C++ object containing an array of char using unique_ptr

Question

I am looking on a way to use unique_ptr to allocate a structure that contains an array of char with a number of bytes that set dynamically to support different types of message.

Assuming:

struct MyMessage
{
    uint32_t      id;
    uint32_t      data_size;
    char          data[4];
};

How can I convert send_message() below to use a smart pointer?

void send_message(void* data, const size_t data_size)
{
    const auto message_size = sizeof(MyMessage) - 4 + data_size;
    const auto msg = reinterpret_cast<MyMessage*>(new char[message_size]);

    msg->id = 3;
    msg->data_size = data_size;
    memcpy(msg->data, data, data_size);

    // Sending the message
    // ...

    delete[] msg;
}

My attempt to use smart point using the code below does not compile:

 const auto message_size = sizeof(MyMessage) - 4 + data_size; const auto msg = std::unique_ptr<MyMessage*>(new char[message_size]); 

Below a complete working example:

#include <iostream>
#include <iterator>
#include <memory>

using namespace std;

struct MyMessage
{
    uint32_t      id;
    uint32_t      data_size;
    char          data[4];
};

void send_message(void* data, const size_t data_size)
{
    const auto message_size = sizeof(MyMessage) - 4 + data_size;
    const auto msg = reinterpret_cast<MyMessage*>(new char[message_size]);
    if (msg == nullptr)
    {
        throw std::domain_error("Not enough memory to allocate space for the message to sent");
    }
    msg->id = 3;
    msg->data_size = data_size;
    memcpy(msg->data, data, data_size);

    // Sending the message
    // ...

    delete[] msg;
}

struct MyData
{
    int  page_id;
    char point_name[8];
};

void main()
{
    try
    {
        MyData data{};
        data.page_id = 7;
        strcpy_s(data.point_name, sizeof(data.point_name), "ab332");
        send_message(&data, sizeof(data));
    }
    catch (std::exception& e)
    {
        std::cout << "Error: " << e.what() << std::endl;
    }
}

Answer 1

The data type that you pass to delete[] needs to match what new[] returns. In your example, you are new[] ing a char[] array, but are then delete[] ing a MyMessage object instead. That will not work.

The simple fix would be to change this line:

delete[] msg;

To this instead:

delete[] reinterpret_cast<char*>(msg);

However, You should use a smart pointer to manage the memory deletion for you. But, the pointer that you give to std::unique_ptr needs to match the template parameter that you specify. In your example, you are declaring a std::unique_ptr whose template parameter is MyMessage* , so the constructor is expecting a MyMessage** , but you are passing it a char* instead.

Try this instead:

// if this struct is being sent externally, consider
// setting its alignment to 1 byte, and setting the
// size of the data[] member to 1 instead of 4...
struct MyMessage
{
    uint32_t      id;
    uint32_t      data_size;
    char          data[4];
};

void send_message(void* data, const size_t data_size)
{
    const auto message_size = offsetof(MyMessage, data) + data_size;

    std::unique_ptr<char[]> buffer = std::make_unique<char[]>(message_size);
    MyMessage *msg = reinterpret_cast<MyMessage*>(buffer.get());    

    msg->id = 3;
    msg->data_size = data_size;
    std::memcpy(msg->data, data, data_size);

    // Sending the message
    // ...
}

Or this:

using MyMessage_ptr = std::unique_ptr<MyMessage, void(*)(MyMessage*)>;

void send_message(void* data, const size_t data_size)
{
    const auto message_size = offsetof(MyMessage, data) + data_size;

    MyMessage_ptr msg(
        reinterpret_cast<MyMessage*>(new char[message_size]),
        [](MyMessage *m){ delete[] reinterpret_cast<char*>(m); }
    );

    msg->id = 3;
    msg->data_size = data_size;
    std::memcpy(msg->data, data, data_size);

    // Sending the message
    // ...
}

Answer 2

This should work, but it is still not clear if accessing msg->data out of bounds is legal (but at least it is not worst than in your original code):

const auto message_size = sizeof(MyMessage) - ( data_size < 4 ? 0 : data_size - 4 );
auto rawmsg = std::make_unique<char[]>( message_size );
auto msg = new (rawmsg.get()) MyMessage;