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cundefined-behavior

Is `y = x = x + 1;` undefined behavior?

Is this code:

y = x = x + 1;

undefined behavior in C?

Solution

  • Answer to your question
    No.

    What will happen
    This will happen:

    int x = 1; /* ASSUME THIS IS SO */
y = x = x + 1;

/* Results: */
y == 2;
x == 2;

    How it compiles
    The same as:

    x += 1;
y = x;

    Why this is not undefined
    Because you are not writing x in the same expression you read it. You just set it to itself + 1, then assign y to the value of x.

    Your future
    If you find the code confusing you can use parentheses for readability:

    y = x = (x + 1);