std::bitset - how to iterate "set" (or "not set") bits?

Question

I'm using bitset in my code:

std::bitset<MAX_INSTRUMENTS_NUMBER_IN_SYSTEM> updatedInstruments;

Often I need to iterate only values that "set" (or "not set"), this is how I do that:

for (int instrumentId = 0; instrumentId < MAX_INSTRUMENTS_NUMBER_IN_SYSTEM; instrumentId++) {
    if (!updatedInstruments[instrumentId]) {
        continue;
    }
    // work
}

Can this iteration be improved to be more readable and possibly faster?

Answer 1

I don't think you can exploit the contiguity of set bits in your code using std::bitset: the interface doesn't provide anything to help in this task, and there's no legal way to access the underlying storage and work directly with it.¹

If instead you can afford to change container, you can find several better alternatives. Here for example is a bitset-like structure that provides "enumerators" for active bits (it seems mostly like working with spans on images), and in the duplicate I linked above there are several other suggestions for data structures more specialized for this use case.

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1. Previously I supposed that iterators might have yielded some performance advantage, but turns out std::bitset doesn't have iterators :-o Also, a similar test performed on std::vector<bool> (which should pack bits more or less the same way) gave a ~2x slowdown using iterators both with g++ and clang++.