Do We Need the `if (this != &rhs)` Check in C Move Assignment Operators?

Move Assignment Operator and if (this != &rhs)

In the copy assignment operator of a class, it is common practice to check if the object being assigned is the invoking object to prevent self-assignment:

Class& Class::operator=(const Class& rhs) {
    if (this != &rhs) {
        // Do the assignment
    }

    return *this;
}

This check is crucial to prevent the object from being assigned to itself, which can lead to unexpected behavior or errors. However, in the case of the move assignment operator, the necessity of this check is debatable.

Do we need the same check for the move assignment operator?

Class& Class::operator=(Class&& rhs) {
    if (this != &rhs) {
        // Do the assignment
    }

    return *this;
}

The question arises whether there can be situations where this == &rhs would be true for the move assignment operator. The answer depends on the interpretation of the move assignment operator.

Interpretations of the move assignment operator:

There are two main interpretations of the move assignment operator:

1. Preserves current value: Some argue that self-move-assignment (where this == &rhs) should preserve the current value of the object. In this case, the if (this != &rhs) check is necessary to prevent accidental self-assignment.
2. Sets to a valid state: Others argue that the self-move-assignment operator should not necessarily preserve the current value but should leave the object in a valid state. In this case, the if (this != &rhs) check is not strictly necessary. However, it can still be useful as an optimization to avoid performing the move operation on self-assignment.

Current and future standards:

The C 11 standard does not explicitly mandate a specific interpretation of the move assignment operator. However, it allows the programmer to define the behavior through the implementation of the move assignment operator.

The proposed C 20 working draft includes a section on move assignment that suggests that self-move-assignment should not crash, but it does not specify whether it should preserve the current value or leave the object in a valid but unspecified state.

Conclusion:

The need for the if (this != &rhs) check in the move assignment operator is a matter of interpretation and specific implementation requirements. If self-move-assignment is expected to preserve the current value, then the check is necessary. Otherwise, it can be omitted as an optimization.

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