Suppose I want to represent a two-dimensional matrix of int as a vector of vectors:
std::vector<std::vector<int> > myVec;
The inner dimension is constant, say 5, and the outer dimension is less than or equal to N. To minimize reallocations I would like to reserve space:
myVec.reserve(N);
What size is assumed for the inner vector? Is this purely implementation dependent? How does this effect the spatial locality of the data? Since the inner dimension is a constant is there a way to tell the compiler to use this constant size? How do these answers change if the inner vector's size changes?
Since your inner dimension is constant, I think you want
std::vector< std::array<int, 5> > vecs;
vecs.reserve(N);
This will give you preallocated contiguous storage, which is optimal for performance.
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