为什么在C中将类型定义为指向未定义结构的指针有效? [英] Why is it valid to define a type as pointer to an undefined struct in C?

问题描述

我正在研究第3方代码库，发现将类型声明为未定义结构的指针显然是有效的.作为最低限度的工作示例，请考虑一个C文件test.c，其中仅包含以下内容:

I was digging into a 3rd party code base and found that it is apparently valid to declare a type as a pointer to an undefined struct. As a minimum working example, consider a C file test.c containing nothing but:

typedef struct foo *bar;

令我惊讶的是，使用该命令可以毫无问题地编译该文件

What surprises me is that this file compiles without any problems using the command

gcc test.c -shared

为什么编译器不抱怨未在任何地方声明struct foo?

Why does the compiler not complain about the struct foo not being declared anywhere?

我的环境是带有gcc的Ubuntu 16.04(Ubuntu 5.4.0-6ubuntu1〜16.04.5)5.4.0 20160609.

My environment is Ubuntu 16.04 with gcc (Ubuntu 5.4.0-6ubuntu1~16.04.5) 5.4.0 20160609.

推荐答案

上面的声明创建了struct foo的转发声明.尽管您无法访问其成员，但是可以在指向它的指针上进行操作.

The declaration above creates a forward declaration of struct foo. Although you can't access its members, you can operate on a pointer to it.

这通常称为不透明类型，用于对库用户隐藏库的实现详细信息.

This is commonly referred to as an opaque type, and is used to hide implementation details of a library from users of the library.

例如，一个库实现可能包含以下内容:

For example a library implementation may contain the following:

lib.c:

struct foo {
  int f1;
};

struct foo *init()
{
    return malloc(sizeof(struct foo));
}

void set1(struct foo *p, int val)
{
    p->f1 = val;
}

int get1(struct foo *p)
{
    return p->f1;
}

void cleanup(struct foo *p)
{
    free(p);
}

该库的头文件可能看起来像这样:

The header file for this library might look like this:

lib.h:

struct foo;

struct foo *init(void);
void set1(struct foo *p, int val);
int get1(struct foo *p);
void cleanup(struct foo *p);

该库的用户将使用init函数创建该结构的实例，并使用set1和get1函数来读取/更新该成员.但是，如果不通过任何一种界面功能，用户将无法创建struct foo的实例或访问成员.

The user of this library would use the init function to create an instance of the struct and the set1 and get1 functions to read / update the member. The user can't however create an instance of struct foo or access the members without going through one of the interface functions.

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