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.

19 s
19

For C++ vs. C90, there’s at least one way to get different behavior that’s not implementation defined. C90 doesn’t have single-line comments. With a little care, we can use that to create an expression with entirely different results in C90 and in C++.

int a = 10 //* comment */ 2 
        + 3;

In C++, everything from the // to the end of the line is a comment, so this works out as:

int a = 10 + 3;

Since C90 doesn’t have single-line comments, only the /* comment */ is a comment. The first / and the 2 are both parts of the initialization, so it comes out to:

int a = 10 / 2 + 3;

So, a correct C++ compiler will give 13, but a strictly correct C90 compiler 8. Of course, I just picked arbitrary numbers here — you can use other numbers as you see fit.

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