Why do some people think x = ++y + y++ is bad?, C++ FAQ

C++ FAQ / Section 39 / FAQ 39.15

Section 39:

39.1	How do I convert a value (a number, for example) to a `std::string`?
39.2	How do I convert a `std::string` to a number?
39.3	Can I templatize the above functions so they work with other types?
39.4	What should be done with macros that contain `if`?
39.5	What should be done with macros that have multiple lines?
39.6	What should be done with macros that need to paste two tokens together? Updated!
39.7	Why can't the compiler find my header file in `#include "c:\test.h"` ?
39.8	What are the C++ scoping rules for `for` loops?
39.9	Why can't I overload a function by its return type?
39.10	What is "persistence"? What is a "persistent object"?
39.11	How can I create two classes that both know about each other?
39.12	What special considerations are needed when forward declarations are used with member objects?
39.13	What special considerations are needed when forward declarations are used with inline functions?
39.14	Why can't I put a forward-declared class in a `std::vector<>`?
39.15	Why do some people think `x = ++y + y++` is bad?
39.16	What's the deal with "sequence points"?

[39.15] Why do some people think x = ++y + y++ is bad?

Because it's undefined behavior, which means the runtime system is allowed to do weird or even bizarre things.

The C++ language says you cannot modify a variable more than once between sequence points. Quoth the standard (section 5, paragraph 4):

Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.