What Operations Are Allowed on Moved-From C Objects?

Allowed Operations on Moved-From Objects

The C standard defines what actions are permissible with objects that have been moved from. These objects are considered to be in an unspecified state, allowing for a limited range of operations.

Standard Reference

Section 17.6.5.15 [lib.types.movedfrom] of the standard states that moved-from objects "shall be placed in a valid but unspecified state."

Non-Restrictive Operations

Objects in an unspecified state can undergo operations that do not have preconditions. Common examples include:

• Destruction
• Assignment
• Constant observers (e.g., get, empty, size)

Restrictive Operations

Operations with preconditions, such as dereferencing or pop_back, cannot be performed directly on moved-from objects, as the object's unspecified state may not satisfy the necessary requirements.

Example: std::swap

The example swap function template relies on assignments to moved-from objects (lines 2 and 3). This is valid because assignment is a non-restrictive operation.

Alternative Constructor Syntax

Regarding line 1, the use of T c = std::move(a); instead of T c(std::move(a)); is for efficiency reasons. Direct initialization requires a copy constructor call, while assignment does not.

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