[Exceptional C++ Style] Item 26 -- Data Formats and Efficiency

[Kloss, Burkhard (IT)]

Lois, 

> Solutions
> 1. Each kind of container in the standard C++ library chooses 
> a different trade-off of space vs performance.
> 
> - A vector<T> stores T objects in a contiguous array and so 
> has no per-element overhead at all.
That is if the vector's capacity is the same as its size.  If you grow a
vector with push_back, you can actually have an overhead of a 100% at
times because the vector doubles its internal memory allocation when it
reallocates.

    vector <double> v;
    for (int i = 0; i < 200; ++i)
    {
        cout << i << ", " << v.size () << ", " << v.capacity () << "\n";
        v.push_back (i);
    }

==> 
0, 0, 0
1, 1, 1
2, 2, 2
3, 3, 4
4, 4, 4
5, 5, 8
6, 6, 8
7, 7, 8
8, 8, 8
9, 9, 16
etc.

Just a minor point, but worth bearing in mind.  Depending on your
situation, it might be better to pre-allocate the capacity in the vector
first.

	Burkhard 
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