Unexpected behavior when placing Unicode text on Clipboard with NULL owner window

user_3382486 0
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I'm trying to place text on the clipboard from a Windows desktop application that I'm working on, but I am getting some behavior I don't understand.

The function I'm using is as follows:

bool Clipboard::CopyText(const XStringW& txt)
{
    size_t memsize = sizeof(wchar_t) * (txt.Length() + 1);
    HGLOBAL glob = GlobalAlloc(GMEM_MOVEABLE, memsize);
    if (glob != NULL)
    {
        void* mem = GlobalLock(glob);
        if (mem != NULL)
        {
            memcpy(mem, (LPCWSTR) txt, memsize);
            GlobalUnlock(glob);

            HANDLE handle = NULL;

            if (OpenClipboard(NULL))
            {
                EmptyClipboard();
                handle = SetClipboardData(CF_UNICODETEXT, glob);
                ASSERT(handle != NULL);
                CloseClipboard();
            }

            return handle != NULL;
        }
        else

What happens after else is not especially important.

If I execute the above block of code, everything actually works as it should. This is strange to me, because according to the documentation:

If an application calls OpenClipboard with hwnd set to NULL, EmptyClipboard sets the clipboard owner to NULL; this causes SetClipboardData to fail.

But I find that it's working as it should.

However, I only added EmptyClipboard() as an experiment. Prior to this, my code was not working properly. Let's say I placed the text "Hello" from a Word document. Then I copied a number 999 from my application. What would happen is, if I tried pasting to Excel or Word (for example), it would paste "Hello". However if I selected "Paste Special" in (say) Excel and pasted as text, it would paste "999". So it would work, but only if a non-default paste was used.

If I add in the EmptyClipboard() (as per my code above), it works perfectly. It clears whatever was there before. But it shouldn't, according to the documentation.

Can anybody help me understand?

clipboard winapi c++
1 Answers

I don't know how to respond to this question with anything other than:

"Because the documentation says so." tm

and leave it at that. The documentation and the examples provided therein specifically state that you must call, in order* (pseudo-code):

if(::OpenClipboard(hwnd)) {
    if(::EmptyClipboard()) {
        //Copy data...
        ::SetClipboardData(/*...*/)'
    }
    ::CloseClipboard();
}

Which also says basically: "Do not call EmptyClipboard without first calling OpenClipboard with a proper handle to a window".

Failing to do that and then wondering why it doesn't work is an exercise in frustration.

*There are cases where responding to specific WM messages should not call ::OpenClipboard but they do not apply here.

user_4211780
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