Why this different behaviour of structure initialization in C and C++?

Question

Code :

#include <stdio.h>

typedef struct{
  int i;
  int tab[];
}mystr;

mystr a = {1,{0,1,2}};

int main(void){
  printf("%d%d%d\n",a.i,a.tab[0],a.tab[1]);
  return 0;
}

C compiling :

$ gcc main.c -o main && ./main
101

C++ compiling :

$ g++ main.c -o main && ./main
main.c:8:27: error: too many initializers for ‘int [0]’
 const mystr a = {1,{0,1,2}};
                           ^

I do understand the problem, it is that tab have no any memory allocated. But why this is ok for C ? Why do C++ doesn't automatically allocate the memory space for the tab ? I Know that structures are similar to objects in C++ but may be there is some subtleties that I do not know about global object instantiation ?

Answer 1

The code is invalid in C. This is not allowed in C

typedef struct{
  int i;
  int tab[];
}mystr;

mystr a = {1,{0,1,2}};

Whatever behavior you observe in C mode is a quirk of your compiler that has nothing to do with the language. The compiler supports the code as a non-standard feature, an extension.

Your code is also invalid in C++, but for a reason that is completely different from what was reported by your compiler. In C++ you are not allowed to have [] arrays as class members at all. The compiler lets it slip through (as an extension again), but fails in another spot later.

So, the answer to your question would be: the compiler you are using has different quirks/extensions in C mode than in C++ mode. Which is not surprising, since these are very different languages.

Answer 2

Everything in the upvoted answer might be accurate, but it dances around the root of your question. You're absolutely right - on debian and alpine docker images, and a gcc invocation that doesn't include -pedantic does compile this and run it without complaint:

root@45c9da7956c8:/# cat t.c; gcc -std=c11 -o t t.c && ./t;
#include <stdio.h>

typedef struct{
  int i;
  int tab[];
}mystr;

mystr a = {1,{0,1,2}};

int main(void){
  printf("%d%d%d\n",a.i,a.tab[0],a.tab[1]);
  return 0;
}
101

Gcc is a very accomodating compiler, and will work with things that can only be described accurately as out of spec. That doesn't necessarily make it a bad compiler - but it does make it a poor test of whether code is standards compliant. -pedantic will help with that.

I'll echo the point made in the question comments that it really is conceptually best to think of C and C++ as completely different languages. At least, I can't think of a case where getting C code to compile on a c++ compiler would have any value. Thinking of them as completely separate is a valid model for how they act in real life.