Rust: offset祼指针操作

offset是偏移元素个数，不是字节数！

bash 复制代码

fn main(){
    let student_a = Student{id:20240001,name:"张三娃".into(),class_id:3,age:14,grade:1};
    let student_b = Student{id:20240002,name:"李四牛".into(),class_id:3,age:15,grade:1};
    let student_c = Student{id:20240003,name:"王二狗".into(),class_id:3,age:14,grade:1};
    let student_d = Student{id:20240004,name:"吴七喜".into(),class_id:3,age:24,grade:1};
    let students= vec![student_a, student_b, student_c, student_d];
    let class = Class{students,class_id:3,grade:1};
    let student_size = std::mem::size_of::<Student>(); // student结构体大小
    let student_align= std::mem::align_of::<Student>();//student内部对齐
    println!("student_size:{:?} student_align:{:?}",student_size,student_align);
    let class_ptr   = &class as *const Class;
    let student_a_ptr = &class.students[0] as *const Student;
    println!("student_a value :{:?}",unsafe{&*student_a_ptr as &Student});
    //let student_b_ptr = unsafe{ student_a_ptr.add(1)}; // 也可以用add这种方式
    // 注意：offset不是偏移字节数，而是偏移元素个数！
    let student_b_ptr = unsafe{ student_a_ptr.offset(1)}; // offset方式
    let offset_between_a_and_b = unsafe{student_a_ptr.offset_from(student_b_ptr)};
    println!("offset_between_a_and_b:{}",offset_between_a_and_b);// -1
    println!("student_b value name :{:?}",unsafe{&*student_b_ptr as &Student}.name);
    //let student_c_ptr = unsafe{ student_a_ptr.add(2)};
    let student_c_ptr = unsafe{ student_a_ptr.offset(2)};
    let student_d_ptr = unsafe{ student_a_ptr.offset(3)};
    println!("student_b ref ptr address : {:p} convert raw ptr address : {:?}",&class.students[1],student_b_ptr);
    println!("student_b ref ptr address : {:p} convert raw ptr address : {:?}",&class.students[2],student_c_ptr);
    println!("student_b ref ptr address : {:p} convert raw ptr address : {:?}",&class.students[3],student_d_ptr);

}
// 某某中学
// 班级
struct Class{
    students: Vec<Student>,//学生
    class_id: u32, //3班
    grade:u32, //高3
}
#[derive(Debug)]
struct Student{
    id: u32, //学号 2024000001；
    name: String, // 姓名，name="张三"
    class_id: u32, //班级，class_id=3,3班
    age: u32, //年龄
    grade: i8, //年级，grade=1,高中一年级
}

输出：

bash 复制代码

student_size:40 student_align:8
student_a value :Student { id: 20240001, name: "张三娃", class_id: 3, age: 14, grade: 1 }
offset_between_a_and_b:-1
student_a value :Student { id: 20240001, name: "张三娃", class_id: 3, age: 14, grade: 1 }
offset_between_a_and_b:-1
offset_between_a_and_b:-1
student_b value name :"李四牛"
student_b ref ptr address : 0x1c2ff7f6f48 convert raw ptr address : 0x1c2ff7f6f48
student_b ref ptr address : 0x1c2ff7f6f70 convert raw ptr address : 0x1c2ff7f6f70
student_b ref ptr address : 0x1c2ff7f6f98 convert raw ptr address : 0x1c2ff7f6f98

其实，你可以通过offset函数源码，可以了解更清楚，在offset内部，会再以bytes的倍数进行check_add。