Memory footprint of 5 byte QByteArray. Is quint64 or QByteArray of 5 bytes more efficient for storage?

Question

Checking size of QByteArray always returns 4 bytes, I'm assuming due to implicit sharing of data in Qt:

int n = 50; //or n = 100, 200
QByteArray arr(n,'a');
cout << sizeof(arr) << endl;
::getchar();

Always prints 4

How can I estimate the actual memory footprint of QByteArray? The question is motivated by efficiently storing large number of 5 byte identifiers - they can either be stored each as quint64 (using 8 bytes for each, 3 bytes are therefore wasted), or as each as QByteArray - but I'm not sure how to estimate overhead in the latter case....

I would like to use these identifiers as key for QMap, so they should each be in their own structure - one long QByteArray won't work...

Answer 1

The actual data for QByteArray (in Qt 4.8) can be found in qbytearray.h and looks like this:

struct Data {
    QBasicAtomicInt ref;
    int alloc, size;
    char *data;
    char array[1];
};

So a quint64 will use less storage if your data fits into it.

Answer 2

sizeof(arr) shows you the pointer size of the object. do arr.squeeze(); then arr.capacity();

But dont keep the squeeze call for final code, what it does is gets rid of any prealocated unused memory by the object, so it reallocates and memcopies (expensive).