Is Operand Evaluation Order Guaranteed in C and C ?

Order of Operand Evaluation in Expressions

When writing expressions, it is important to understand the order in which the operands will be evaluated. This can impact the outcome of the expression, especially when side effects are involved.

C and C

In both C and C , the order of evaluation for operands is unspecified. This means that the compiler is free to evaluate the operands in any order it chooses.

For example, consider the expression a b. It is not guaranteed that a will be evaluated before b. The compiler may choose to evaluate b first, and then store the result in a temporary variable before evaluating a.

The standard for C specifically states that "the order of evaluation of function arguments is unspecified," and for normal operators, "the order of evaluation of operands of individual operators and subexpressions of individual expressions [...] is unspecified."

C 11 and Beyond

The wording for operand evaluation order has changed slightly in C 11 and later versions. The second statement now says that the order is "unsequenced" rather than unspecified. However, this is essentially the same concept. The compiler is still not required to evaluate operands in any particular order.

Conclusion

It is important to be aware that the order of operand evaluation is unspecified in C and C . This can impact the outcome of expressions, especially when side effects are involved. When writing expressions, it is best to avoid relying on a specific evaluation order.

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