Why is `i = i 1;` now legal in C 17?

C 17 Legalizes i = i 1; Due to Revised Operator Evaluation Order

In C 11, the assignment i = i 1; was considered undefined behavior. However, in C 17, it became a valid expression. To understand this change, we need to delve into the revised rules for operator evaluation.

Pre-C 17 Undefined Behavior

In C 11, the sequencing of operand evaluations for assignment operators was defined as follows:

• The value computations of the operands were sequenced before the assignment operation.
• Side effects on a scalar variable were unsequenced relative to both other side effects and value computations.

In i = i 1;, the post-increment i exhibits side effects (modifying the value of i), which could occur interleaved with the assignment operación. This potential for unsequenced side effects led to undefined behavior.

C 17's Revised Sequencing Rule

C 17 introduced a new sequencing rule for assignment operators:

• The right operand is sequenced before the left operand.

This revised rule applies to both the value computations and side effects of the right operand.

Legalization of i = i 1;

In i = i 1;, the right operand is i 1. The evaluation of this expression involves the postfix increment of i within the parentheses, followed by the addition of 1. According to the new rule, this entire evaluation is sequenced and completed before the assignment to i.

Therefore, in C 17, the act of modifying i (side effect) is completed before the actual assignment, eliminating the potential for unsequenced side effects. This revised sequencing makes the i = i 1; expression legal in C 17.

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