C Vocabulary in Reference to structures

Question

Consider my examples: questions are outlined in the comments.

struct jeff {        //jeff is the tag name correct?
   int age;
   int weight;
};

typedef struct {   // Is it correct to say that this structure has no "tag" name?
  int age;
  int weight;      
}jeff;         // This is an alias for an anonymous structure correct? Not its tag name.

typedef struct jeff {     // Is it correct to say that this struct has a tag name jeff
  int age;               // and an alias Jeffery?
  int weight;
 } Jeffery;

These questions are really just concerned with correct semantics in regards to the C language.

One final Q:

struct {
 int age;
 int weight;
}jeff;         // Why would one want a struct with no name/alias. I don't see the benefit.

[This](http://stackoverflow.com/q/3772968/1410711) might be helpful... — Recker, Feb 24 '13 at 23:40

score 4 · Accepted Answer · edited Feb 25 '13 at 01:01

4

jeff is the tag name correct?

Correct.

Is it correct to say that this structure has no "tag" name?

Exactly.

This is an alias for an anonymous structure correct? Not its tag name.

AFAIK it's called a typedef, but nevertheless, no structure tag here, only a typename.

Is it correct to say that this struct has a tag name jeff and an alias Jeffery?

If you wish so.

Why would one want a struct with no name/alias?

Well, just for fun, perhaps? Maybe it's a temporary variable somewhere and it isn't used anywhere else in the code, at all, so it would have been superfluous to define a struct type for it. (Anyway I don't like this style....)

edited Feb 25 '13 at 01:01

Jonathan Leffler

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answered Feb 24 '13 at 23:44

Perhaps for the sake of hacking generic arguments, one might use an anonymous struct in a macro: `#define generic_foo(...) foo(&(struct { int x; char *y; double z; }){ .x=0, .y=NULL, .z=0.0, __VA_ARGS__ })` followed by: `generic_foo(.x=10);` – autistic Feb 24 '13 at 23:54
1

The nameless struct can be useful to avoid "parallel indexing" of arrays, eg `struct { char *ptr; size_t len; } strings[MAX_ARGS];` – wildplasser Feb 25 '13 at 00:02

score 1 · Answer 2 · answered Feb 25 '13 at 01:12

Use for anonymous structures

One place where an (array of an) anonymous structure can be useful is in code where the data is not needed outside the one function.

const char *info_lookup(int value)
{
    static const struct { int number; char *name; } list[] =
    {
        {  1, "twenty-seven"  },
        { 13, "unfortunately" },
        { 27, "won"           },
        ...
    };
    enum { NUM_LIST_ITEMS = sizeof(list) / sizeof(list[0]) };

    for (i = 0; i < NUM_LIST_ITEMS; i++)
    {
        if (value <= list[i].number)
            return(list[i].name);
    }
    return(0);
}

I also sometimes use it for running tests where the structure captures the test information:

static const struct
{
    const char *ver1;
    const char *ver2;
    int         result;
} test[] =
{
    {   "7.0.4.27", "7.0.4.17", +1 },
    {   "7.0.4.23", "7.0.4.17", +1 },
    {   "7.0.4.23", "7.0.4.27", -1 },
    {   "7.0.4.23", "7.0.5.07", -1 },
    ...20+ tests omitted...
};

enum { NUM_TESTS = DIM(test) };

static const char *result(int i)
{
    if (i < 0)
        return("<");
    else if (i > 0)
        return(">");
    else
        return("=");
}

int main(void)
{
    size_t j;
    int fail = 0;

    for (j = 0; j < NUM_TESTS; j++)
    {
        int r1 = version_compare(test[j].ver1, test[j].ver2);
        int r2 = version_compare(test[j].ver2, test[j].ver1);
        const char *pass_fail = "PASS";
        char extra[32] = "";
        if (r1 != test[j].result)
        {
            pass_fail = "FAIL";
            fail++;
            snprintf(extra, sizeof(extra), " Expected %s", result(test[j].result));
        }
        assert(r1 == -r2);
        printf("%s: %-10s  %s  %s%s\n",
               pass_fail, test[j].ver1, result(r1), test[j].ver2, extra);
    }

    if (fail == 0)
    {
        printf("== PASS == %d tests passed\n", NUM_TESTS);
        return(0);
    }
    else
    {
        printf("!! FAIL !! %d out of %d tests failed\n", fail, NUM_TESTS);
        return(1);
    }
}

It is only appropriate when nothing outside the file needs to know about the structure, and when there's only one variable of the type (or you can declare all the variables of the type in a single declaration, but I usually have just one declarator per declaration, so that amounts to one variable of the type).

If you need to refer to the type more than once, then it needs a name — either the structure tag or the typedef name or both.

C Vocabulary in Reference to structures

2 Answers2

Use for anonymous structures