Nested structures

Question

The following code compiles on a C++ compiler.

#include <cstdio>
int main()
{
    struct xx
    {
        int x;
        struct yy
        {
            char s;
            struct xx *p;
        };
        struct yy *q;
    };
}

Would there be any difference in behavior while compiling with a C compiler?
i.e. would there be any compiler error?

Answer 1

The code in your post is obviously incomplete, just declarations, so it is hard to say anything conclusive.

On obvious difference is that in C++ the inner struct type will be a member of outer struct type, while in C language both struct types will be members of the same (enclosing) scope. (BTW, was it your intent to declare them locally in main?).

In other words, in C++ the code that follows would have to refer to the structs as xx and xx::yy, while in C they would be just xx and yy. This means that the further code would look different for C and C++ and if it will compile in C++, it wouldn't compile in C and vice versa.

Added: C language prohibits declaring struct types inside other structs without declaring a member of that type. So, you struct yy declaration is illegal in C and will produce compiler diagnostic message. If you wanted your code to become legal in both C and C++, you'd have to combine the struct yy declaration with some data member declaration. In your case that could be pointer q:

struct xx {
        int x;
        struct yy {
                char s;
                struct xx *p;
        } *q;
};

The above is legal in both C and C++ (taking into account the differences I explained earlier), but your original declaration is not legal in C.

Answer 2

C does not permit you to nest a type declaration inside a function definition. Also, to eliminate the warning about "does not declare anything, you should merge the declarations of type struct yy and member q. The following compiles with gcc with max warnings on:

struct xx
{
        int x;
        struct yy
        {
                char s;
                struct xx *p;
        } *q;
};

int main()
{
  return 0;
}