Are Moved-From Vectors Always Empty in C ?

Emptiness of Moved-From Vectors: Exploring the Options

In the realm of C programming, vectors are versatile data structures often leveraged for their efficient memory management. However, when it comes to understanding the behavior of moved-from vectors, programmers may encounter questions about their emptiness.

Standard Definition and Requirements

The C standard provides general guidelines for the behavior of objects after they've been moved from, as stated in N3485 17.6.5.15 [lib.types.movedfrom]/1:

"Objects of types defined in the C standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state."

While this provision doesn't explicitly exclude vectors, it also doesn't provide specific guidance on their behavior.

Examining the Move Constructor

The move constructor for vectors, vector::vector(vector&& v), is required to have constant complexity. This necessitates the transfer of resources from the source vector v to the new vector *this, leaving v in an empty state.

Exploring Move Assignment and Allocator Influence

The move assignment operator, vector& vector::operator=(vector&& v), exhibits more complexity and is influenced by the vector's allocator. Three scenarios emerge:

1. Allocator Propagation Enabled:
   When allocator_traits::propagate_on_container_move_assignment::value is true, the move assignment operator deallocates elements and memory, transfers ownership from v, and leaves both vectors empty.
2. Allocator Propagation Disabled (Same Allocator):
   Similar to the previous case, the move assignment operator resembles Case 1 but retains the allocators intact. However, it follows the same pattern, resulting in empty vectors.
3. Allocator Propagation Disabled (Different Allocators):
   In this case, the move assignment cannot transfer resources and resorts to moving elements individually. This requires the ability to move-assign or copy-assign elements, potentially leaving the moved-from vector v containing moved-from elements. Some implementations may choose to explicitly clear v, but this is not mandated by the standard.

Conclusion

While vectors typically become empty after being moved from, there are exceptions depending on the allocator's behavior and the ability to move or copy assign elements. Implementations have the freedom to adopt different approaches, as long as they adhere to the general requirements for moved-from objects. Understanding these scenarios is crucial for programmers when working with vectors and move semantics in C applications.

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