Question

C and C++ have many differences, and not all valid C code is valid C++ code.
(By “valid” I mean standard code with defined behavior, i.e. not implementation-specific/undefined/etc.)

Is there any scenario in which a piece of code valid in both C and C++ would produce different behavior when compiled with a standard compiler in each language?

To make it a reasonable/useful comparison (I’m trying to learn something practically useful, not to try to find obvious loopholes in the question), let’s assume:

• Nothing preprocessor-related (which means no hacks with #ifdef __cplusplus, pragmas, etc.)
• Anything implementation-defined is the same in both languages (e.g. numeric limits, etc.)
• We’re comparing reasonably recent versions of each standard (e.g. say, C++98 and C90 or later)
  If the versions matter, then please mention which versions of each produce different behavior.

Answer 1

The following, valid in C and C++, is going to (most likely) result in different values in i in C and C++:

int i = sizeof('a');

See Size of character ('a') in C/C++ for an explanation of the difference.

Another one from this article:

#include <stdio.h>

int  sz = 80;

int main(void)
{
    struct sz { char c; };

    int val = sizeof(sz);      // sizeof(int) in C,
                               // sizeof(struct sz) in C++
    printf("%d\n", val);
    return 0;
}