Why does a=(b++) have the same behavior as a=b++?

Question

I am writing a small test app in C with GCC 4.8.4 pre-installed on my Ubuntu 14.04. And I got confused for the fact that the expression a=(b++); behaves in the

Answer 1

Parentheses are entirely syntactic. They just group expressions and they are useful if you want to override the precedence or associativity of operators. For example, if you use parentheses here:

a = 2*(b+1);

you mean that the result of b+1 should be doubled, whereas if you omit the parentheses:

a = 2*b+1;

you mean that just b should be doubled and then the result should be incremented. The two syntax trees for these assignments are:

   =                      =
  / \                    / \
 a   *                  a   +
    / \                    / \
   2   +                  *   1
      / \                / \
     b   1              2   b

a = 2*(b+1);            a = 2*b+1;

By using parentheses, you can therefore change the syntax tree that corresponds to your program and (of course) different syntax may correspond to different semantics.

On the other hand, in your program:

a1 = (b1++);
a2 = b2++;

parentheses are redundant because the assignment operator has lower precedence than the postfix increment (++). The two assignments are equivalent; in both cases, the corresponding syntax tree is the following:

    =
   / \
  a   ++ (postfix)
      |
      b

Now that we're done with the syntax, let's go to semantics. This statement means: evaluate b++ and assign the result to a. Evaluating b++ returns the current value of b (which is 10 in your program) and, as a side effect, increments b (which now becomes 11). The returned value (that is, 10) is assigned to a. This is what you observe, and this is the correct behaviour.