deref

Deref, short for dereference, is the operation of accessing the value stored at the address referred to by a pointer or reference. Dereferencing is central to pointer-based programming, enabling read and write access to the data located at a given memory location.

In C and C++, the dereference operator is the asterisk *. If p is a pointer to an int, int x = *p; assigns the value at p to x, and *p = 7; stores 7 into the pointed-to location. The address-of operator & creates the pointer. Dereferencing requires the pointer to point to valid memory; dereferencing a null or dangling pointer yields undefined behavior.

Safety and lifetime considerations are important across languages. Many environments enforce checks or separate value access from memory management to reduce risks. Dereferencing a null pointer or a freed object can lead to crashes or unpredictable behavior.

Rust provides a language-specific model for dereferencing. The Deref trait describes how a type behaves when dereferenced with the * operator. Types like Box<T>, Rc<T>, and &T implement Deref to provide access to the underlying T, and the compiler performs deref coercions to let you call methods on a value through an enclosing smart pointer without explicit dereferencing. Dereferencing raw pointers via *ptr is unsafe and requires an unsafe block.

Other contexts also use the idea of dereferencing. In Lisp, a deref-like operation can read the value stored in a reference. While exact syntax and safety rules vary, the core concept remains: dereferencing retrieves the value identified by a name, reference, or pointer.