pass structure to function via pointer

Question

I'm using Win32 API and the _beginthreadex call to run a thread the following way:

struct StructItem
{
   std::string title;
   int amount;
};

StructItem structItems[33];
unsigned int id;
HANDLE thread = (HANDLE)_beginthreadex(NULL, 0, my_thread, (void*)structItems, 0, &id);

And this is my thread:

unsigned int __stdcall my_thread(void *p)
{
    for (int i = 0; i < 20; i++)
    {           
        // todo: print struct.title
        Sleep(1000);
    }

    return 0;
}

As far as I understood the *p is a pointer to my list of structures, since I passed them to the 4th argument in the _beginthreadex call, but I can't understand how can I cast the *p so that I can access the array of structs from within the thread?

@πάνταῥεῖ but it's an array of structs, instead of a single struct — 0x29a
– 0x29a, Commented Feb 25, 2019 at 14:20
@0x29a Do you really need to use the Win32 threads? Since C++11, there's std::thread. — Ted Lyngmo
– Ted Lyngmo, Commented Feb 25, 2019 at 14:26
@TedLyngmo I had trouble getting the std::thread to run in the background, it kept hanging the main UI and I could not solve it, I will give it another shot, thanks! — 0x29a
– 0x29a, Commented Feb 25, 2019 at 14:28
@0x29a See here: stackoverflow.com/questions/1461432/what-is-array-decaying — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Feb 25, 2019 at 14:32

molbdnilo · Accepted Answer · 2019-02-25 14:21:42Z

7

Since the array decays into a StructItem* (the location of the array's first element) when you pass it as an argument, cast it back to StructItem*.

unsigned int __stdcall my_thread(void *p)
{
    auto items = static_cast<StructItem*>(p);
    for (int i = 0; i < 20; i++)
    {           
        std::cout << items[i].title << '\n';
        Sleep(1000);
    }
    return 0;
}

Note that the cast to void* is entirely unnecessary.

answered Feb 25, 2019 at 14:21

molbdnilo

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Comments

Sumit Jha · Accepted Answer · 2019-02-25 14:26:28Z

1

You could cast void pointer to the pointer type of your struct, and then dereferencing that pointer expression to get the element at a particular offset:

*((StructItem*) p); /* first element */
*((StructItem*) p+ 1); /* second element */

Its a c-style method. However I would rather prefer C++ style which is already answered.

answered Feb 25, 2019 at 14:26

Sumit Jha

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