如何获得内部结构的尺寸

Question

I'm trying to get size of inner structure ie struct B . But I'm getting compilation error:

prog.c: In function 'main': prog.c:10:53: error: expected ')' before ':' token printf("%d | %d", sizeof(struct A), sizeof(struct A::struct B));

Following is my code:

#include <stdio.h>

struct A
{
        struct B{};
};

int main() {
    printf("%d | %d", sizeof(struct A), sizeof(struct A::struct B));
    return 0;
}

Could you suggest that how I can achieve this in C?

UPDATED

Answer from @Jabberwocky solves my above problem. But What about following code. This can also be found here :

#include <stdio.h>

struct A
{
    struct B{};
};

struct B
{};

int main() {
    printf("%d | %d", sizeof(struct A), sizeof(struct B), sizeof(struct A::struct B));
    return 0;
}

In this case I'm getting compilation error as following:

prog.c:8:8: error: redefinition of 'struct B'
struct B
^
prog.c:5:10: note: originally defined here
struct B{};
^
prog.c: In function 'main':
prog.c:12:71: error: expected ')' before ':' token
printf("%d | %d", sizeof(struct A), sizeof(struct B), sizeof(struct A::struct B));

Here how I can diffrentiate between struct B and struct A::struct B

Answer 1

#include <stdio.h>

struct A
{
        struct B{};   // this should not compile anyway in C as C
                      // does not allow empty structs
                      // but this would compile: struct B{int a;};
};

int main() {
    printf("%d | %d", sizeof(struct A), sizeof(struct B));
                                           // just write struct B
    return 0;
}

Working sample:

#include <stdio.h>

struct A
{
  int b;
  struct B { int a; };
};

int main() {
  printf("%d | %d", sizeof(struct A), sizeof(struct B));
  return 0;
}

Possible output on a 32 bit system:

8 | 4

Answer 2

Be aware that C support for nested structures is logical only. Each structure exists on its own.

#include <stdio.h>
struct A {                   // same as
    int bar;                 // struct B { int foo };
    struct B { int foo; } b; // struct A { int bar; struct B b; };
};

int main(void) {
    struct A a;
    a.bar = 42;
    a.b.foo = -1;
    printf("a.bar is %d; a.b.foo is %d\n", a.bar, a.b.foo);

    struct B b; /* struct B is visible outside struct A */
    b.foo = 666;
    printf("b.foo - a.bar is %d\n", b.foo - a.bar);
    return 0;
}

https://ideone.com/rHaanj