typedef 向前声明,C语言结构的typedef与前向声明

川长思鸟来 2022-08-30 11:16 134阅读 0赞

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I was using typedef for structures everywhere in my application. I then started to refactor into several

header files when it started to get clunky. I noticed I needed to forward declare Object, and Klass.

Well, to my surprise, I couldn't forward declare Object, or Klass. This is because, as you can see in the

Object and Klass structure, I'm using a typedef of Object and Klass.

//Klass.h

typedef struct Klass Klass\_t;

struct Klass

void (\*\_initialize)(Object\_t\* object, Klass\_t\* klass);

\};

//Object.h

typedef struct Object Object\_t;

struct Object

Klass\_t\* \_klass;

\};

At first, using typedef was great. But trying to forward declare Object:

struct Object\_t;

Doesn't work as I would need to rewrite by function declarations as:

void (\*\_initialize)(struct Object\_t\* object, Klass\_t\* klass);

So I decided to just typedef Object inside the Klass.h header file:

typedef struct Object Object\_t;

Well when all header files are included into my Main.c file, it complians:

Object.h:5: error: redefinition of typedef 'Object\_t'

So, I then decided to just drop all struct typedefs and explicity declare my structures.

Is there a way to typedef a structure and forward declare in another file without explicitly using struct Object?

I want to keep structure typedefs inside the header file where

the structure is declared. If I have to group all typedefs inside one header file then I would rather not use typedef at all. Anyways, thanks for your time.

解决方案

Remember that a typedef is just an alternative name for a type. There's a reason the Linux kernel doesn't use typedefs for structure types, and you're demonstrating it.

There are a couple of ways around your problem. I note that C11 does allow multiple occurrences of the same typedef, but I'm assuming you're stuck with an older compiler that does not support that.

TL;DR

Even though the Linux kernel doesn't use typedefs, I usually do use typedefs, but I mostly avoid mutually referential structure types (I can't think of any code where I used such a type). And, like Jens Gustedt notes in his answer, I almost invariably use the notations:

typedef struct SomeTag SomeTag;

so that the type name and the structure tag are the same (they're in different namespaces). This operation is not necessary in C++; when you define struct SomeTag or class SomeTag, the name SomeTag becomes a type name without the need for an explicit typedef (though a typedef does no harm other than revealing that the author is more experienced in C than C++, or the code originated as C code).

I also observe that names starting with an underscore are best treated as 'reserved for the implementation'. The rules are a little more complex than that, but you run a risk of the implementation usurping your names — and being within its rights to usurp your names — when you use names starting with an underscore, so don't. Likewise, POSIX reserves type names ending \_t for the implementation if you include any POSIX headers (such as when you aren't compiling in strict Standard C only mode). Avoid creating such names; they'll hurt you sooner or later. (I've not fixed the code below to deal with either of these issues: caveat emptor!).

In the code fragments below, I'm mostly ignoring the code that prevents multiple inclusions (but you should have it in your code).

Extra header

typedefs.h:

typedef struct Object Object\_t;

typedef struct Klass Klass\_t;

klass.h

\#include "typedefs.h"

struct Klass

void (\*\_initialize)(Object\_t \*object, Klass\_t \*klass);

\};

object.h

\#include "typedefs.h"

struct Object

Klass\_t \*\_klass;

\};

This works because the two type names Klass\_t and Object\_t are declared before they're used.

Use struct Object in prototype

klass.h

typedef struct Klass Klass\_t;

struct Object;

struct Klass

void (\*\_initialize)(struct Object \*object, Klass\_t \*klass);

\};

Or, for consistency, it might even use:

void (\*\_initialize)(struct Object \*object, struct Klass \*klass);

object.h

\#include "klass.h"

struct Object

Klass\_t \*\_klass;

\};

This works because (within broad limits — basically, if the types are defined at file scope, not inside a function) struct Object always refers to the same type, regardless of whether all the details are fully defined yet.

GCC 4.8.2

Under all of -std=c89, -std=c99 and -std=c11, GCC 4.8.2 accepts replicated typedefs, as in the code below. It requires -std=c89 -pedantic or -std=c99 -pedantic to get errors about the repeated typedefs.

Even without the -pedantic option, GCC 4.5.2 rejects this code; however, GCC 4.6.0 and later versions accept it without the -pedantic option.

klass.h

\#ifndef KLASS\_H\_INCLUDED

\#define KLASS\_H\_INCLUDED

typedef struct Klass Klass\_t;

typedef struct Object Object\_t;

struct Klass

void (\*\_initialize)(Object\_t \*object, Klass\_t \*klass);

\};

\#endif /\* KLASS\_H\_INCLUDED \*/

object.h

\#ifndef OBJECT\_H\_INCLUDED

\#define OBJECT\_H\_INCLUDED

typedef struct Klass Klass\_t;

typedef struct Object Object\_t;

struct Object

Klass\_t \*klass;

\};

\#endif /\* OBJECT\_H\_INCLUDED \*/

consumer.c

\#include "klass.h"

\#include "object.h"

Klass\_t k;

Object\_t o;

You'll have to decide whether that's a risk you're willing to take for your code — how important is portability, and to which versions of C (and which C compilers) must it be portable.

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