How do I iterate equal values with the standard library?

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This is basically Range v3’s group_by: group_by(v, std::equal_to{}). It doesn’t exist in the C++17 standard library, but we can write our own rough equivalent:

template <typename FwdIter, typename BinaryPred, typename ForEach>
void for_each_equal_range(FwdIter first, FwdIter last, BinaryPred is_equal, ForEach f) {
    while (first != last) {
        auto next_unequal = std::find_if_not(std::next(first), last,
            [&] (auto const& element) { return is_equal(*first, element); });

        f(first, next_unequal);
        first = next_unequal;
    }
}

Usage:

for_each_equal_range(v.begin(), v.end(), std::equal_to{}, [&] (auto first, auto last) {
    for (; first != last; ++first) {
        // Do something with each element.
    }
});

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