Why Did \'i = i 1;\' Become a Legitimate Operation in C 17?

Why i = i 1; Became Legal in C 17

In C 11, the code snippet "i = i 1;" invoked undefined behavior. However, in C 17, this code became legally acceptable. This change stems from the enhanced sequencing guarantees introduced in C 17.

According to the C 11 standard [N3337 basic.exec], the sequencing of side effects is unsequenced unless explicitly specified. In the example code, the assignment operator (=) and the postfix increment operator ( ) both have side effects that can lead to undefined behavior if not correctly sequenced.

In C 17, a new sentence was added to [N4659 expr.ass] that clarifies the sequencing rules for the assignment operator:

The right operand is sequenced before the left operand.

This additional statement ensures that all side effects within the right operand are executed before any side effects within the left operand, including the assignment itself.

In the case of "i = i 1;", the right operand (i 1) consists of the postfix increment operator (i ) and the constant 1. The sequencing guarantees introduced in C 17 ensure that the postfix increment is performed and its side effect occurs before the assignment is executed. This eliminates the undefined behavior that was present in C 11.

Therefore, the enhanced sequencing guarantees in C 17 have made "i = i 1;" legal by ensuring that the side effects of the right and left operands are correctly sequenced, preventing undefined behavior.

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