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?

can u pls post your include statements too and macros if u used any? — arunb2w, Feb 06 '14 at 18:23
@rullof : ya but data members do not make any difference in this case — kevin gomes, Feb 06 '14 at 18:32
Please add some members to make the question more clear for people answering. — rullof, Feb 06 '14 at 18:33
The warning is not related to the fact that the `address` struct doesn't have any variables, but the fact that its declaration statement doesn't define any variables. — juan.facorro, Feb 06 '14 at 18:42
@juan.facorro : but after the `struct address`, I have defined a variable. — kevin gomes, Feb 06 '14 at 18:44
@kevingomes Yes, you have, but not in the `struct address {};` statement, which precedes `struct address a1;`, this is why the compiler warns you about a statement that doesn't declare a variable in the `emp` struct. — juan.facorro, Feb 06 '14 at 18:47
@juan.facorro : then why the compiler does not show any warning for `struct emp` as I have also not created any variable of `struct emp` type. — kevin gomes, Feb 06 '14 at 18:49
@kevingomes Just updated the question to explain why the `emp` is not causing a warning. — juan.facorro, Feb 06 '14 at 18:54
wow..........may I know the reason for the downvote???????????? — kevin gomes, Feb 06 '14 at 18:55
There is no reason for defining a structure inside another as it's no longer accessible outside of the parent structure scope. (I didn't down voted). — rullof, Feb 06 '14 at 19:01
@rullof : I know that, but my question was not about the application of defining a structure within another. — kevin gomes, Feb 06 '14 at 19:03
You should stick to the role otherwise you should expect errors. — rullof, Feb 06 '14 at 19:06

juan.facorro · Accepted Answer · 2014-02-06T18:50:24.377

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 {};
};

Antti Haapala -- Слава Україні · Answer 2 · 2018-03-18T23:36:21.227

The reason the warning is displayed is that the first excerpt is not proper C - it has a constraint violation that a standards-compliant C compiler must produce a diagnostisc message for. It violates the C11 6.7.2.1p2:

Constraints

A struct-declaration that does not declare an anonymous structure or anonymous union shall contain a struct-declarator-list.

Meaning that it is OK to write

struct foo {
    struct {
          int a;
    };
};

since the inner struct declares an anonymous structure, i.e. it is not named.

But in your example the struct address has a name - address - and therefore it must have a declarator list after the closing brace - declarator list being for example a1 as in your example, or more complex foo, *bar, **baz[23][45].

I don't think so, what you quote means (in a not very precise way): a struct with a name must has members which have not only types but also declarators(variable names), what you quote has nothing to do with this issue. — , Jul 16 '19 at 07:31

score 5 · Answer 3 · answered Feb 06 '14 at 18:53

The syntax of a structure definition is:

struct identifier {
    type member_name;

    // ...

};

If you add an identifier just after the closing curly brace, you're declaring a variable with that defined struct.

In your first example the compiler consider the address struct as member type. it's like if you writes:

struct identifier {

    type ; // No member name is specified
    type a1;

    // ...

}

But in the second example you specified the member name:

struct identifier {

    type a1; // Member name specified

    // ...

}

And here is an example of the warning: http://ideone.com/KrnYiE.

score 0 · Answer 4 · answered Jun 11 '20 at 22:08

0

Going to also throw this out there that this error may also show up if you don't properly forward declare types found in other namespaces.

https://stackoverflow.com/a/19001722/1330381

Example

namespace A {

// forward declarations
namespace X {
namespace Y {
class MyType;
}
}

namespace B {

class Foo() {
    void methodBar(const X::Y::MyType& mt);
}
}

answered Jun 11 '20 at 22:08

jxramos

7,356
6
57
105

This is not C and doesn't solve the question. – vmemmap Jul 13 '22 at 15:34

score 0 · Answer 5 · answered Oct 01 '22 at 23:02

0

In my case, i had the same warning because the structure name was previously used and changing the structure name solved the problem.

answered Oct 01 '22 at 23:02

seyyed

63
6

declaration does not declare anything : warning?

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