在struct中使用函数指针-C编译器

Question

I found an answer how to make function pointer in struct but I am still curious about its operation. Can anybody explain this clearly?

Here is my question,

This code runs properly but...

char func_1(void);    
char func_2(char);

struct mStruct
{
    char name[100]; 
    int age;
    char (*func_1)(void);
    void (*func_2)(char);
};

void init_struct(struct mStruct *pStruct)<-why this void function is necessary? 
{
    if(pStruct == NULL) {
        pStruct = malloc(sizeof(struct mStruct));
    }

    (*pStruct).age = 25;
    (*pStruct).func1 = &func1;
    (*pStruct).func2 = &func2;
 }

 char func_1(void)
 {
   ... ;
  }    
 char func_2(char)
 {
   ... ;
  }

I already tried to eliminate the init_struct function, but all tries failed. My gcc compiler only accepts it as above. Does anybody know another way to initialize the struct without using a function or why it is only acceptable as a void function?

Answer 1

Actually, besides the title, your question seems to have nothing to do with function pointers.

You have a struct mStruct with some members. For your question, it does not really matter what these members are.

With this struct, you don't need an initialization function. You can always simply use the struct and initialize it:

struct mStruct s;
s.member = 20;

But, this comes with a cost: You repeat yourself and when you want to change the struct (say, add another member), you will have to change lot of places. That's bad and can be fixed with an initialization function:

void init_struct(struct mStruct *pStruct)
{
    pStruct->member = 20;
}

struct mStruct s;
init_struct(&s);

Now, you should not add other responsibilities to init_struct than to initialize it. In your example, you also allocate memory for the struct, in a buggy way. Instead, use another function for that:

struct mStruct* create_struct()
{
    struct mStruct *pStruct = (struct mStruct*) malloc(sizeof(struct mStruct));
    init_struct(pStruct);
    return pStruct;
}

struct mStruct *pStruct = create_struct();
do_something_with(pStruct);
free(pStruct); // Don't forget this!

If you need more cleanup than just freeing the memory, write yet another function for it:

void destroy_struct(struct mStruct *pStruct)
{
    cleanup(pStruct);
    free(pStruct);
}

struct mStruct *pStruct = create_struct();
do_something_with(pStruct);
destroy_struct(pStruct); // Don't forget this!