downcasting

Downcasting is the explicit conversion of a reference or pointer from a base class to a derived class type. It is often necessary when code stores objects in a collection of base type but later needs access to derived-specific members. Downcasting is contrasted with upcasting, which safely converts a derived object to its base type and is usually implicit.

In languages with runtime type information, downcasting can be checked at runtime. Java requires a cast and may throw ClassCastException if the object is not an instance of the target type. C++ offers dynamic_cast for pointers and references; on pointers it returns nullptr if the dynamic type is not compatible, and on references it throws std::bad_cast. static_cast performs an unchecked conversion that can lead to undefined behavior if the cast is invalid. C# provides both an explicit cast and the as operator; the explicit cast may throw InvalidCastException, while as returns null when the cast is not possible.

Slicing risk: in languages like C++, downcasting by value can cause object slicing; storing a derived object in a base object may lose derived data. Safe downcasting typically uses pointers or references. Performance: downcasts require RTTI checks and may be slower than upcasting.

Best practices: minimize downcasting; prefer virtual dispatch and polymorphism; use type checks when necessary; encapsulate type-specific behavior with design patterns such as visitor.