Re: newb help: making sense of typedef enum...

From: Chris Torek (nospam_at_torek.net)
Date: 05/13/04 

Date: 13 May 2004 05:57:43 GMT

In article <939f49f8.0405122057.56b5d67d@posting.google.com>
Chris <iamlevis3@hotmail.com> writes:
>I'm trying to make sense of the following. ...
>typedef enum { DOUBLE_LIST, INT_LIST } DATA_TYPE;

The typedef here is something of a red herring.

The best thing to do, as far as I am concerned, for understanding
purposes, is to remove all the typedefs from C code. In this case,
that also requires adding an enumeration tag, giving:

    enum list_enum { DOUBLE_LIST, INT_LIST };

This defines a new type, "enum list_enum", with two enumeration
members. The two enumeration members are just names for integer
constants, numbered sequentially starting from 0 (in this case).

All the typedef does, once it is re-inserted, is make an alias
-- another name -- for the type "enum list_enum":

    typedef enum list_enum DATA_TYPE;

Now you can type "DATA_TYPE" wherever you would otherwise have to
type "enum list_enum". (In fact, there is no requirement that the
typedef come second, and if you want to use typedefs with linked-list
structures, I recommend putting the typedef line first. I prefer
not to use typedef at all, myself, though.)

>typedef struct {
> DATA_TYPE type;
> int number_values;
>} section_t;

Here is another one of these silly mixed-up typedef lines again.
Assuming you *want* to use typedefs (for whatever strange reason),
I still claim it is better to write the typedef first, *then* the
struct declaration. Of course, you will have to add a tag, just
as before:

    typedef struct section section_t;
    struct section {
        DATA_TYPE type; /* or: enum list_enum type; */
        int number_values;
    };

Again, the typedef does not really add anything -- the "real"
type is "struct section"; "section_t" is just an alias for this,
an extra name that lets you skip the word "struct".

It is worth noting here that both "section_t" and "DATA_TYPE"
are spelled specially: the former ends in "_t", and the latter
is in all-capital-letters. When using typedefs, it *is* a good
idea to mark them specially, because they kind of goof up the
syntax of C programs. Ordinary declarations will have an
instantly recognizable keyword in front:

    int i;
    struct S somestruct;
    void f(void);
    double (d); /* unnecessary parentheses, but for illustration */

But suppose you see:

    zog (x);

Is this a declaration of the variable "x" of type "zog", where
"zog" is a typedef-alias for some other type? Or is it a call
to the function zog(), passing the value of x? If all your
typedefs are consistently marked, you can tell:

    zog_t (x); /* this one is a variable declaration */
    mog (y); /* this one is a call to a function */

In this case, one typedef uses an "_t" suffix, and the other uses
all-uppercase. It seems a bit odd to mix conventions like this.

>Now, I typed this stuff into a small program and ran it though gcc.
>gcc had no complaints, but I'm puzzled as to how, in the struct,
>DATA_TYPE can be a valid type, since in the enum there is DOUBLE_LIST
>and INT_LIST... nothing that seems like a type that a compiler would
>understand without more information.

Enumeration members are just aliases for integer constants. Even
if the enumeration does not have a tag, you can even use this to
define constants -- real, true constants, not read-only "const"
variables -- that you can use wherever you would otherwise use a
constant:

    enum { MAX_LEVELS = 30 };

    int some_table[MAX_LEVELS];

The original enumeration did not have a tag, but its type -- an
otherwise un-nameable type -- is "captured" by the surrounding
typedef:

    typedef enum /* no tag */ { MEMBER } alias;

Now "alias" is an alias for the missing enumeration tag. The
members are just integer constants, and in this case just 0 and
1 respectively. So:

    if (i == INT_LIST)

means exactly the same thing as:

    if (i == 1)

To declare i as a variable with the enumerated type, you might
normally write either:

    enum the_tag i;

or:

    alias i;

but here the tag is missing, so you have to use the typedef-alias. 

-- 
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W)  +1 801 277 2603
email: forget about it   http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

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