Question about a struct declaration

[heavyz]

In C, if i want to declare a struct, there are 3 methods to do so:

1: struct dummy { ... };
2: typedef struct { ... } dummy;
3: typedef struct _dummy { ... } dummy;

Usually i use the first method to declare my structs, but in many open
source projects (such as libxml2), the third method is used. I am just
curious about the difference.. Would somebody tell me which method is
recommended, and why? Thanks in advance.

ZHENG Zhong

[Andrey Tarasevich]

Keith Thompson wrote:

>
But the real problem with

typedef struct dummy *dummy;

is that the identifier "dummy" is used both for the struct tag and for
a *pointer to* the struct type. It's common to use the same identifier
for a struct tag and a pointer to the struct, as in:

typedef struct dummy dummy;

But making "struct dummy" a struct and "dummy" a pointer will only
cause confusion, whether you're trying to write dual-language code or
not. Pick two different names.

I'd say that in the implementation hiding situations giving the same
name to the exposed type and to the hidden-struct tag is not a problem,
but (quite the opposite) a way to remind the user, who's thoughtlessly
trying to use 'struct dummy' in his code, that there's something else
named 'dummy' he should really be using instead.

I think the generic word "dummy" obscured the problem in this case.
In real life, the name(s) would be more descriptive, perhaps something
like:

typedef struct widget_info *handle;

No, _that_ would be a bad idea. Giving a descriptive name to something
that is supposed to be hidden is a way to encourage the user to start
making unwarranted assumptions and even attempt break the implementation
protection. There's no reason for the user to know anything
"descriptiv e" about what's hiding behind the 'handle' (besides the
information given in the specification).

--
Best regards,
Andrey Tarasevich

[John Bode]

On Apr 16, 4:04 am, heavyz <heavyzh...@gma il.comwrote:

In C, if i want to declare a struct, there are 3 methods to do so:

1: struct dummy { ... };
2: typedef struct { ... } dummy;
3: typedef struct _dummy { ... } dummy;

Usually i use the first method to declare my structs, but in many open
source projects (such as libxml2), the third method is used. I am just
curious about the difference.. Would somebody tell me which method is
recommended, and why? Thanks in advance.

ZHENG Zhong

Using the typedef name (methods 2 and 3) allows you to sort-of de-
emphasize the "struct-ness" of dummy; you're basically telling the
user of that type to think in terms of dummy as an atomic entity (much
like a scalar), not as a collection of items. See FILE in stdio.h as
an example; we're not expected to know anything about the details of
FILE items, just to pass pointers to them between the various stdio
functions.

As others have pointed out, identifiers with leading underscores
(_dummy) are reserved for the implementation, and should not be used
in application code.