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10.4 Postincrement and Ordering

The ordering requirements for the postincrement and postdecrement operations (see Postincrement/Postdecrement) are loose: those side effects must happen “a little later,” before the next sequence point. That still leaves room for various orders that give different results. In this expression,

z = x++ - foo ()

it’s unpredictable whether x gets incremented before or after calling the function foo. If foo refers to x, it might see the old value or it might see the incremented value.

In this perverse expression,

x = x++

x will certainly be incremented but the incremented value may be replaced with the old value. That’s because the incrementation and the assignment may occur in either oder. If the incrementation of x occurs after the assignment to x, the incremented value will remain in place. But if the incrementation happens first, the assignment will put the not-yet-incremented value back into x, so the expression as a whole will leave x unchanged.

The conclusion: avoid such expressions. Take care, when you use postincrement and postdecrement, that the specific expression you use is not ambiguous as to order of execution.