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Ownership model

Terra’s ownership model ensures every resource has exactly one owner at any time, akin to Rust’s single-ownership principle, preventing data races, dangling pointers, and double-free errors. This model guarantees safety in sequential and shared memory parallel contexts through strict resource transfer and access rules.

L-Values, R-Values, and B-Values​

  • L-Values: Values explicitly allocated with a var statement (e.g., var x : T or var x = ...). They represent named, persistent objects with a defined lifetime.
  • R-Values: Temporary objects, typically resulting from function calls. They are short-lived and exist only within their expression.
  • B-Values: Reference objects (&T), enabling borrowing. They allow the caller to retain ownership while the callee operates on the resource without transferring it.

Passing by value or by reference​

Passing objects to functions or in an assignment can be done in two ways:

  1. By value: Transfers ownership to the receiver.
    • L-values: Use the __copy method if defined, duplicating the resource, or default to transferring ownership using the autogenerated __move.
    • R-Values: Always transfer ownership using the autogenerated __move.
    • The __move__ directive, e.g. __move__(a), can be used to transfer ownership via __move, limiting unnecessary copies.
    • Non-managed objects are trivially copied bitwise.
  2. By Reference (B-Values): Grants temporary access via &T. No ownership transfer occurs; the original owner retains responsibility for cleanup via __dtor in case of managed types.

Planned safety features​

Terra’s single-ownership model enables compile-time verification of resource safety for sequential and parallel programs. The following enhancements are planned to strengthen this model:

  1. Initialization Tracking:

    • Tracks variable initialization at compile time.
    • Flags use of uninitialized variables (e.g., after a move) as compile errors.
    • Skips __dtor for uninitialized objects, improving safety and efficiency.

  2. Constant References (const&):

    • Enforces read-only access (const& T) recursively at compile time.
    • Allows safe, unsynchronized sharing in parallel programs.