Comments in C89 and C++
In C89 comments are enclosed between /* and */. // is not standard, altought many compilers support it. However, you can force your compiler not to recognise it. This is the key for the sequent hack: when compiled as C it prints 10, when compiled as C++, it prints 100.
#include <stdio.h>
int main(){
int a = 100//**/10
;
printf("%d\n", a);
return 0;
}
How is that possible?
In particular expression 100//**/10 evaluates to "10". /**/ is a comment and is not considered, so it remains 100/10 (remember that // is not a comment delimiter).
When compiling as a C++ file, everything after // is a comment, and so 100//**/10 evaluates to 100.
This does not work with C99 anymore. // comments are valid in C99.
How to force C89 mode
To force gcc to compile it as strict C89 use -pedantic -std=C89.
I chose to use the .c extension, since using .cc the compiler seems to force C++ compilation (unless you specify the language).
In fact when you compile a C++ file with gcc (ggc, not g++), it understands it is a C++ file and compiles it as a C++ file. However, you will see errors. They are not compiling errors, they are linking ones.
For example:
/usr/bin/ld: Undefined symbols:
std::basic_string<char, std::char_traits<char>, std::allocator<char> >::find(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned long) const
...
operator delete[](void*)
operator delete(void*)
operator new[](unsigned long)
operator new(unsigned long)
___cxa_allocate_exception
___cxa_begin_catch
___cxa_end_catch
___cxa_rethrow
___cxa_throw
___gxx_personality_v0
restFP
saveFP
collect2: ld returned 1 exit status
gcc does not link with libstdc++ library. You have to specify -lstdc++ and everything works. However, in this case, when compiling the C++ version, use g++.
