An explanation about Sequence points

Question

Lately, I have seen a lot of questions being asked about output for some crazy yet syntactically allowed code statements like like i = ++i + 1 and i=(i,i++,i)+1;. Frankly realistically speaking hardly anyone writes any such code in actual programing.To be frank I have never encountered any such code in my professional experience. So I usually end up skipping such questions here on SO. But lately the sheer volume of such Q's being asked makes me think if I am missing out some important theory by skipping such Q's. I gather that the such Q's revolve around Sequence points. I hardly know anything about sequence points to be frank and I am just wondering if not knowing about it is a handicap in some way. So can someone please explain the theory /concept of Sequence points, or If possible point to a resource which explains about the concept. Also, is it worth to invest time in knowing about this concept/theory?

Answer 1

The simplest answer I can think of is:

C++ is defined in terms of an abstract machine. The output of a program executed on the abstract machine is defined ONLY in terms of the order that "side effects" are performed. And Side effects are defined as calls into IO library functions, and changes to variables marked volatile.

C++ compilers are allowed to do whatever they want internally to optimize code, but they cannot change the order of writes to volatile variables, and io calls.

Sequence points define the c/c++ program's heartbeat - side effects before the sequence point are "complete" and side effects after the sequence point have not yet taken place. But, side effects (or, code that can effect a side effect indirectly( within a sequence point can be re-ordered.

Which is why understanding them is important. Without that understanding, your fundamental understanding of what a c++ program is (And how it might be optimized by an agressive compiler) is flawed.

Answer 2

See http://en.wikipedia.org/wiki/Sequence_point.

It's a quite simple concept, so you don't need to invest much time :)