How to add the ordered unique index specifies in multi_index_container

Question

I'm using boost:multi_index_container. following is the code

#include <string>
#include <boost/multi_index_container.hpp>
#include "boost/multi_index/global_fun.hpp"
#include "boost/multi_index/hashed_index.hpp"
#include <boost/multi_index/sequenced_index.hpp>
#include "boost/multi_index/mem_fun.hpp"
#include "boost/multi_index/member.hpp"
#include "boost/multi_index/ordered_index.hpp"
#include "boost/multi_index/indexed_by.hpp"

using namespace std;

class uri
{
public:
    string str;
    uri(string s) :str(s)
    {
    }
    string getUri(){ return str; }

};


struct UriWithBase
{
    UriWithBase(uri u1, uri u2) :link(u1), baseLink(u2){}
    uri link;
    uri baseLink;
};



int main()
{
    typedef boost::multi_index::multi_index_container <
        UriWithBase*,
        boost::multi_index::indexed_by <
        boost::multi_index::sequenced < >
        >
    > UriMultiIndex;

    UriMultiIndex m_uris;

    uri urib1("www.google.com");
    uri uri1("www.news.google.com");

    UriWithBase *ubptr = new UriWithBase(uri1, urib1);

    m_uris.push_back(ubptr);
    return 0;
}

Now I want to add a ordered unique index specifies in multi_index_container. This index is for unique order of elements in the container depends upon the first element of "UriWithBase" class i.e. uri link. I tried following but not worked. boost::multi_index::ordered_unique<

    boost::multi_index::tag<uris_by_uri>,
BOOST_MULTI_INDEX_CONST_MEM_FUN(UriWithBasePtr, const uri&, (*(UriWithBasePtr)).link)>

Answer 1

There a few ways to add an ordered index:

1. by using the ordered_unique specifier together with a custom comparator

   class uri
   {
   public:
       string str;
       uri(string s) :str(s)
       {
       }
       string getUri() const { return str; }
   };
   
   struct UriWithBase
   {
       UriWithBase(uri u1, uri u2) :link(u1), baseLink(u2){}
       uri link;
       uri baseLink;
   };
   
   struct CUriComparator {
       bool operator()(const uri &lh, const uri &rh) const {
           return lh.str < rh.str;
       }
   };
   
   struct uris_by_uri_and_crawl_state {};
   
   typedef boost::multi_index::multi_index_container <
       UriWithBase*,
       boost::multi_index::indexed_by <
         boost::multi_index::sequenced<>,
         boost::multi_index::ordered_unique <
           boost::multi_index::tag<uris_by_uri_and_crawl_state>,
           BOOST_MULTI_INDEX_MEMBER(UriWithBase, uri, link),
           CUriComparator
         >
       >
   > UriMultiIndex;
   

2. by defining a compare operator of the uri class

   class uri
   {
   public:
       string str;
       uri(string s) :str(s)
       {
       }
       string getUri(){ return str; }
       bool operator < (const uri &rh) const    {
           return this->str < rh.str;
       }
   };
   
   struct UriWithBase
   {
       UriWithBase(uri u1, uri u2) :link(u1), baseLink(u2){}
       uri link;
       uri baseLink;
   };
   
   struct uris_by_uri_and_crawl_state {};
   
   typedef boost::multi_index::multi_index_container <
       UriWithBase*,
       boost::multi_index::indexed_by <
         boost::multi_index::sequenced<>,
           boost::multi_index::ordered_unique <
               boost::multi_index::tag<uris_by_uri_and_crawl_state>,
               BOOST_MULTI_INDEX_MEMBER(UriWithBase, uri, link)
           >
       >
   > UriMultiIndex;
   

3. by declaring a method (UriWithBase::getLinkUri) in the UriWithBase class which returns the link's str member value.

   class uri
   {
   public:
       string str;
       uri(string s) :str(s)
       {
       }
       string getUri() const { return str; }
   };
   
   struct UriWithBase
   {
       UriWithBase(uri u1, uri u2) :link(u1), baseLink(u2){}
       uri link;
       uri baseLink;
   public:
       string getLinkUri(void) const {
           return link.getUri();
       }
   };
   
   struct uris_by_uri_and_crawl_state {};
   
   typedef boost::multi_index::multi_index_container <
       UriWithBase*,
       boost::multi_index::indexed_by <
           boost::multi_index::sequenced<>,
           boost::multi_index::ordered_unique <
               boost::multi_index::tag<uris_by_uri_and_crawl_state>,
               BOOST_MULTI_INDEX_CONST_MEM_FUN(UriWithBase, string, getLinkUri)
           >
       >
   > UriMultiIndex;
   

Answer 2

class uri
{
public:
    string str;
    uri(string s) :str(s)
    {
    }

    // note we made this const
    string getUri() const { return str; }
};

inline bool operator<(const uri& x,const uri& y)
{
  return x.getUri()<y.getUri();
}

...

typedef boost::multi_index::multi_index_container <
    UriWithBase*,
    boost::multi_index::indexed_by <
    boost::multi_index::sequenced < >,
    boost::multi_index::ordered_unique<
        boost::multi_index::tag<uris_by_uri_and_crawl_state>,
        boost::multi_index::member<UriWithBase,uri,&UriWithBase::link>
    >
    >
> UriMultiIndex;