C中的半继承:这个代码片段如何工作？

#include<stdio.h>

typedef struct {
    int x;
    int y;
} Point;

typedef struct {
    Point base;
    int z;
} Point3D;

int main() {
    Point3D point = {.base.x = 1, .base.y = 2, .z = 3};
    printf("Point: (%d, %d, %d)\n", point.base.x, point.base.y, point.z);
    return 0;
}