Well, besides the fact that this is one of this super-theoretical examples, yes – it is possible. But do the math on yourself: (probably) 8 bytes per int multiplied by one billion plus overhead for memory allocation. Now compare this number to your memory available.

I have 48GB of RAM.
8 bytes multiplied by 1 billion is only 8GB of RAM required.

Does Qt use ‘int’ internally to pass the size of new memory allocations for growing a QVector?

The int is 4 bytes on x86_64 so it’s rather 4GB of RAM.

Do you get the exception on

There is no exception using:

Also, there is no exception if I use std::vector and not QVector.

Do you allocate the vector like you show in the example? Could you try this:

However, I think I would not use the Qt container classes for such data blobs. This is the point where the copy-on-write of the Qt containers becomes a burden rather than a benefit.

I think moderator meant as follows?

Or in case the vector size is unknown up-front:

Thanks for the suggestion however it still throws an exception.

It will even throw an exception just doing a reserve as follows:

I have investigated the problem and it seems to be as follows:

1. QVector<T>::malloc(int aalloc) is called by vec.reserve(), where aalloc is 1 billion.
2. In QVector<T>::malloc we call:

The first argument ‘size’ is:
4 + (1000000000 – 1) * 4 = 4000000000 which is greater than the positive range of a signed int.

Looking at the declaration of QVectorData::allocate we see that the first argument is an int:

So it looks like it has an overflow error.

Note: even 550 million messages throws the exception, but below this the allocate passes, presumably because the size is less than the positive range of an int.

andrewjlouth wrote:

I have 48GB of RAM.
8 bytes multiplied by 1 billion is only 8GB of RAM required.

Does Qt use ‘int’ internally to pass the size of new memory allocations for growing a QVector?

Yes. And asking the OS for a contiguous chunk of 8GB isn’t something I would get rid off with an “only”.

this has nothing to do with int and long ?
can you try with:

vec.reserve(1000000000L);

deimos wrote:

this has nothing to do with int and long ?
can you try with:

vec.reserve(1000000000L);

Actually, no. QVector (and all other container classes) only work with integers (int, not long or long long). So, I guess it is just outside of the Qt containers specs to try to strore that many items in one. I would recommend using a different container class for that, from outside of Qt.

[edit: answer put outside quote, Eddy]