Passing an object to a thread in C++

Question

I have a project that creates an object of a class stored in a header file. I need this object passed into a thread from the main function.

Currently I'm using hChatThread = CreateThread(NULL,0,ChatFunc,"1",0,&dwChatThreadId); as a way of creating a thread, and it seems I cannot change the fact that the called function can only have LPVOID as a type. If I could somehow change this, I could then pass the object as an argument like so: hChatThread = CreateThread(NULL,0,ChatFunc,myObject,0,&dwChatThreadId);

But it seems this is next to impossible.

Any ideas?

Thread:

DWORD WINAPI ChatFunc(LPVOID randparam)
{
    ServerMain(myObject); //need to pass object again here to be used in other functions
    return 0;
}

Main:

int main(int argc, char *argv[])
{
    myObject w;
    w.exec();
    hChatThread = CreateThread(NULL,0,ChatFunc,"1",0,&dwChatThreadId);
    return 0;
}

Answer 1

You can use LPVOID lpParameter to pass a pointer to the object, and then cast it back. Something like this:

int main(int argc, char *argv[])
{
    myObject w;
    w.exec();
    hChatThread = CreateThread(NULL,0,ChatFunc,&w,0,&dwChatThreadId);

    // wait for the thread to terminate,
    // otherwise w might get destroyed while the thread
    // is still using it (see comments from @MartinJames)

    return 0;
}

DWORD WINAPI ChatFunc(LPVOID param)
{
     myObject* p = static_cast<myObject*>(param);
    ServerMain(p); //need to pass object again here to be used in other functions
    return 0;
}

Answer 2

The easiest way would be to make that Object a global object in the header file, and given that you included the code:

#include "headerFilesName.h"

in the file where your main function is. Then I would declare another object inside the file with the main function and just pass it its value

headerFileName myNewObject;

with this object of the class of the header file, you should be able to access the other object you created initially.

Answer 3

There is only one flexible solution - dynamically allocate 'w' with operator new, pass the pointer into CreateThread, cast it back in the thread function and, when/if the thread function exits/returns and the object instance is no longer required, delete it.

All the other solutions either don't work reliably, have messy globals or are inflexible.