C / C++ / MFC

#define MY_MESSAGE (WM_APP + 1)

class Packet {
public:
    Packet();
    ~Packet();
    // ... data members
    int m1;
};

...

// allocate a packet, set its contents

Packet *packet = new Packet;
packet->m1 = 123456;

// here's how you post a message

CString strWndName = "My wnd";
CString strClassName = "Wnd class";
HWND hwnd = ::FindWindow(strClassName,strWndName);

::PostMessage(hwnd,MY_MESSAGE,0,(LPARAM)packet);

afx_msg LRESULT OnMyMessage(WPARAM wParam,LPARAM lParam);

BEGIN_MESSAGE_MAP( MyWindowClass, <window base class> )
...
ON_MESSAGE(MY_MESSAGE,OnMyMessage)
END_MESSAGE_MAP()
...

LRESULT MyWindowClass::OnMyMessage(WPARAM wParam,LPARAM lParam)
{
    Packet *packet = (Packet *)lParam;

    // ... use information in packet

    delete packet;

    return (LRESULT)0;
}