Question

I am experiencing a problem with the vector container. I am trying to improve the performance of inserting a lot of elements into one vector.

Basically I am using vector::reserve to expand my vector _children if needed:

if (_children.capacity() == _children.size())
{
     _children.reserve(_children.size() * 2);
}

and using vector::at() to insert a new element at the end of _children instead of vector::push_back():

_children.at(_children.size()) = child;

_children has already one element in it, so the first element should be inserted at position 1, and the capacity at this time is 2.

Despite this, an out_of_range error is thrown. Can someone explain to me, what I misunderstood here? Is it not possible to just insert an extra element even though the chosen position is less than the vector capacity? I can post some more code if needed.

Thanks in advance.

/mads

Answer 1

Increasing the capacity doesn’t increase the number of elements in the vector. It simply ensures that the vector has capacity to grow up to the required size without having to reallocate memory. I.e., you still need to call push_back().

Mind you, calling reserve() to increase capacity geometrically is a waste of effort. std::vector already does this.