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Editorial Team
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Asked: 2026-05-23T20:55:47+00:00

I encountered a line of code: int a = 10; int b = 40;

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I encountered a line of code:

int a = 10;
int b = 40;
a = a + b - (b = a);
cout << a << "   " << b << endl;

I cannot understand what happens in this code.
Can anyone explain for me?

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1 Answer

  1. Editorial Team

    Undefined behavior. the value of b is changed and used for computation without an intervening sequence point. The results of the program are unpredictable – it can print anything or crash, or do do some nasty system calls.

    Except where noted, the order of evaluation of operands of individual
    operators and subexpressions of individual expressions, and the order
    in which side effects take place, is unspecified.53) Between the
    previous and next sequence point a scalar object shall have its stored
    value modified at most once by the evaluation of an expression.
    Furthermore, the prior value shall be accessed only to determine the
    value to be stored. The requirements of this paragraph shall be met
    for each allowable ordering of the subexpressions of a full
    expression; otherwise the behavior is undefined.

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