C++ Map with Multiple Keys.md

I recently had need for a C++ collection which could be accessed with multiple keys. Specifically, I had an object that I wanted to look up either by name or id (in O(1) time). One way to do this is to create two unordered_maps – one keyed by id and another by name, but that is error-prone. I ended up finding a boost class which will do this. It took me a while get it working, so I thought I would share the result.

Person.h

//The object being stored in the collection
struct Person
{
    int Id;
    std::string Name;
};

PersonCache.h

//A consumer of the collection
class PersonCache
{
private:
	PersonMap m_People;

public:
	PersonCache();
	~PersonCache();

    bool IsEmpty() const
    {
    	return m_People.empty();
    }
    
    Person* GetPersonByName(std::string lowerCaseName) const
    {
    	auto& indexByName = m_People.get<IndexByName>();
    	auto iter = indexByName.find(lowerCaseName);
    	if (iter != indexByName.end()) return &iter.get_node()->value();
    	
    	return nullptr;
    }
    
    Person* GetPersonById(const int id) const
    {
    	auto& indexById = m_People.get<IndexById>();
    	auto iter = indexById.find(id);
    	if (iter != indexById.end()) return &iter.get_node()->value();
    
    	return nullptr;
    }
    
	PersonCache::iterator begin()
    {
    	return m_People.begin();
    }
    
    PersonCache::iterator end()
    {
    	return m_People.end();
    }
};

PersonMap.h

//the collection...

//These structs just allow us to name the indexes
struct IndexById {};
/// <summary>
/// This index uses a lower case key extractor, so any lookup against this index should submit a lower case string.
/// </summary>
struct IndexByName {};

/// <summary>
/// When keys are pulled out for comparison, they will be converted to lower case.
/// Built from examples here: http://boost.2283326.n4.nabble.com/multi-index-index-based-on-case-insensitive-strings-td2565253.html
/// That link also provides a more generic, reusable way to do this which is more verbose.
/// </summary>
struct LowerCaseKeyExtractor
{
	typedef std::string result_type;

	std::string operator()(const Person& person) const
	{
		return boost::algorithm::to_lower_copy(person.Name);
	}
};

//This was implemented using ideas from https://stackoverflow.com/questions/39510143/how-to-use-create-boostmulti-index/39510606#39510606
//There is a good explanation of each part there.  See also https://www.boost.org/doc/libs/1_64_0/libs/multi_index/doc/tutorial/basics.html
typedef 
	boost::multi_index_container<
		Person, //The data type stored
		boost::multi_index::indexed_by< //List of indexes
			boost::multi_index::hashed_unique<
				boost::multi_index::tag<IndexById>, //Give the index a name
				boost::multi_index::member<Person, int, &Person::Id> //Use Id as the key
			>,
			boost::multi_index::hashed_non_unique<
				boost::multi_index::tag<IndexByName>, //Give the index a name
				LowerCaseKeyExtractor
			>
		>
	> PersonMap;