Initialize string type elements in a struct

问题

I am trying to dynamically store the contents of a pointer into a struct. I defined the struct as follows:

struct msg3 {
    uint16_t     msgSize;  // 2 bytes
    uint16_t     msgType;  // 2 bytes
    uint32_t     symbolIndex;  // 4 bytes
};

I then proceed to initialize a struct of type msg3 using code below. (Packet is of type const u_char*).

const msg3* msg3_ = reinterpret_cast<const msg3*>(packet);

The above works fine, and sizeof(msg3) accurately returns 8.

Following the first 8 bytes in packet, there is 11 bytes of null-terminated ASCII characters.

I try to add a string as an element of the struct to capture the following 11 bytes of ASCII characters that are null-terminated:

struct msg3 {
    uint16_t     msgSize;  // 2 bytes
    uint16_t     msgType;  // 2 bytes
    uint32_t     symbolIndex;  // 4 bytes
    std::string  symbol;     // 11 bytes
};

Under this definition of struct (with the additional symbol element), sizeof(msg3) returns 32 when I want it to return 19. Plus, when I try to access msg3_.symbol, I get a segmentation fault. Why is this happening?

Essentially, I am trying to replicate

std::string symbol = (char*)packet + 8;

within the definition of the struct.

来源：https://stackoverflow.com/questions/62725064/initialize-string-type-elements-in-a-struct