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i made boost multi index set of several indices most of them are ordered non unique.

now i need to make for loop over the set using iterator for ordered non unique index but i need to skip the structs with same non unique value.

in other words i need to loop only using the unique values of this ordered non unique key.

is there any way to identify only the unique values in non unique key and use this unique values to loop??

i did not put code because it is the method in general i am looking for.

i know about std::unique but i do not want to make another container as the original one is being modified during making the container which make synchronization difficult for me.

Joaquín M López Muñoz
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ahmed allam
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1 Answers1

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You may iterate through container using upper_bound function of container. Here's working example:

struct MyStruct {
    int group_id, id;
};

struct ByGroupID {};

using StructContainer = boost::multi_index_container<
    MyStruct,
    boost::multi_index::indexed_by<
        boost::multi_index::ordered_non_unique<
            boost::multi_index::tag<ByGroupID>,
            boost::multi_index::member<MyStruct, int, &MyStruct::group_id>
        >
    >
>;

int main() {
    StructContainer my_structs;

    my_structs.insert({1, 1});
    my_structs.insert({1, 2});
    my_structs.insert({1, 3});
    my_structs.insert({2, 4});
    my_structs.insert({2, 5});
    my_structs.insert({3, 6});

    auto &by_group_id = my_structs.get<ByGroupID>();

    for (auto it = by_group_id.begin(); it != by_group_id.end();) {
        std::cout << it->group_id << "\n";
        it = by_group_id.upper_bound(it->group_id);
    }
}

It outputs 1, 2, 3.

pure cuteness
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