Object-Oriented Fortran

Fortran 2003 added type extension, type-bound procedures, and polymorphism to the derived-type system, letting programs express object-oriented designs without abandoning Fortran's array-oriented performance strengths. A base type declares TYPE :: shape with a CONTAINS section binding a procedure like area, and any type extending it, TYPE, EXTENDS(shape) :: circle, inherits shape's components and procedures while adding its own (radius) and optionally overriding area with its own circle-specific implementation, following the same is-a relationship familiar from languages like C++ or Java.

Extended types automatically inherit every component and type-bound procedure of their parent, and can override an inherited procedure by redeclaring the same binding name with a new implementation, as long as the interface is type-compatible. The parent's components remain accessible in the child, and a child instance can be treated as a value of the parent type in contexts requiring a CLASS(shape) argument, which is the essence of Liskov-substitutability that makes polymorphic collections of shapes work correctly regardless of which concrete extension each element actually is.

Declaring a variable as CLASS(shape), rather than TYPE(shape), makes it polymorphic: it can hold any extension of shape at runtime, and calling s%area() dispatches dynamically to whichever concrete type's implementation is actually stored, exactly like virtual dispatch in C++. When you need to act on type-specific components not present in the base type, SELECT TYPE (s) constructs a runtime type-selection block with TYPE IS (circle) and TYPE IS (rectangle) branches, letting you safely downcast and access circle-specific components like radius only within the branch where the runtime type has been confirmed to match.

An ABSTRACT type, declared TYPE, ABSTRACT :: shape, cannot be instantiated directly and typically pairs a DEFERRED type-bound procedure, PROCEDURE(area_interface), DEFERRED :: area, with an abstract interface declaring only the signature; every concrete extension must supply its own implementation of that deferred binding or itself remain abstract. This mirrors an abstract base class or pure virtual function in C++, enforcing at compile time that no one can create a bare, meaningless 'shape' object and forcing every concrete shape to define what area() actually means for it.