MoveOnlyconst

MoveOnlyconst is a term used in some C++ codebases to describe a wrapper or design pattern that enforces move-only semantics while preserving const-correctness. It is not a standard library type or formalized concept in the C++ standard, but it appears in discussions and implementations aimed at controlling ownership and moves in APIs.

Definition and purpose

MoveOnlyconst refers to a type or wrapper around a value that cannot be copied but can be moved, with explicit behavior when the wrapper or its value is const-qualified. The goal is to prevent accidental copies and to document and enforce ownership transfer semantics. Because moving from a const object is ill-formed in standard move semantics, a MoveOnlyconst-aware design typically relies on revealing move capability only through non-const contexts.

Semantics

A typical MoveOnlyconst implementation deletes the copy constructor and copy assignment operator, while providing a move constructor and move assignment operator. Access to the underlying value is offered through accessors that respect const-correctness, such as returning a const reference for read-only use and a non-const rvalue or reference for legitimate moves from non-const objects. If an instance is const, moving from it is disallowed, ensuring that const objects remain unmodified and unmoved.

Implementation approaches

One approach is a templated wrapper class that stores a value of type T and explicitly disables copying while enabling moving only for non-const instances. The wrapper may provide methods or conversion operators to access T in a controlled way, such as T& or const T&, and an rvalue access path for move operations. Some designs use explicit move helpers to signal intent and to prevent moves on const-qualified wrappers.

Use cases and relationships

MoveOnlyconst patterns are useful when APIs require strict ownership transfer and when objects should not be duplicated. They complement standard move-only types (like unique_ptr) and containers that hold move-only values by making ownership transfer behavior explicit. It is a design idiom rather than a standard feature, related to move semantics, const-correctness, and ownership models in modern C++.