qt vector reallocation during append()

user4592357

i was looking through qt vector append() source and i noticed that reallocation during insertion doesn't work alike std::vector. the latter allocates a new memory of size size * 1.5 while qt vector allocates a new memory of size + 1. why is this difference? also, this makes append linear, so what's the good?

SGaist

Hi,

That's a question better asked on the interest mailing list. You'll find there Qt's developers/maintainers. This is forum is more user oriented.

Christian Ehrlicher

@user4592357 said in qt vector reallocation during append():

while qt vector allocates a new memory of size + 1

This is absolutely wrong. It just passes the new size to an internal function named realloc. What you miss is that the second parameter is set to QArrayData::Grow which will do the right thing.

user4592357

@Christian-Ehrlicher
okay, so what's the size of the new vector, after re-allocation?

Christian Ehrlicher

@user4592357 said in qt vector reallocation during append():

what's the size of the new vector

The size is oldSize+1, the reseved size can be seen with capacity()

/edit: simply following the calls you will get to this function: qCalculateGrowingBlockSize