Different output for different compiler - C and C++

Question

Can you think of 'a program' which gives 'different outputs for a C and a C++ compilers' (yet gives consistent output under the same language)?

This is a stupid interview question – BЈовић Mar 29 '11 at 07:47 — BЈовић, Mar 29 '11 at 07:47

score 20 · Answer 1 · answered Mar 29 '11 at 03:33

20

Incompatibilities between ISO C and ISO C++

A common example is sizeof('A'), which is usually 4 in C but always 1 in C++, because character constants like 'A' have the type int in C but the type char in C++:

#include <stdio.h>

int main(void)
{
    printf("%d\n", sizeof('A'));
}

answered Mar 29 '11 at 03:33

Adam Rosenfield

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operator `sizeof` returns `size_t` not `int`. So your code contains error. http://stackoverflow.com/questions/940087/whats-the-correct-way-to-use-printf-to-print-a-size-t – UmmaGumma Mar 29 '11 at 19:20
And `sizeof (int)` could be 1. – Keith Thompson Jan 02 '12 at 18:48

score 20 · Accepted Answer · answered Mar 29 '11 at 04:23

20

This program produces 12 in C++ or C99, and 6 in C89:

#include <stdio.h>

int main()
{
    int a = 12//**/2;
    ;

    printf("%d\n", a);
    return 0;
}

answered Mar 29 '11 at 04:23

caf

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although exact, I look at it more as a hack :) Nice one though. – Matthieu M. Mar 29 '11 at 06:40
1

That doesn't really answer the question, though, does it? It produces the same output in C and C++ (C99 is, after all, the current C standard). – Keith Thompson Oct 17 '11 at 05:09
@KeithThompson: Yes, [ecik's answer](http://stackoverflow.com/questions/5467576/different-output-for-different-compiler-c-and-c/5469636#5469636) is a more accurate match for the question. – caf Oct 17 '11 at 05:15

score 9 · Answer 3 · answered Mar 29 '11 at 03:32

9

int main() { return sizeof 'a'; }

answered Mar 29 '11 at 03:32

Ben Voigt

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But not perfectly portable. `sizeof 'a'` could be 1 in C if `sizeof (int) == 1` (which is possible only if `CHAR_BIT >= 16`). – Keith Thompson Oct 17 '11 at 05:05

score 5 · Answer 4 · answered Mar 29 '11 at 03:47

5

typedef char X;
int main() {
    struct X { double foo; }
    printf("%d\n", sizeof(X));
    return 0;
}

answered Mar 29 '11 at 03:47

Chris Dodd

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Missing `#include `. `"%d"` requires an `int`, not a `size_t`. A conforming implementation could have `sizeof (struct X) == 1` (with, for example, `CHAR_BIT == 64`), fixable by giving `struct X` two `char` members instead of a `double`. – Keith Thompson Jan 02 '12 at 18:47

score 4 · Answer 5 · edited Oct 17 '11 at 05:12

4

From wikipedia, altered to produce consistent output in each language:

extern int T;

int size(void)
{
    struct T {  int i;  int j;  };

    return sizeof(T) == sizeof(int);
    /* C:   return 1
    *  C++: return 0
    */
}

edited Oct 17 '11 at 05:12

caf

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answered Mar 29 '11 at 08:12

ecik

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This returns a different status depending on the language, but it doesn't produce any *output*. – Keith Thompson Feb 19 '15 at 15:58

score 3 · Answer 6 · answered Mar 29 '11 at 03:27

3

int class;

Will not compile in C++ and will compile in C.

answered Mar 29 '11 at 03:27

Seva Alekseyev

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Wow!! many ideas all in one place... nice.. – Aditya369 Mar 29 '11 at 05:37
1

The question was about program output, not compiler output. – Björn Pollex Mar 29 '11 at 08:17

score 2 · Answer 7 · answered Oct 17 '11 at 05:07

2

#include <stdio.h>
int main(void)
{
#ifdef __cplusplus
    puts("C++");
#else
    puts("C");
#endif
    return 0;
}

answered Oct 17 '11 at 05:07

Keith Thompson

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The preprocessor is part of the language? – harper Oct 17 '11 at 05:45
@harper: Yes. It's section 6.10 of the C99 standard, section 16 of the C++ standard. – Keith Thompson Oct 17 '11 at 06:02

Different output for different compiler - C and C++

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