How to Calculate the memory occupied by a QMap object

[wysota]

for the QMap<qint32,qint32> 10,000 items, it took about 256 KB however, it should take about 80KB = ( 10,000 * 8 ) .

No, it would take more, 80kB is just an estimation.

and for the QMap<qint32,QString> 10,000 items, it took about 960 KB however, it should take about 468KB = ( 10,000 * 8 + 40*10,000 ) .

QString is much heavier than int, you can't consider raw data as a good estimation in this case, especially that QString keeps its data as unicode (which immediately doubles memory usage). There is also a matter of using p-impl.

[ahmedb]

I understand that 80KB is just an estimation, but I got 256KB which is much larger than expected !! and for the QString I already calculated the size assuming every character is two bytes length. and that was not useful as well.

anyway, I needed some equation by which I can get near the real size of the object. but it looks harder than I thought.

Thanks again for your help.

[squidge]

You can't rely on the results from Windows Task Manager. For example, If you allocate memory but don't write to it, its not regarded as part of your process. The actual allocation will occur when you access that memory.

Under Windows, you can use the GetProcessMemoryInfo API to estimate the amount of memory usage before and after the allocation of an object, but this is again subject to the compiler actually allocating memory from the system, rather than from it's own heap. When you then delete this memory it may not be returned to the system straight away.

A loop allocating 10,000 items and checking the process memory information each time would give you a decent estimate however.