What are the Validity and Allowable Operations on Moved-From Objects in C 11?

Validity of Moved-From Objects in C 11

Understanding the validity of moved-from objects in C 11 can be complex. This article aims to clarify the concept, particularly in the context of pimpl objects.

Moved-From Objects

When an object is moved from, its state becomes unspecified. The standard library defines the validity criteria for standard library types, ensuring that moved-from objects are placed in a valid but unspecified state. However, for custom types, it is the developer's responsibility to define and document the valid state and allowable operations for moved-from objects.

Example: Pimpl Idiom

In the example pimpl idiom, the impl_ member points to a dynamic allocation. After a move operation, the pointer in the moved-from object is set to nullptr. This renders the do_stuff() method invalid, as it attempts to dereference the impl_ pointer.

Invariant vs. Valid State

The invariant that a Foo object can always do_stuff() is no longer valid after being moved from. In general, moved-from objects may not satisfy all invariants that apply to their non-moved counterparts.

Option 1: Checked Operation

One option is to explicitly check if the impl_ pointer is nullptr in do_stuff() and initialize it if necessary. However, this introduces a potential performance overhead.

Option 2: Define Invalid State

Alternatively, the code can explicitly define that moved-from Foo objects are in an invalid state and that calling do_stuff() on them will result in an error.

Concepts and Moved-From Objects

It's important to note that the concepts defined by the standard library do not consider moved-from objects. To meet the requirements of concepts, moved-from objects of custom types must still fulfill the concept requirements.

Conclusion

Understanding the validity of moved-from objects is crucial for implementing move semantics correctly. Developers should carefully consider the implications of move operations on the state of their objects and define the expected behavior accordingly.

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