导致类型“struct A”定义不完整的循环引用

Question

The cyclic reference looks like this. A -> B -> C -> A

I've forward declared A within C to resolve the earlier issue but then IntelliSense is not working or compiler is unable to reach out the fields of A.

Edit included guards.

ah

#ifndef CYCLIC_A_H
#define CYCLIC_A_H

#include "B.h"

typedef struct A A;

struct A {
    B *b;
    int value;
};

#endif //CYCLIC_A_H

bh

#ifndef CYCLIC_B_H
#define CYCLIC_B_H

#include "C.h"

typedef struct B B;

struct B {
    C *c;
};

#endif //CYCLIC_B_H

c.h

#ifndef CYCLIC_C_H
#define CYCLIC_C_H

// #include "A.h" // this causes issue as well

struct A;

typedef struct C C;

struct C {
    struct A *a;
};

#endif //CYCLIC_C_H

main.c

#include "C.h"

int main() {
    struct C *c = malloc(sizeof(C));
    c->a->value = 42; // problem

    return 0;
}

Answer 1

In your code, the #include "Ch" incorporates the text from that file (and only that file) into the body of the main.c file. The trouble then is that there is no actual definition of the A structure provided.

Now, while you can declare (and even do some stuff with) a pointer to an undefined structure - so long as you have declared it as a struct - (as you have in C.h ), you cannot access any of that structure's members without the full definition.

So, when you change the header that you #include , from C.h to Ah , the pre-processor inserts the text from that header and all headers it itself includes into the body of main.c . Thus, the contents of all three headers are thus incorporated, and you then have the full definitions of the A , B and C structures.

Note: As pointed out in the comments, you also need to complete your "// problem" line, with a proper assignment: c->a->value = 42; // (or whatever) c->a->value = 42; // (or whatever) .