C - function inside struct

Question

Im trying to assign a function inside a struct, so far I have this code:

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.*

Which is the correct way to do it ?

Answer 1

It can't be done directly, but you can emulate the same thing using function pointers and explicitly passing the "this" parameter:

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.

Answer 2

As others have noted, embedding function pointers directly inside your structure is usually reserved for special purposes, like a callback function.

What you probably want is something more like a virtual method table.

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_t structure. 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_t interface to be able to augment how the operations behave.

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.

Answer 3

How about this?

#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;
} 

Answer 4

This will only work in C++. Functions in structs are not a feature of C.

Same goes for your client.AddClient(); call ... this is a call for a member function, which is object oriented programming, i.e. C++.

Convert your source to a .cpp file and make sure you are compiling accordingly.

If you need to stick to C, the code below is (sort of) the equivalent:

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);

}

Answer 5

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.

It is not necessary to reinvent object-orientation from scratch for a C program, which is not an object oriented language.

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.

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.

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.

A C function often returns only a status byte or int and that is often ignored. In your case a proper form might be

 /* add client to struct, return 0 on success */
 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

Answer 6

Suppose you have the following struct:

struct Object {
    int field;
}

In C, there is no good way to write object.Add(1) and make Add use/change fields of object. You have two options:

a) Abandon the wish to write object.Add(1) and write idiomatic C code instead. Trying to find a way to write object.Add(1) in C using some tricks will just make the code more complex, more bug prone, and is not worth it.

The equivalent way to do it in C is to have a separate function accept a pointer to the object:

void Add(struct Object *object, int amount) {
    object->field += amount;
}

And call this function as follows:

struct Object object;
Add(&object, 1);

b) Use another programming language. For example, in C++, you can define a method:

struct Object {
    int field;
    void Add(int amount) {
        field += amount;
    }
}

and use it in your code:

Object object;
object.add(1);

The C++ compiler will make it work on your behalf. In fact, as long as inheritance is not involved, the C++ code will be equivalent to the function definition and its invocation mentioned in (a).

Answer 7

You can pass the struct pointer to function as function argument. It called pass by reference.

If you modify something inside that pointer, the others will be updated to. Try like this:

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 *client)
{
        /* this will change the original client value */
        client.password = "secret";
}

int main()
{
    client_t client;

    //code ..

    AddClient(&client);
}