C++ Initialization of array in structure

Question

I would like initiated size of array in structure when I constructing object:

class someclass{
public:
  someclass(int init_msgsz) { this->msgsz = init_msgsz; }

private:
   int msgsz;
   struct msgbuf {
      long msqtype;
      uint8_t msgtext[msgsz];
   } send_message_buf, receive_message_buf;
};

But something is wrong :-( anybody help me?
Note this structure is from: msgrcv, msgsnd - message operations

Update:

#include <iostream>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <stdint.h> //uint8_t == unsigned char
#include <string.h>

class communication
{
public:
    communication(key_t &init_key, int &init_msgflg, size_t init_msgsz);
    ~communication();

    int send_msg(uint8_t *send_msg);
    int receive_msg(uint8_t *rec_msg);

private:
    int msqid;
    key_t key;
    int msgflg;
    int msgsz;
    size_t buf_length;

    struct msgbuf {
        long    msqtype;
        uint8_t    *msgtext;
        } send_message_buf, receive_message_buf;
    int get_msgid();
};

communication::communication(key_t &init_key, int &init_msgflg, size_t init_msgsz)
{
    this->key = init_key;
    this->msgflg = init_msgflg;
    this->msgsz = init_msgsz;
    get_msgid();

    send_message_buf.msgtext = new uint8_t[this->msgsz];
    receive_message_buf.msgtext = new uint8_t[this->msgsz];
}
communication::~communication()
{
    delete [] send_message_buf.msgtext;
    delete [] receive_message_buf.msgtext;
    send_message_buf.msgtext = NULL;
    receive_message_buf.msgtext = NULL;
}

int communication::get_msgid()
{
    if ((this->msqid = msgget(this->key, this->msgflg )) < 0)
    {
        std::cout << "\n[erro]...msgget\n";
        return -1;
    }
    return 0;
}

int communication::send_msg(uint8_t *send_msg)
{
    std::copy ( send_msg, send_msg+this->msgsz,
    this->send_message_buf.msgtext);
    this->buf_length = sizeof(this->send_message_buf.msgtext);
    this->send_message_buf.msqtype = 1;

    if (msgsnd(this->msqid, &send_message_buf, this->buf_length, IPC_NOWAIT) < 0)
    {
        std::cout << "\n[erro]...msgsnd\n";
        return -1;
    }

    return 0;
}

int communication::receive_msg(uint8_t *rec_msg)
{
    if (msgrcv(this->msqid, &receive_message_buf, this->msgsz+1, 1, 0) < 0) 
    {
        std::cout << "\n[erro]...msgrcv\n";
        return -1;
    }

    std::copy ( this->receive_message_buf.msgtext,       
    this->receive_message_buf.msgtext+this->msgsz, rec_msg);

    return this->msgsz;
}


int main()
{

    size_t msgsz = 16;
    int msgflg = IPC_CREAT | 0666;
    key_t keySpi = 1001;

    unsigned int msg_size;
    uint8_t receive_msg[msgsz];
    uint8_t send_msg[] = {0xFF, 0xAA, 0xAB, 0xFF, 0xAA, 0xAB, 0xFF, 0xAA, 0xAB, 0xFF, 0xAA, 0xAB, 0xFF, 0xAA, 0xAB, 0xFF};

    communication msgSpi(keySpi, msgflg, msgsz);
    msgSpi.send_msg(send_msg);
    msg_size = msgSpi.receive_msg(receive_msg);

    std::cout << "I got msg with size: " << msg_size << std::endl;
    for (int i = 0; i < msg_size; i++)
        std::cout << (int)i << ": " << (int)receive_msg[i] << std::endl;

    std::cout << "\n";
    return 0;
}

Compilation OK, but I can't delete mem in destructor.

You should post the compiler error message as part of the question. — BoBTFish
– BoBTFish, Commented Dec 16, 2015 at 12:19
pointers <i>send_message_buf.msgtext</i> and <i>receive_message_buf.msgtext</i> are pointed to same adress — Adrian Peniak
– Adrian Peniak, Commented Dec 16, 2015 at 13:53

Cory Kramer · Accepted Answer · 2015-12-16 12:18:10Z

7

The size of an array must be known at compile time, not run time. Therefore you'd have to have either a dynamic array or a std::vector

struct msgbuf {
    long msqtype;
    std::vector<uint8_t> msgtext;
} send_message_buf, receive_message_buf;

answered Dec 16, 2015 at 12:18

Cory Kramer

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2 Comments

Adrian Peniak Over a year ago

It works with template parameter: template <int msgsz> struct msgbuf { long msqtype; uint8_t msgtext[msgsz]; };// send_message_buf, receive_message_buf; Thank you for fast response.

Cory Kramer Over a year ago

Again that method will only work if msgsz is known at compile time, it won't work if that value is only known at runtime.

TartanLlama · Accepted Answer · 2015-12-16 12:23:32Z

1

If init_msgsz is known at compile-time, you can make it a template parameter:

template <std::size_t Msgsz>
class someclass{
   static constexpr std::size_t msgsz = Msgsz;
   struct msgbuf {
      long msqtype;
      std::array<uint8_t, Msgsz> msgtext;
   } send_message_buf, receive_message_buf;
};

answered Dec 16, 2015 at 12:23

TartanLlama

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