Get hwnd by process id c++

HWND g_HWND=NULL;
BOOL CALLBACK EnumWindowsProcMy(HWND hwnd,LPARAM lParam)
{
    DWORD lpdwProcessId;
    GetWindowThreadProcessId(hwnd,&lpdwProcessId);
    if(lpdwProcessId==lParam)
    {
        g_HWND=hwnd;
        return FALSE;
    }
    return TRUE;
}
EnumWindows(EnumWindowsProcMy,m_ProcessId);

A single PID (Process ID) can be associated with more than one window (HWND). For example if the application is using several windows.
The following code locates the handles of all windows per a given PID.

void GetAllWindowsFromProcessID(DWORD dwProcessID, std::vector <HWND> &vhWnds)
{
    // find all hWnds (vhWnds) associated with a process id (dwProcessID)
    HWND hCurWnd = NULL;
    do
    {
        hCurWnd = FindWindowEx(NULL, hCurWnd, NULL, NULL);
        DWORD dwProcID = 0;
        GetWindowThreadProcessId(hCurWnd, &dwProcID);
        if (dwProcID == dwProcessID)
        {
            vhWnds.push_back(hCurWnd);  // add the found hCurWnd to the vector
            wprintf(L"Found hWnd %d\n", hCurWnd);
        }
    }
    while (hCurWnd != NULL);
}

You can use EnumWindows and GetWindowThreadProcessId() functions as mentioned in this MSDN article.