On 26/05/2021 21:39, Alberto Mardegan wrote:
Now, I haven't dug very deep in QString and QVariant, I preferred to let the experts talk first :-) Does the code in the QSQLite plugin look correct to you?
That depends: when does SQLite exactly stop using the string data passed to it exactly?
Is it expected that QString::utf16() makes a memory allocation? And is it correct that this happens even when the QString is const?
In the overwhelming majority of cases, utf16() won't make any allocation. There's just one exception -- if the QString was built via fromRawData(). In that case, in order to ensure NUL termination, utf16() does actually "detach" (even if it's a const method).
(Fun fact: utf16() is one of the few const methods in Qt value classes which isn't thread safe.)
It's a bit interesting that SQLite takes pointer+size, so does not seem to require NUL termination -- for that use case, there's unicode()/constData() + size(), that never reallocates (unlike utf16()).
And why destroying a const QVariant causes the deallocation of the data created by QString::utf16()??
QString::utf16 doesn't "create" data: it modifies the string *itself* so that it becomes detached and NUL terminated. When you destroy that QString, you'll be deallocating the data (provided there are no more shallow copies around).
Similarly, when you destroy a QVariant, you destroy the object it's holding (provided there are no further shallow copies around).
So yes, in the general case, destroy QVariant -> destroy the QString contained -> deallocate memory.
HTH, -- Giuseppe D'Angelo | giuseppe.dang...@kdab.com | Senior Software Engineer KDAB (France) S.A.S., a KDAB Group company Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com KDAB - The Qt, C++ and OpenGL Experts
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