On 28/04/2020 19.41, Giuseppe D'Angelo via Development wrote:
Il 28/04/20 21:45, Matthew Woehlke ha scritto:
* QList gets adapted so that its internal array allocates 3 *
sizeof(void*) per element, so that e.g. Q6StringList won't require a
per-item allocation. This would then also avoid allocations for other
datatypes, e.g. QStringView, QImage, maybe QVariant and QColor; but of
course waste a ton of space for the ones which remain small (most of Qt
implicitly shared datatypes).
Uh... can't it allocate sizeof(T) if T meets some criteria? IOW, I don't
see the second case penalizing smaller types unless the implementation
is poorly done.
This way of working is the *key* of QList design. QList is always a
vector of void* (so it's nice for Qt pimpled types).
...?
This allows to type erase the entire management of the vector (it's
always a vector of void*), reducing the amount of template code that
needs to be instantiated.
Okay, I'm going to *assume* that what you're trying to say, poorly, is
that QList<T> for relocatable T, sizeof(T) ≤ sizeof(void*), is
implemented in terms of casting the bits of T into an intptr_t, which is
further mangled into a void*.
Just because that's how it's implemented *now* doesn't preclude a new,
ABI-breaking version from being implemented as void*[N] for N less than
some small value. It would increase the code overhead *somewhat*, but
not unmanageably so (all allowable instances could still be explicitly
instantiated), and that is probably worth it for the difference in space
efficiency.
--
Matthew
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