How to pass std::thread as thread id in PostThreadMessage?-CodePudding

How could I pass the id of a std::thread thread as the id into PostThreadMessage?

Like suppose, I have a thread:

// Worker Thread

auto thread_func = [](){
    while(true) {
        MSG msg;
        if(GetMessage(&msg, NULL, 0, 0)) {
            // Do appropriate stuff depending on the message
        }
    }
};

std::thread thread_not_main = std::thread(thread_func);

And then I want to send a message from my main thread to the above thread so that I can handle the message in a non-expensive way. So as to not interrupt the main thread.

Like:

 // Main Thread

 while(true) {
     MSG msg;
     while(GetMessage(&msg, NULL, 0, 0)) {
          TranslateMessage(&msg);
          if(msg.message == WM_PAINT) {
              PostThreadMessage(); // How do I pass the thread id into the function?
          } else {
               DispatchMessage(&msg);
          }
     }
}

The summary of the problem is that PostThreadMessage requires a thread-id to be passed in as a parameter, now std::thread::get_id doesn't provide it in a "DWORD convertible format". So then I can't pass the thread's id as a parameter.

My question is: How would I pass the thread id as a parameter to PostThreadMessage?

CodePudding user response：

You can get the underlying, "Windows-style" thread handle for a std::thread object by calling its native_handle() member function. From that, you can retrieve the thread's ID by calling the GetThreadId WinAPI function, and passing that native handle as its argument.

Here's a short code snippet that may be what you'd want in the case you've outlined:

    auto tHandle = thread_not_main.native_handle(); // Gets a "HANDLE"
    auto tID = GetThreadId(tHandle);                // Gets a DWORD ID
    if (msg.message == WM_PAINT) {
        PostThreadMessage(tID, msg, wParam, lParam);
    }
    else {
        DispatchMessage(&msg);
    }
    //...

CodePudding user response：

Solution:

You can use the native_handle() member function to get the underlying platform-specific handle of the thread, and then pass that to PostThreadMessage:

std::thread thread_not_main = std::thread(thread_func);

...

PostThreadMessage(thread_not_main.native_handle(), WM_PAINT, 0, 0);