Different C ++ and C functions

1. In C ++ function must have a return type, and C defaults to int.
2. C ++ and return the actual return type must be the same type of return, or an implicit conversion may be performed, C and no such limitation can be arbitrarily return.
3. Can not have a C ++ function is called without parameter arguments, C can be used in any type, any number of parameters invoked.

   f(void)
   {char a = 0;
       double d = 0;
       if(a == 'a')
       {
           return &a;
       }
       else
       {
           return d;
       }
   }
   int main ()
   {
       int i = 0;
       double d = 0;
       double *pd = &d;
       f(i, pd);
       return 0;
   }

   Compiling pure C implementations, a warning will be given, but by default the compiler (many compilers except as provided by the warning as errors). But given C ++ compiler error. vs2015 with compiler error message is given as follows: