ville.voutilainen at gmail.com
Mon Mar 20 13:37:41 CET 2017
On 20 March 2017 at 14:13, Robin Burchell <robin.burchell at crimson.no> wrote:
>> And when size matters, have this in mind:
>> sizeof(std::vector<int>) == 32
>> sizeof(QVector<int>) == 8
>> sizeof(QList<int>) == 8
> To provide a more helpful response than Marc's: I assume that you are
> aware that the Qt types just contain a pointer to a thing which contains
> the actual data (unlike std::vector), right?
libstdc++ vector is 24, it seems as if msvc's vector is just padded to
an alignment boundary(*). I don't
see anything particularly alarming in that size.
(*) it might be remotely possible that it fails to remove storage for
an empty allocator, but I kinda doubt that.
> That is, QVector<int> is actually (snipped from source):
> template <typename T>
> class QVector
> typedef QTypedArrayData<T> Data;
> Data *d;
> Where QTypedArrayData<T> is a QArrayData, which is:
> struct Q_CORE_EXPORT QArrayData
> QtPrivate::RefCount ref;
> int size;
> uint alloc : 31;
> uint capacityReserved : 1;
> qptrdiff offset;
> If I'm adding it up right, this gives me a total cost of 32 bytes on my
> system. Although the real cost is actually higher I'd guess, due to the
> two being allocated in separate pieces (given that malloc usually has
> some kind of book-keeping overhead).
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