declaration does not declare anything : warning?

Question

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

int main()
{
    struct emp
    {
        struct address
        {
              int a;
        };
        struct address a1;
    };
}

This code shows a warning:-

warning : declaration does not declare anything (enabled by default)

Where as the following code shows no warning

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

int main()
{
    struct emp
    {
        struct address
        {
             int a;
        }a1;
    };
}   

Why 'warning' is displayed in the first code only?

Answer 1

The reason why the compiler is showing the warning is because it doesn't see a name for the variable of type address you defined for the emp struct, even though you do declare something using address on the next line, but I guess the compiler is not smart enough to figure that out.

As you showed, this produces a warning:

struct emp {
  struct address {}; // This statement doesn't declare any variable for the emp struct.
  struct address a1;
};

But not this:

struct emp {
  struct address {} a1; // This statement defines the address struct and the a1 variable.
};

Or this:

struct address {};

struct emp {
  struct address a1; //the only statement declare a variable of type struct address
};

The struct emp {} doesn't show any warnings since this statement is not inside a struct defintion block. If you did put it inside one of those then the compiler will show a warning for that as well. The following will show two warnings:

struct emp {
  struct phone {};
  struct name {};
};