隐藏C结构定义 [英] Hiding C struct definition

问题描述

下面是我的设置：

在public.h：

#ifndef PUBLIC_H_
#define PUBLIC_H_

#include "func.h"

/*extern typedef struct _my_private_struct PRIVATE_;*/
typedef struct _my_private_struct PRIVATE_; /* Thanks to larsmans and Simon Richter */
#endif

在struct.h

#ifndef STRUCT_H_
#define STRUCT_H_

struct _my_private_struct {
    int i;
};
#endif

在func.h：

#ifndef FUNC_H_
#define FUNC_H_
#include "struct.h"

/* typedef struct _my_private_struct PRIVATE_; */
extern PRIVATE_ * get_new(int);
#endif

在func.c：

#include <stdlib.h>
#include "func.h"

PRIVATE_ * get_new(int i)
{
    PRIVATE_ *p = (PRIVATE_ *) malloc(sizeof(PRIVATE_));
    if (p == NULL) return NULL;

    p->i = i;

    return p; 
}

在main.c中：

#include "public.h"

int main(int argc, char ** argv)
{
    PRIVATE_ *p = get_new(2);
    return 0;
}

当我编译的文件与海湾合作委员会，我得到这个错误：

When I compile those file with GCC I'm getting this error:

OLD编译错误

多个存储类声明指定

编译错误后编辑

预期=，，，;，前ASM或__attribute__''*'标记

expected '=', ',', ';', 'asm', or '__attribute__' before '*' token

有人能帮助我/解释为什么我得到这个问题以及如何解决它？

Can someone help me out/explain why I'm getting this and how to fix it?

推荐答案

其他答案涉及您的问题pretty好。但是，请允许我加入他们，并回答您的最新评论：

The other answers cover your problem pretty well. However, allow me to add to them and answer your latest comment:

我收到编译错误：在public.h：重新定义的typedef PRIVATE _...

I'm getting compile error: in public.h: redefinition of typedef PRIVATE_...

虽然该错误是不言自明，它可能不太清楚为什么会发生的事情。考虑当包括public.h会发生什么：

While the error is self-explanatory it's probably less clear why that's happening. Consider what happens when you include public.h:

#include "struct.h"
#include "func.h"
typedef struct _my_private_struct PRIVATE_;

如果您通过跟踪这一点，完全展开preprocessor，这就是你会得到：

If you trace through this and fully expand the preprocessor, this is what you'll get:

// struct.h
struct _my_private_struct
{
    int i;
};

// func.h
typedef struct _my_private_struct PRIVATE_;
extern PRIVATE_ * get_new(int);

// public.h
typedef struct _my_private_struct PRIVATE_;

为什么你遇到了问题应该现在是显而易见的。如果没有func.h typedef的，因为它没有看到私人但是你的 get_new 原型失败。 OTOH，如果你离开你已经两次将它定义typedef的。

It should now be obvious why you're running into problems. Without the typedef in func.h, your get_new prototype fails because it hasn't seen PRIVATE yet. OTOH, if you leave the typedef in you've defined it twice.

此外，它看起来像你想保持这一结构与其他code和模块专用。即使你做修复生成错误，你还没有真正实现了封装。试想一下：

Also, it looks like you're trying to keep that structure private from other code and modules. Even if you do fix the build errors you haven't really achieved that encapsulation. Consider this:

int main()
{
    PRIVATE_ *p = get_new(2);
    p->i = 1337;        // HAHA, I just modified your private i.
                        // what are you going to do about it?
}

如果你想在C数据隐私考虑一个不透明的指针设计。我建议你​​重组来源是这样的：

If you want data privacy in C consider an opaque pointer design. I recommend restructuring your source like this:

// public.h
#ifndef PUBLIC_H_
#define PUBLIC_H_

#include "func.h"

#endif

---

// func.h
#ifndef FUNC_H_
#define FUNC_H_

struct PRIVATE_NOT_ACCESSIBLE;
typedef struct PRIVATE_NOT_ACCESSIBLE myint_t;

// declare your struct methods here
myint_t* get_new(int);
// ..

#endif

---

// func.c
#include <stdlib.h>
#include "func.h"

// define this only with functions 
// that's suppose to work with its internal data
struct PRIVATE_NOT_ACCESSIBLE
{
    int i;
};

myint_t * get_new(int i)
{
  // ...
}

现在，如果你试试这个：

Now if you try this:

#include "public.h"

int main()
{
    myint_t *p = get_new(2);
    p->i = 1337;            // Aw, doesn't work anymore :(
}

编辑：：要回答下面的OP的评论

To answer the OP's comment below.

如果你有一个以上的编译单元来实现私有结构的方法，你还可以把它通过移动专用的定义到一个专门的工作头：

If you have private struct's methods implemented in more than one compilation unit you can still make it work by moving private's definition to a dedicated header:

// func_implementation.h
#include "func.h"
struct PRIVATE_NOT_ACCESSIBLE
{
    int i;
};
// internal methods, helper functions you don't want exposed should go here too.
// eg.
void helper_method(myint_t *);

这是实现你的结构私人对象的源文件将包括func_implementation.h。使用私人将包括func.h外部客户code只。

Source files that implement your struct private 'object' will include 'func_implementation.h'. External client code that uses private will include 'func.h' only.

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