C++ 结构体内存对齐

定义了两个结构体

 typedef struct Cmd {
    uint8_t ua;
    uint8_t ub;
    uint8_t uc;

    uint32_t ue;
} Cmd_t;


typedef struct Cmd_tag {
    uint8_t value;
    uint8_t data[1]; // 将 data 定义为指向 Cmd_t 结构体的指针
} tag_t;

在实际使用中，看见前人的代码是，new 一块内存放两个结构体。我对内存大小有些疑惑。代码如下

void test2()
{
    int size = 9;
    unsigned char* pdata = new unsigned char[size];
    memset(pdata, 0, size);
    // 解释 pdata 为 tag_t* 类型的指针
    tag_t* pCmd = (tag_t*)pdata;
    pCmd->value = 1;
    // 将 pdata + 1 解释为 Cmd_t* 类型的指针，并将其赋给 data 成员
    Cmd_t*  p= (Cmd_t*)(pdata + 1);

    p->ua = 10;
    p->ub = 10;
    p->uc = 10;
    p->ue = 10;


  
    std::cout << "value: " << static_cast<int>(pCmd->value) << std::endl;
    std::cout << "Address of pCmd->value: " << static_cast<void*>(&pCmd->value) << std::endl;

    std::cout << "data->a: " << static_cast<int>(p->ua) << std::endl;
    std::cout << "Address of p->a: " << static_cast<void*>(&p->ua) << std::endl;

    std::cout << "data->ub: " << static_cast<int>(p->ub) << std::endl;
    std::cout << "Address of p->ub: " << static_cast<void*>(&p->ub) << std::endl;

    std::cout << "data->uc: " << static_cast<int>(p->uc) << std::endl;
    std::cout << "Address of p->uc: " << static_cast<void*>(&p->uc) << std::endl;

    std::cout << "data->ue: " << p->ue << std::endl;
    std::cout << "Address of p->ue: " << static_cast<void*>(&p->ue) << std::endl;

    delete[] pdata;
}

输出如下
在这里插入图片描述
解释：

 std::cout << "Size of tag_t: " << sizeof(tag_t) << " bytes" << std::endl;
 std::cout << "Size of Cmd_t: " << sizeof(Cmd_t) << " bytes" << std::endl;
// Size of tag_t: 2 bytes
 //Size of Cmd_t: 8 bytes