MontagFTB had a problem which, upon investigation, allegedly was caused by a subtle "fact": "The parent specified in CreateWindowEx is both the parent of the window and the owner of the window, but when you call SetParent it only sets the parent of the window, not the owner." MontagFTB then concluded that some messages were sent to the parent and others were sent to the owner.

This is a faulty diagnosis. We'll look at the correct diagnosis next time, but today's topic is parents and owners. Actually, parent and owner windows were already covered by my 2005 PDC talk, Five Things Every Win32 Programmer Should Know, so for most of you, today's topic is a review. (And I included the topic in the talk specifically so I wouldn't have to blog about it, but obviously that plan didn't work out.)

A window can be created as a child window (WS_CHILD set) or a top-level window (WS_CHILD not set). A child window has a parent, which you specify when you call CreateWindowEx, and which you can change by calling SetParent. A top-level window, on the other hand, has no parent. Its parent is NULL.

Ownership is a concept that relates top-level windows. A top-level window can optionally have an owner, which is also specified when you call CreateWindowEx, and which you can change by a complicated mechanism described in my talk.

Note that changing a window's parent or owner is not a normal operation. You usually create a window with a specific parent or owner and leave it that way for the lifetime of the window.

Now, a window can have a parent, or it can have an owner, or it can have neither, but it can never have both.

What would it mean for a window to have both a parent and an owner? Well, in order to have a parent, the window must itself be a child. But in order to have an owner, the window must be top-level. And top-level windows and child windows are mutually exclusive (and collectively exhaustive), because you either have the WS_CHILD style (which makes you a child) or you don't (which makes you top-level). Since people like tables so much, here's a table:

Child windowTop-level window
The Parent window is...non-NULL NULL
The Owner window is...N/A NULL or non-NULL

The box for "The Owner window of a Child window..." is marked N/A because the question is meaningless. Ownership is a relationship among top-level windows.

By analogy, consider the people at a school for children. They can be separated into two groups, students and teachers. (We'll treat non-teaching staff as teachers with no students.)

Each student is assigned to a teacher. Each teacher might or might not have another teacher as a mentor. Several students can be assigned the same teacher, but every student must be assigned to some teacher. Similarly, several teachers might have the same mentor, but some teachers won't have a mentor at all, and some mentors might themselves have mentors.

It's impossible for a person to have both a teacher and a mentor, because having a teacher applies only to students, and having a mentor applies only to teachers. Teachers don't attend classes (they lead the classes) so they don't have a teacher. But they might have mentors. Asking for a student's mentor is a meaningless question because students don't have mentors; teachers do.

Since a window cannot have both a parent and an owner, the CreateWindowEx function takes a single HWND parameter which is either the parent or owner of the window being created, depending on what type of window you're creating. If you're creating a child window, then the parameter represents the parent window; if you're creating a top-level window, then the parameter represents the owner window.

A similar overloading of parameters happens with the HMENU: If you're creating a child window, then the parameter represents the child window identifier; if you're creating a top-level window, then the parameter represents the window menu. Because only top-level windows can have menus, and only child windows can have child window identifiers.

If this parameter overloading bothers you, you can write your own helper functions:

HWND CreateChildWindowEx(
    DWORD dwExStyle,
    LPCTSTR lpClassName,
    LPCTSTR lpWindowName,
    DWORD dwStyle,
    int x,
    int y,
    int nWidth,
    int nHeight,
    HWND hWndParent,
    UINT_PTR id,
    HINSTANCE hInstance,
    LPVOID lpParam
)
{
 // A child window must have the WS_CHILD style
 if (!(dwStyle & WS_CHILD)) {
  SetLastError(ERROR_INVALID_PARAMETER);
  return NULL;
 }
 return CreateWindowEx(
    dwExStyle,
    lpClassName,
    lpWindowName,
    dwStyle,
    x,
    y,
    nWidth,
    nHeight,
    hWndParent,
    reinterpret_cast<HMENU>(id),
    hInstance,
    lpParam);
}

HWND CreateTopLevelWindowEx(
    DWORD dwExStyle,
    LPCTSTR lpClassName,
    LPCTSTR lpWindowName,
    DWORD dwStyle,
    int x,
    int y,
    int nWidth,
    int nHeight,
    HWND hWndOwner,
    HMENU hMenu,
    HINSTANCE hInstance,
    LPVOID lpParam
)
{
 // A top-level window must not have the WS_CHILD style
 if (dwStyle & WS_CHILD) {
  SetLastError(ERROR_INVALID_PARAMETER);
  return NULL;
 }
 return CreateWindowEx(
    dwExStyle,
    lpClassName,
    lpWindowName,
    dwStyle,
    x,
    y,
    nWidth,
    nHeight,
    hWndOwner,
    hMenu,
    hInstance,
    lpParam);
}

There's more to parent windows and owner windows than what I've written here; I refer you to my talk (or other documentation) for more details.

Next time, we'll look at what MontagFTB is really seeing.