How to index and query STL map containers by multiple keys?

Question

I came across one requirement where the record is stored as

Name :  Employee_Id  :  Address

where Name and Employee_Id are supposed to be k

Answer 1

If EmployeeID is the unique identifier, why use other keys? I would use EmployeeID as the internal key everywhere, and have other mappings from external/human readable IDs (such as Name) to it.

Answer 2

Boost.Multiindex

This is a Boost example

In the above example an ordered index is used but you can use also a hashed index:

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <string>
#include <iostream>

struct employee
{
    int         id_;
    std::string name_;
    std::string address_;

    employee(int id,std::string name,std::string address):id_(id),name_(name),address_(address) {}

};

struct id{};
struct name{};
struct address{};
struct id_hash{};
struct name_hash{};


typedef boost::multi_index_container<
    employee,
    boost::multi_index::indexed_by<
        boost::multi_index::ordered_unique<boost::multi_index::tag<id>,  BOOST_MULTI_INDEX_MEMBER(employee,int,id_)>,
        boost::multi_index::ordered_unique<boost::multi_index::tag<name>,BOOST_MULTI_INDEX_MEMBER(employee,std::string,name_)>,
        boost::multi_index::ordered_unique<boost::multi_index::tag<address>, BOOST_MULTI_INDEX_MEMBER(employee,std::string,address_)>,
        boost::multi_index::hashed_unique<boost::multi_index::tag<id_hash>,  BOOST_MULTI_INDEX_MEMBER(employee,int,id_)>,
        boost::multi_index::hashed_unique<boost::multi_index::tag<name_hash>,  BOOST_MULTI_INDEX_MEMBER(employee,std::string,name_)>
    >
> employee_set;

typedef boost::multi_index::index<employee_set,id>::type employee_set_ordered_by_id_index_t;
typedef boost::multi_index::index<employee_set,name>::type employee_set_ordered_by_name_index_t;
typedef boost::multi_index::index<employee_set,name_hash>::type employee_set_hashed_by_name_index_t;

typedef boost::multi_index::index<employee_set,id>::type::const_iterator  employee_set_ordered_by_id_iterator_t;
typedef boost::multi_index::index<employee_set,name>::type::const_iterator  employee_set_ordered_by_name_iterator_t;


typedef boost::multi_index::index<employee_set,id_hash>::type::const_iterator  employee_set_hashed_by_id_iterator_t;
typedef boost::multi_index::index<employee_set,name_hash>::type::const_iterator  employee_set_hashed_by_name_iterator_t;


int main()
{
    employee_set employee_set_;

    employee_set_.insert(employee(1, "Employer1", "Address1"));
    employee_set_.insert(employee(2, "Employer2", "Address2"));
    employee_set_.insert(employee(3, "Employer3", "Address3"));
    employee_set_.insert(employee(4, "Employer4", "Address4"));

    // search by id using an ordered index 
    {
        const employee_set_ordered_by_id_index_t& index_id = boost::multi_index::get<id>(employee_set_);
        employee_set_ordered_by_id_iterator_t id_itr = index_id.find(2);
        if (id_itr != index_id.end() ) {
            const employee& tmp = *id_itr;
            std::cout << tmp.id_ << ", " << tmp.name_ << ", "  << tmp .address_ << std::endl;
        } else {
            std::cout << "No records have been found\n";
        }
    }

    // search by non existing id using an ordered index 
    {
        const employee_set_ordered_by_id_index_t& index_id = boost::multi_index::get<id>(employee_set_);
        employee_set_ordered_by_id_iterator_t id_itr = index_id.find(2234);
        if (id_itr != index_id.end() ) {
            const employee& tmp = *id_itr;
            std::cout << tmp.id_ << ", " << tmp.name_ << ", "  << tmp .address_ << std::endl;
        } else {
            std::cout << "No records have been found\n";
        }
    }

    // search by name using an ordered index
    {
        const employee_set_ordered_by_name_index_t& index_name = boost::multi_index::get<name>(employee_set_);
        employee_set_ordered_by_name_iterator_t name_itr = index_name.find("Employer3");
        if (name_itr != index_name.end() ) {
            const employee& tmp = *name_itr;
            std::cout << tmp.id_ << ", " << tmp.name_ << ", "  << tmp .address_ << std::endl;
        } else {
            std::cout << "No records have been found\n";
        }
    }

    // search by name using an hashed index
    {
        employee_set_hashed_by_name_index_t& index_name = boost::multi_index::get<name_hash>(employee_set_);
        employee_set_hashed_by_name_iterator_t name_itr = index_name.find("Employer4");
        if (name_itr != index_name.end() ) {
            const employee& tmp = *name_itr;
            std::cout << tmp.id_ << ", " << tmp.name_ << ", "  << tmp .address_ << std::endl;
        } else {
            std::cout << "No records have been found\n";
        }
    }

    // search by name using an hashed index but the name does not exists in the container
    {
        employee_set_hashed_by_name_index_t& index_name = boost::multi_index::get<name_hash>(employee_set_);
        employee_set_hashed_by_name_iterator_t name_itr = index_name.find("Employer46545");
        if (name_itr != index_name.end() ) {
            const employee& tmp = *name_itr;
            std::cout << tmp.id_ << ", " << tmp.name_ << ", "  << tmp .address_ << std::endl;
        } else {
            std::cout << "No records have been found\n";
        }
    }

    return 0;
}

Answer 3

If you want to use std::map, you can have two separate containers, each one having adifferent key (name, emp id) and the value should be a pointer the structure, so that you will not have multiple copies of the same data.

