Get values from a boost::multi_index

Question

I have created a boost::multi_index successfully and inserted values too. I have two hashed indices to the multi_index. Both are member functions, but one is unique and the other one is non-unique.

I am trying to figure out the way to get the values from the container using the values of the hashes. I could not understand how I should do that. I searched online, and I see there are a lot of people have asked this question. But I dont understand what needs to be done. I saw a few solutions in C++11 but I dont use C++11 and I dont understand whats being done. Can someone please explain to me how to use it? Given below is my code,

#include "stdafx.h"
#include<multi_index_container.hpp>
#include<boost/multi_index/hashed_index.hpp>
#include<boost/multi_index/mem_fun.hpp>
#include<boost/multi_index/tag.hpp>



class RetClass
{
    int a, b;
};

class StoreMe
{
    RetClass ex;
    std::string exStr;
    int id;
public:
    void setId(RetClass a) 
    {
        ex = a;
    };


    virtual const RetClass& getId() const { return ex; }
    virtual std::string getIdString() const { return exStr; }
    int getUniqueId() const { return id; }
};

struct IndexByStringId{};
struct IndexByUniqueId{};

typedef boost::multi_index_container<
    StoreMe,
    boost::multi_index::indexed_by<
        boost::multi_index::hashed_unique<
            boost::multi_index::tag<IndexByStringId>,
            boost::multi_index::const_mem_fun<StoreMe, std::string,     &StoreMe::getIdString> 
        >,
        boost::multi_index::hashed_non_unique<
            boost::multi_index::tag<IndexByUniqueId>,
            boost::multi_index::const_mem_fun<StoreMe, int,     &StoreMe::getUniqueId> 
        >
    >
> mi_storeMe;

int _tmain(int argc, _TCHAR* argv[])
{
    return 0;
}

I want to be able to,

1. Get the values non-unique Id maps to
2. Get the value (if it exists) that the unique Id maps to

Please let me know the correct/simplest way to get this done. Also I don't use C++11.

Answer 1

Here's how you'd retrieve from the string-based index:

mi_storeMe container;

std::string needle = whatToSearchFor();
auto iterator = container.get<IndexByStringId>().find(needle);
if (iterator != container.get<IndexByStringId>().end())
  found(*iterator);
else
  notFound();

For the ID-based index, it's very similar:

mi_storeMe container;

RetClass needle = whatToSearchFor();
auto range = container.get<IndexByUniqueId>().equal_range(needle);
processRangeFromTo(range.first, range.second);

The answer uses auto from C++11 so that I can avoid spelling out the types involved. If you do not have access to C++11, please do the type deduction yourself by reading Boost's multi_index documentation. I cannot vouch for correctness, but I believe the iterator type can be spelled as

mi_storeMe::index<IndexByStringId>::type::iterator

---

Tangentially related: how to do printf-style debugging of multi-index containers without C++11.

First off, remember thatwhile you don't have auto, you still have type deduction in templates. No need to spell out types if a function template can deduce them:

template <class It>
void printContainerItems(It from, It to) {
  for (; from != to; ++from)
    print(*from);
}

printContainerItems(container.begin(), container.end());

Second, you can easily iterate over an index:

const mi_Container::index<IndexByIdString>::type& index = container.get<IndexByIdString>();
for (
  mi_Container::index<IndexByIdString>::type::iterat‌​or it = index.begin(), end = index.end();
  it != end;
  ++it
)
{
  operate_on(*it);
}