Does C Specify the Order of Operand Evaluation in Expressions?

Order of Evaluation of Operands in C

When evaluating expressions involving multiple operands, the order in which these operands are assessed can impact the result. In C , the evaluation order of both user-defined and standard operators is crucial for understanding program behavior.

For user-defined types, the expression a b is treated as a function call, and the C standard explicitly states that the order of argument evaluation is unspecified. This implies that the compiler is free to evaluate a and b in any sequence it deems suitable.

As for standard operators, the standard also emphasizes that the evaluation order of operands and subexpressions is unspecified. This allows compilers to optimize code by reordering the evaluation of expressions without altering their final value. Notably, this behavior applies to both C and C languages.

For example, consider the expression x = y z;. The compiler may choose to first evaluate y, followed by z, and then perform the addition. Alternatively, it could assess z, then y, and then calculate the sum. The order is irrelevant as long as the result remains unchanged.

Therefore, in C , the evaluation order of operands is unspecified for both user-defined and standard operators. This flexibility permits compilers to optimize code performance while maintaining correctness.

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