Create multiple indexes into a large collection of objects with smart pointers

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粉色の甜心 2021-01-16 18:15

I am creating multiple indexes (ie, that use different keys) into a large collection of objects. The objects can change, and the collection can shrink and grow. My thought

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温柔的废话 (楼主)

2021-01-16 18:51

Here's one way.

std::vector to hold the data items (to ensure that the addresses don't change when the vector resizes) and then containers holding reference_wrappers (copyable references) to make the indexes.

compilable example:

#include 
#include 
#include 
#include 
#include 
#include 
#include 

struct Thing {
    Thing(std::string name, int value)
    : _name { std::move(name) }
    , _value { value }
    {}

    const std::string& name() const {
        return _name;
    }

    void write(std::ostream& os) const {
        os << "{ " << _name << " : " << _value << " }";
    }    
private:
    std::string _name;
    int _value;
};

inline std::ostream& operator<<(std::ostream& os, const Thing& t) {
    t.write(os);
    return os;
}

struct multi_index
{
    using multi_by_name_index = std::multimap>;

    void add_thing(std::string name, int value) {

        // todo: checks to ensure that indexes won't be violated

        // add a new thing to the main store
        _main_store.emplace_back(new Thing{std::move(name), value});

        // store a reference to it in each index
        auto& new_thing = *(_main_store.back().get());
        _name_index.emplace(new_thing.name(), new_thing);
    }

    using multi_by_name_range = std::pair;
    multi_by_name_range get_all_by_name(const std::string name) const
    {
        return _name_index.equal_range(name);
    }

private:
    std::vector> _main_store;
    std::multimap> _name_index;
};

using namespace std;

int main()
{
    multi_index mi;

    mi.add_thing("bob", 8);
    mi.add_thing("ann", 4);
    mi.add_thing("bob", 6);

    auto range = mi.get_all_by_name("bob");
    for( ; range.first != range.second ; ++range.first) {
        cout << range.first->second << endl;
    }

   return 0;
}

expected output:

{ bob : 8 }                                                                                                                             
{ bob : 6 }

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