Answer 4

Example with tew keys:

#include <memory>
#include <map>
#include <iostream>

template <class KEY1,class KEY2, class OTHER >
class MultiKeyMap {
  public:
  struct Entry
  {
    KEY1 key1;
    KEY2 key2;
    OTHER otherVal;
    Entry( const KEY1 &_key1,
           const KEY2 &_key2,
           const OTHER &_otherVal):
           key1(_key1),key2(_key2),otherVal(_otherVal) {};
    Entry() {};
  };
  private:
  struct ExtendedEntry;
  typedef std::shared_ptr<ExtendedEntry> ExtendedEntrySptr;
  struct ExtendedEntry {
    Entry entry;
    typename std::map<KEY1,ExtendedEntrySptr>::iterator it1;
    typename std::map<KEY2,ExtendedEntrySptr>::iterator it2;
    ExtendedEntry() {};
    ExtendedEntry(const Entry &e):entry(e) {};
  };
  std::map<KEY1,ExtendedEntrySptr> byKey1;
  std::map<KEY2,ExtendedEntrySptr> byKey2;

  public:
  void del(ExtendedEntrySptr p)
  {
    if (p)
    {
      byKey1.erase(p->it1);
      byKey2.erase(p->it2);
    }
  }

  void insert(const Entry &entry) {
    auto p=ExtendedEntrySptr(new ExtendedEntry(entry));
    p->it1=byKey1.insert(std::make_pair(entry.key1,p)).first;
    p->it2=byKey2.insert(std::make_pair(entry.key2,p)).first;
  }
  std::pair<Entry,bool> getByKey1(const KEY1 &key1) 
  {
    const auto &ret=byKey1[key1];
    if (ret)
      return std::make_pair(ret->entry,true);
    return std::make_pair(Entry(),false);
  }
  std::pair<Entry,bool> getByKey2(const KEY2 &key2) 
  {
    const auto &ret=byKey2[key2];
    if (ret)
      return std::make_pair(ret->entry,true);
    return std::make_pair(Entry(),false);
  }
  void deleteByKey1(const KEY1 &key1)
  {
    del(byKey1[key1]);
  }
  void deleteByKey2(const KEY2 &key2)
  {
    del(byKey2[key2]);
  }
};


int main(int argc, const char *argv[])
{
  typedef MultiKeyMap<int,std::string,int> M;
  M map1;
  map1.insert(M::Entry(1,"aaa",7));
  map1.insert(M::Entry(2,"bbb",8));
  map1.insert(M::Entry(3,"ccc",9));
  map1.insert(M::Entry(7,"eee",9));
  map1.insert(M::Entry(4,"ffffd",9));
  map1.deleteByKey1(7);
  auto a=map1.getByKey1(2);
  auto b=map1.getByKey2("ffffd");
  auto c=map1.getByKey1(7);
  std::cout << "by key1=2   (should be bbb ): "<< (a.second ? a.first.key2:"Null") << std::endl;
  std::cout << "by key2=ffffd (should be ffffd ): "<< (b.second ? b.first.key2:"Null") << std::endl;
  std::cout << "by key1=7   (does not exist): "<< (c.second ? c.first.key2:"Null") << std::endl;
  return 0;
}

Output:

by key1=2   (should be bbb ): bbb
by key2=ffffd (should be ffffd ): ffffd
by key1=7   (does not exist): Null

Answer 5

C++14 std::set::find non-key searches solution

This method saves you from storing the keys twice, once one the indexed object and secondly on as the key of a map as done at: https://stackoverflow.com/a/44526820/895245

This provides minimal examples of the central technique that should be easier to understand first: How to make a C++ map container where the key is part of the value?

#include <cassert>
#include <set>
#include <vector>

struct Point {
    int x;
    int y;
    int z;
};

class PointIndexXY {
    public:
        void insert(Point *point) {
            sx.insert(point);
            sy.insert(point);
        }
        void erase(Point *point) {
            sx.insert(point);
            sy.insert(point);
        }
        Point* findX(int x) {
            return *(this->sx.find(x));
        }
        Point* findY(int y) {
            return *(this->sy.find(y));
        }
    private:
        struct PointCmpX {
            typedef std::true_type is_transparent;
            bool operator()(const Point* lhs, int rhs) const { return lhs->x < rhs; }
            bool operator()(int lhs, const Point* rhs) const { return lhs < rhs->x; }
            bool operator()(const Point* lhs, const Point* rhs) const { return lhs->x < rhs->x; }
        };
        struct PointCmpY {
            typedef std::true_type is_transparent;
            bool operator()(const Point* lhs, int rhs) const { return lhs->y < rhs; }
            bool operator()(int lhs, const Point* rhs) const { return lhs < rhs->y; }
            bool operator()(const Point* lhs, const Point* rhs) const { return lhs->y < rhs->y; }
        };
        std::set<Point*, PointCmpX> sx;
        std::set<Point*, PointCmpY> sy;
};

int main() {
    std::vector<Point> points{
        {1, -1, 1},
        {2, -2, 4},
        {0,  0, 0},
        {3, -3, 9},
    };
    PointIndexXY idx;
    for (auto& point : points) {
        idx.insert(&point);
    }
    Point *p;
    p = idx.findX(0);
    assert(p->y == 0 && p->z == 0);
    p = idx.findX(1);
    assert(p->y == -1 && p->z == 1);
    p = idx.findY(-2);
    assert(p->x == 2 && p->z == 4);
}