Question

Considering following array and two series of assignments:

char charArray[3];

charArray[0]='a';
charArray[1]='b';
charArray[2]='c';

char & charRef1=charArray[0];
charRef1='a';
char & charRef2=charArray[1];
charRef2='b';
char & charRef3=charArray[2];
charRef3='c';

Does C++ standard dictate whether these two series of assignments should be implemented identically or differently by the compiler?

Answer 1

No, the standard makes no requirements that the implementation details must be the same. 1.9/1:

The semantic descriptions in this
International Standard define a
parameterized nondeterministic
abstract machine. This International
Standard places no requirement on the
structure of conforming
implementations. In particular, they
need not copy or emulate the structure
of the abstract machine. Rather,
conforming implementations are
required to emulate (only) the
observable behavior of the abstract
machine as explained below.

So only the “observable behavior” has to be the same. Observable behavior is defined in 1.9/6:

The observable behavior of the
abstract machine is its sequence of
reads and writes to volatile data and
calls to library I/O functions.

The exact instructions used to achieve this are not “observable behavior”, and in your example since the array is not volatile, the order of writes isn’t observable either. In fact, unless you use the array later, the writes themselves aren’t observable. It would be legal for the implementation’s optimizer to successfully remove the entire code snippet in one case but not the other, although perhaps surprising that it could manage only one.