std::multimap<int, std::pair<std::function<...>, bool>> target_subs{};

//...
// various insertions and deletions...
//...

for (auto sub = begin(target_subs); sub != end(target_subs); ) {
    if (sub->second.second) {
        (sub++)->second.first(evt);

    } else {
        sub->second.first(evt);
        ++sub;
    }
}

size_t valid_passes{0};

for (auto sub = begin(target_subs); sub != end(target_subs); ) {
    if (sub->second.second) {
        (sub++)->second.first(evt);
        if (target_subs.size() < valid_passes) { break; } // Not very useful...
        // possible to erase all elements and insert the same number back in

    } else {
        sub->second.first(evt);
        ++sub;
    }

    ++valid_passes;
}