Question

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Undefined, unspecified and implementation-defined behavior

I’m trying to deepen my understanding of undefined behavior in C++. Suppose a C++ compiler will intentionally detect some cases of undefined behavior – for example, modifying the variable twice between two sequence points:

x++ = 2;

Once that imaginary compiler reliably detects such a situation it will say emit ten totally random machine instructions into the produced machine code.

According to C++ standard, wherever something is classified as UB there’re no requirements on what happens. Will the described imaginary compiler be conformant to the C++ standard?

Answer 1

Yes. The standard imposes no requirements, so it can do whatever it wants:

undefined behavior

behavior, such as might arise upon use of an
erroneous program construct or
erroneous data, for which this
International Standard imposes no
requirements.

Just as a note, that is undefined behavior, but it’s not necessarily a good example. On g++ 4.4.1, it will refuse to compile with:

error: lvalue required as left operand
of assignment

because the result of a post-increment is not an lvalue.