C++ 操作结构的原始数据

Question

我有一个看起来像这样的简单结构:

struct Object 
{
    int x_;
    double y_;
};

我正在尝试操作对象的原始数据，这就是我所做的:

int main()
{
    Object my_object;
    unsigned char* raw_data = reinterpret_cast<unsigned char*>(&my_object);

    int x = 10;
    memcpy(raw_data, &x, sizeof(x));
    raw_data += sizeof(x);

    double y = 20.1;
    memcpy(raw_data, &y, sizeof(y));


    Object* my_object_ptr = reinterpret_cast<Object *>(raw_data);


    std::cout << *(my_object_ptr).x << std::endl;    //prints 20       (expected 10)
    std::cout << *(my_object_ptr).y << std::endl;    //prints Rubbish  (expected 20.1)
}

我期待上面的代码能工作，，，

真正的问题是什么？这可能吗？

Answer 1

您需要使用 offsetof macro .还有一些问题，最重要的是你修改了 raw_data 指针，然后将修改后的值转换回 Object* 指针，导致未定义的行为。我选择删除 raw_data 修改(替代方案是不将其强制转换，而是直接检查 my_object)。这是给你的固定代码，注释中有解释:

#include <iostream>
#include <cstring> // for memcpy
#include <cstddef> // for offsetof macro

struct Object 
{
    int x_;
    double y_;
};

int main()
{
    Object my_object;
    unsigned char* raw_data = reinterpret_cast<unsigned char*>(&my_object);

    int x = 10;
    // 1st memcpy fixed to calculate offset of x_ (even though it is probably 0)
    memcpy(raw_data + offsetof(Object, x_), &x, sizeof(x));
    //raw_data += offsetof(Object, y_); // if used, add offset of y_ instead of sizeof x

    double y = 20.1;
    // 2nd memcpy fixed to calculate offset of y_ (offset could be 4 or 8, depends on packing, sizeof int, etc)
    memcpy(raw_data + offsetof(Object, y_), &y, sizeof(y));

    // cast back to Object* pointer
    Object* my_object_ptr = reinterpret_cast<Object *>(raw_data);


    std::cout << my_object_ptr->x_ << std::endl;    //prints 10
    std::cout << my_object_ptr->y_ << std::endl;    //prints 20.1
}