Side effects in C

Question

I thought that my understanding of side effects in programming languages was OK.

I think this is a great definition from wikipedia:

"in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world."

However, I read this in the same link(yes, I know that is probably not the best place to look for examples):

"One common demonstration of side effect behavior is that of the assignment operator in C++. For example, assignment returns the right operand and has the side effect of assigning that value to a variable. This allows for syntactically clean multiple assignment:"

int i, j;
i = j = 3;

Why do they consider that a side-effect? It is the same as two simple assignment statements to 2 local variables.

Thanks in advance.

The quoted text is incorrect. The value of an assignment operation is the value assigned, which, in a simple assignment, is the value of the right side converted to the type of the left side. So, if d is a double with value 3.5 and x is an int, the value of x = d is 3, not 3.5. — Eric Postpischil
Note that the quote is about C++, which is quite different from C. — Bo Persson

Eric Postpischil Eric Postpischil · Accepted Answer · 2013-04-25T18:05:54

You can use an assignment expression as a value:

double d = 3.5;

int x, y;

printf("%d", x = d); // Prints "3".

y = (x = d) * 5; // Sets y to 15.

double z = x = d; // Sets z to 3 (not 3.5).

The value produced by x = d, is its main effect. The changing of the value of x is a side effect.

Side effects in C

3 Answers