Are unspecified and undefined behavior required to be consistent between compiles of the same program with the same compiler in the same environment?

Question

Let\'s pretend my program contains a specific construct the C++ Standard states to be unspecified behavior. This basically means the implementation has to do something reasonabl

Answer 1

I don't know about unspecified behaviour (but judging from the name, maybe it does the same bad/evil thing everywhere just no one really knows what exactly it does). But for the undefined behavior, i think this one could behave VERY differently across platforms or compilers. I've seen some really strange coredumps on Solaris which did not occur on Ubuntu etc.

Answer 2

It's worth noting that the implementation of the specified behaviour of the C++ Standard is not 100% identical across compilers, even today. Given this, it's not reasonable to expect that unspecified or undefined behaviour is compiler-agnostic. You have the best chance of writing portable code if you just stick to the Standard.

Answer 3

Undefined behavior can vary between runs of the same program, and even between execution of the same code in the same run of the program. As an example, the value of an uninitialized (automatic) variable is undefined, and then its actual value is just whatever value that happened to be at that place in memory. Obviously, this can vary.

EDIT:

This goes for unspecified behavior too. For example, the order of evaluation of function arguments is unspecified, so if they have side effects, those side effects can occur in any order. This may print "Hi!Ho!" or "Ho!Hi!":

f( printf("Hi!"), printf("Ho!") );

This can vary between executions, too. As the standard says: "An instance of the abstract machine can thus have more than one possible execution sequence for a given program and a given input." The difference is that with undefined behavior, anything can happen: the computer can explode, reformat the disk, or whatever. If the behavior is unspecified, the computer is not allowed to explode.

There is also implementation-defined behavior, such as the value of sizeof(int). This must be the same at all times, for the same compiler.