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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?
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std::vector::reserveRequests that the vector capacity be at least enough to contain n elements. If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater).
If the allocated memory capacity in the vector is large enough to contain the new elements, no additional allocations for the vector are needed. So no, then it won't reserve memory.
C++ Vector Library - reserve() FunctionThe C++ function std::vector::reserve() requests to reserve vector capacity be at least enough to contain n elements. Reallocation happens if there is need of more space.
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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