this question was asked to my friend in an interview. "can you write any program that will compile in C but not in C++". is there any such program?
This pretty much invalidates any other answer we might come up with, right?
Manuel
2010-02-12 12:53:29
Not really - you can break C++ programs in more ways, by combining some differences in creative ways. But all the not-so-creative answers are in there.
MSalters
2010-02-12 13:34:57
A:
Depending on which version of C, I guess I would've chickened out and put
int main() {
/* This is invalid in pre-C99! */
}
:)
Roel
2010-02-12 12:43:10
MSalters
2010-02-12 13:27:14
+5
A:
#include <stdlib.h>
int main()
{
char* p = malloc(10);
return 0;
}
visitor
2010-02-12 12:47:48
because of the implicit void* to char* conversion that a C++ compiler won't swallow: http://codepad.org/2WFJP61O
Manuel
2010-02-12 12:56:32
+2
A:
int main(int new, char **class) {}
Compiles in C, but not in C++ because in C++ new and class are keywords and can't be used as identifiers.
For C89, you also need return 0; in the body of the function. Using C99 was a bit of a cheat anyway, because C99 has loads of features which aren't in C++. Here I use complex types, stdint, restrict and VLAs:
#include <complex.h>
#include <stdint.h>
typedef double _Complex dc;
int funny_average(dc * restrict a, dc * restrict b, uint16_t n) {
dc array[n+2];
memset(array, 0, sizeof array);
array[0] = *a;
array[1] = *b;
return normal_average(array, n + 2);
}
You get the idea. Some, all, or none of these might be supported as extensions by any given C++ implementation, but they aren't standard (Actually, uint16_t is optional in C99, so strictly speaking this code might not compile as C either).
Steve Jessop
2010-02-12 12:47:51
+3
A:
int main()
{
int x[-2 + sizeof('a')];
return 0;
}
Ooh, ooh!
#ifdef __cplusplus
FAIL TO COMPILE HORRIFICALLY!
#else
int main() { return 0; }
#endif
Kaz Dragon
2010-02-12 13:04:29
Strictly speaking, that might not compile as C either, since `sizeof(int)` might be <= 2. But it has a good chance.
Steve Jessop
2010-02-12 13:10:31
Yes, sizeof('a') == sizeof(int) in C and sizeof('a') == sizeof(char) in C++, but nothing prevent sizeof(int) to be 1 or 2 in C.
AProgrammer
2010-02-12 13:12:50
It would work if sizeof(int) was 2 as well, because you can have zero-sized arrays in C++ at least.But yes, failure if sizeof(int) == sizeof(char).
Kaz Dragon
2010-02-12 13:22:30
+28
A:
Why complicate things?
int main() {
int class = 7;
}
Thomas Padron-McCarthy
2010-02-12 13:22:35
A:
This is a shot:
main(int argc,char**argv){
char *p=malloc(0);
return 0;
}
Best regards, Tom.
tommieb75
2010-02-12 13:27:44
malloc(0) is actually implementation dependent in C, and I doubt that any C compilers check it at compile time, which was the question. C++ reference the C standard WRT malloc(), so I don't see how this addresses the question there either. Of course, if you are talking about void * conversions, two answers have already covered this.
anon
2010-02-12 14:01:53
The use of implicit declaration of `main` as a function that returns an `int` was actually a first. +1 !
RaphaelSP
2010-04-12 14:00:00
A:
int main(int argc, char * * argv)
{
int status;
while (argc > 0)
{
status = main(argc - 1, argv);
}
return EXIT_SUCCESS;
}
In C, the main function can be executed from inside the program.
(Critics: If this is not true, note the version of C where the specification changed.)
Thomas Matthews
2010-02-12 17:39:27