortran.