TITLE: Increment operator and lvalues



PROBLEM: pcwu@csie.nctu.edu.tw (Pei-Chi Wu)

	In ANSI C, both the return type of prefix ++/-- and postfix ++/--  
are not lvalues. But in C++, the prefix ++/-- are redefined as returning 
lvalue. So you can write code like this:
	int i; 
	++ ++ i;
but cannot write:
	int i;
	i ++ ++;
(Increment and decrement can only be applied on lvalue.)
	What is the reason the language make prefix and postfix different?
	Why do the committee make C++ incompatible with ANSI C without meaningful
improvements or advantages?




RESPONSE: ark@europa.att.com (Andrew Koenig), AT&T Bell Laboratories

Because the result of the expression

	++i

is `i' after incrementing, which can be viewed as equivalent to

	(++i, i)

Since `i' is an lvalue, this expression can reasonably be treated as an
lvalue.  On the other hand, the result of the expression

	i++

must be a copy of the old value of `i,' since `i' no longer has that value.
Since it is a copy, it must be a compiler-generated temporary, which is
not an lvalue.

The behavior you describe is not an incompatibility.  Incompatibilies
are cases where valid C programs behave differently in C++.  Since
neither i++ nor ++i is an lvalue in C, it is impossible to write a
C program that detects the C++ behavior.  Thus this behavior is a
pure extension to C, much like almost all of the rest of C++.