C语言 C - 结构内的函数
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C - function inside struct
提问by xRed
Im trying to create a function inside a structure, so far i have this code:
我试图在结构内创建一个函数，到目前为止我有这个代码：
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
pno AddClient()
{
/* code */
}
};
int main()
{
client_t client;
//code ..
client.AddClient();
}
Error: client.h:24:2: error: expected ‘:', ‘,', ‘;', ‘}' or ‘attribute' before ‘{' token.
错误：client.h:24:2：错误：在“{”标记之前应为“:”、“、”、“;”、“}”或“属性”。
Which is the correct way to do it ?
哪个是正确的方法？
回答by FatalError
It can't be done directly, but you can emulate the same thing using function pointers and explicitly passing the "this" parameter:
它不能直接完成，但您可以使用函数指针并显式传递“this”参数来模拟相同的事情：
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
pno (*AddClient)(client_t *);
};
pno client_t_AddClient(client_t *self) { /* code */ }
int main()
{
client_t client;
client.AddClient = client_t_AddClient; // probably really done in some init fn
//code ..
client.AddClient(&client);
}
It turns out that doing this, however, doesn't really buy you an awful lot. As such, you won't see many C APIs implemented in this style, since you may as well just call your external function and pass the instance.
然而，事实证明，这样做并没有真正给你带来很多好处。因此，您不会看到许多以这种方式实现的 C API，因为您也可以只调用外部函数并传递实例。
回答by jxh
As others have noted, embedding function pointers directly inside your structure is usually reserved for special purposes, like a callback function.
正如其他人所指出的，直接在结构中嵌入函数指针通常保留用于特殊目的，例如回调函数。
What you probably want is something more like a virtual method table.
您可能想要的是更像虚拟方法表的东西。
typedef struct client_ops_t client_ops_t;
typedef struct client_t client_t, *pno;
struct client_t {
/* ... */
client_ops_t *ops;
};
struct client_ops_t {
pno (*AddClient)(client_t *);
pno (*RemoveClient)(client_t *);
};
pno AddClient (client_t *client) { return client->ops->AddClient(client); }
pno RemoveClient (client_t *client) { return client->ops->RemoveClient(client); }
Now, adding more operations does not change the size of the client_tstructure. Now, this kind of flexibility is only useful if you need to define many kinds of clients, or want to allow users of your client_tinterface to be able to augment how the operations behave.
现在，添加更多操作不会改变client_t结构的大小。现在，这种灵活性仅在您需要定义多种客户端或希望允许client_t界面用户能够增强操作行为方式时才有用。
This kind of structure does appear in real code. The OpenSSL BIO layer looks similar to this, and also UNIX device driver interfaces have a layer like this.
这种结构确实出现在实际代码中。OpenSSL BIO 层看起来与此类似，而且 UNIX 设备驱动程序接口也有这样一个层。
回答by QSQ
This will only work in C++. Functions in structs are not a feature of C.
这仅适用于 C++。结构体中的函数不是 C 的特性。
Same goes for your client.AddClient(); call ... this is a call for a member function, which is object oriented programming, i.e. C++.
同样适用于您的 client.AddClient(); call ...这是对成员函数的调用，是面向对象的编程，即C++。
Convert your source to a .cpp file and make sure you are compiling accordingly.
将您的源代码转换为 .cpp 文件并确保您进行了相应的编译。
If you need to stick to C, the code below is (sort of) the equivalent:
如果你需要坚持使用 C，下面的代码是（有点）等价的：
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
};
pno AddClient(pno *pclient)
{
/* code */
}
int main()
{
client_t client;
//code ..
AddClient(client);
}
回答by rogal
How about this?
这个怎么样？
#include <stdio.h>
typedef struct hello {
int (*someFunction)();
} hello;
int foo() {
return 0;
}
hello Hello() {
struct hello aHello;
aHello.someFunction = &foo;
return aHello;
}
int main()
{
struct hello aHello = Hello();
printf("Print hello: %d\n", aHello.someFunction());
return 0;
}
回答by Chris Reid
You are trying to group code according to struct. C grouping is by file. You put all the functions and internal variables in a header or a header and a object ".o" file compiled from a c source file.
您正在尝试根据结构对代码进行分组。C 分组是按文件的。你把所有的函数和内部变量放在一个头文件或一个头文件和一个从 ac 源文件编译的对象“.o”文件中。
It is not necessary to reinvent object-orientation from scratch for a C program, which is not an object oriented language.
没有必要为 C 程序从头开始重新发明面向对象，C 程序不是面向对象的语言。
I have seen this before. It is a strange thing. Coders, some of them, have an aversion to passing an object they want to change into a function to change it, even though that is the standard way to do so.
我以前见过这个。这是一件奇怪的事情。编码人员，其中一些人，厌恶将他们想要更改的对象传递为一个函数来更改它，即使这是这样做的标准方法。
I blame C++, because it hid the fact that the class object is always the first parameter in a member function, but it is hidden. So it looks like it is not passing the object into the function, even though it is.
我责怪 C++，因为它隐藏了类对象始终是成员函数中的第一个参数的事实，但它是隐藏的。所以看起来它没有将对象传递给函数，即使它是。
Client.addClient(Client& c); // addClient first parameter is actually
// "this", a pointer to the Client object.
C is flexible and can take passing things by reference.
C 很灵活，可以通过引用传递事物。
A C function often returns only a status byte or int and that is often ignored. In your case a proper form might be
AC 函数通常只返回一个状态字节或 int 并且经常被忽略。在您的情况下，适当的形式可能是
err = addClient( container_t cnt, client_t c);
if ( err != 0 )
{ fprintf(stderr, "could not add client (%d) \n", err );
addClient would be in Client.h or Client.c
addClient 将在 Client.h 或 Client.c 中
