Random array generation with no duplicates

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You start off filling a container with consecutive elements beginning at 0

std::iota(begin(vec), end(vec), 0);

then you get yourself a decent random number generator and seed it properly

std::mt19937 rng(std::random_device{}());

finally you shuffle the elements using the rng

std::shuffle(begin(vec), end(vec), rng);

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On some implementations random_device doesn’t work properly (most notably gcc on windows) and you have to use an alternative seed, i.e. the current time → chrono.

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