What Operations Are Permissible on Moved-From Standard Library Objects in C ?

Operations on Moved-From Objects

Despite the common misconception that moved-from objects can only be destroyed, the C standard specifies that moved-from objects of standard library types are placed in an unspecified state, allowing for a limited scope of operations. This state arises after an object is moved from, enabling operations that typically do not have preconditions.

Unconditional Operations

The following operations can generally be applied to moved-from objects without encountering preconditions:

• Destruction: Deallocating the object's memory.
• Assignment: Setting a moved-from object as the value of another object.
• Constant observations: Accessing information such as getter functions, array sizes, and emptiness booleans.

Conditional Operations

In contrast, the following operations may require the object not to be in an unspecified state and hence are generally not permissible on moved-from objects:

• Dereference: Accessing the object's internals via pointers or references.
• Mutation: Making changes to the object's fields.

Regarding std::swap

In the example swap function template, the assignment operations (lines 2 and 3) are valid even though they involve moved-from objects because assignment generally does not have preconditions.

Finally

The standard detailing unspecified-state objects for standard library types is located at 17.6.5.15 [lib.types.movedfrom].

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