Vector of vectors, reserve

c++ stdvector
OSE picture OSE · Sep 24, 2013 · Viewed 17.5k times · Source

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?

Answer

Ben Voigt picture Ben Voigt · Sep 24, 2013

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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