Incompatibility between C and C++ code

Question

The given C code

#include <stdio.h>
int x = 14; 
size_t check()
{
   struct x {};
   return sizeof(x); // which x
}
int main()
{
    printf("%zu",check()); 
    return 0;
}

gives 4 as output in C on my 32 bit implementation whereas in C++ the code

#include <iostream>
int x = 14;    
size_t check()
{
   struct x {};
   return sizeof(x); // which x    
}
int main()
{
    std::cout<< check(); 
    return 0;
}

outputs 1. Why such difference?

Answer 1

In C++ class declaration struct x {}; introduces the name x into the scope of check and hides x (previously declared as int at file scope). You get 1 as the output because size of empty class cannot be zero in C++.

In C, an inner scope declaration of a struct tag name never hides the name of an object or function in an outer scope.You need to use the tag name struct to refer to the typename x (struct). However you can't have an empty struct in C as it violates the syntactical constraints on struct(however gcc supports it as an extension).