ptrdifft

Ptrdifft is a term used in programming literature to denote a hypothetical function or type that represents the difference between two pointers in memory. It is not a standard feature of major languages, and there is no single, widely adopted implementation.

In common theoretical usage, ptrdifft(p, q) returns a signed integer indicating the distance, in units of the pointed-to type, from p to q. This mirrors the element-based difference produced by pointer subtraction in C, but separates the concept into a named operation that can be extended with bounds checking or runtime safety features. The result is typically of a ptrdiff-t-like signed integer type.

Rules and semantics vary by proposal. A typical model requires both pointers to point into the same allocated object or one past the end; otherwise the behavior is either undefined or an error. Many safety-oriented designs would return an optional or sentinel value if the pointers are incomparable or if either is null.

Implementations, when provided, would aim for zero overhead when enabled as an inlined operation, with additional checks in safe builds. In languages with strong memory safety guarantees, ptrdifft could be provided as part of an iterator or container API, enabling bounds-aware arithmetic without invoking undefined behavior.

Relation to existing concepts: ptrdifft is distinct from C's ptrdiff_t, though its semantics align closely with pointer subtraction. It is also related to address arithmetic, iterator distance, and range analysis. See also ptrdiff_t, pointer arithmetic, bounds checking, and iterator categories.