Understanding the increment operator in C

Question

Why is the following illegal in C?

y = (w + x)++;

According to my book, this is illegal, but I don't see why.

Answer 1

In i++, the value of i is changed. After execution, i's value is one plus its previous value. You can't store a value in w+x, though, and so you can't do any of the following, which all (if they worked) would have more or less the same effect:

w+x = w+x+1;
w+x += 1;
(w+x)++;

Something that can be placed on the left hand side of an assignment is typically called an lvalue (l is for left). The concise way of saying this is that ++ can only be applied to lvalues, and w+x isn't an lvalue. You can read more about lvalues (and other kinds of values) in this question and its answers:

• What are rvalues, lvalues, xvalues, glvalues, and prvalues?

Answer 2

According to Dennis M. Ritchie's book: "The C Programming Language":

2.8 Increment and Decrement Operators

(page 44)

The increment and decrement operators can only be applied to variables; an expression like (i + j)++ is illegal. The operand must be a modifiable lvalue of arithmetic or pointer type.

Because an expression

i++;

is equivalent to:

i = i + 1;

So an expression like:

(i + j)++;

something like equivalent to:

(i + j) =  (i + j) + 1;  // meaning less

That looks meaning less, we can't modify expression.

Related: An interesting bug one may like to know about in gcc 4.4.5 is that expression j = ++(i | i); compiles that should produce l-value error. Read: j = ++(i | i); and j = ++(i & i); should an error: lvalue?

Read about modifiable lvalue from (1) Expression must be a modifiable lvalue error and (2) msdn docs: Lvalues and Rvalues