C在定义之前声明结构

Question

The following is the source code that is problem:

#include <stdio.h>

typedef struct Foo
{
    int a;
    Bar b;
} Foo;

typedef struct Bar
{
    int a;
    int b;
} Bar;

int main(void)
{
    Foo f;
    f.a = 1;
    f.b.a = 2;
    f.b.b = 3;
    printf("%d %d %d\n", f.a, f.b.a, f.b.b);

    return 0;
}

I want to declare the Bar struct in the Foo struct that keep order of the struct declarations ( Foo struct is first). But I can't, it occurs some errors(One is error: unknown type name 'Bar' ) in the compile time.

How to do it?

Answer 1

The compiler needs to be able to determine the size of Foo . If Bar is unknown at the time Foo is defined the compiler can not determine the size of Foo . The only way around this is using a pointer since all pointers have the same size.

You can use a forward declaration of the structure and then reference it as a pointer. This means that Foo can never automatically allocate the memory for Bar . As a consequence the memory has to be allocated separately.

If you can avoid this do not do it.

#include <stdio.h>
#include <stdlib.h>

typedef struct Bar Bar;
typedef struct Foo Foo;

struct Foo
{
    int a;
    Bar * b;
};

struct Bar
{
    int a;
    int b;
};

void dynamic(void)
{
    Foo f;

    f.a = 1;
    f.b = (Bar*)malloc(sizeof(Bar));
    f.b->a = 2;
    f.b->b = 3;
    printf("%d %d %d\n", f.a, f.b->a, f.b->b);

    free(f.b);
}

void automatic(void)
{
    Foo f;
    Bar b;

    f.a = 1;
    f.b = &b;
    f.b->a = 2;
    f.b->b = 3;
    printf("%d %d %d\n", f.a, f.b->a, f.b->b);
}

int main(void)
{
    dynamic();
    automatic();
}

Answer 2

The Bar struct declaration has to precede the Foo struct declaration - in case you do not want to use pointers and allocate memory. In C, if you have another struct as a member in a struct, the other struct has to be declared before so as the compiler can see it.

Answer 3

Tada!

#define Bar struct { int a; int b; }

typedef struct Foo
{
    int a;
    Bar b;
} Foo;

#undef Bar

typedef struct Bar
{
    int a;
    int b;
} Bar;

int main(void)
{
    Foo f;
    f.a = 1;
    f.b.a = 2;
    f.b.b = 3;
    printf("%d %d %d\n", f.a, f.b.a, f.b.b);

    return 0;
}

(please don't do this)