nested unordered_maps in c++

Question

I'm in the process of re-writting a program I have in Java to C++. I am having a lot of trouble with the complex data structure I am using:

unordered_map< string, unordered_map<string, list<string> > >

It took me a while but I was eventually able to figure out how to add 'items' (for lack of a better word) to the unordered_map. However, I come to you because I cannot figure out how to retrieve the items that I put in it using unordered_map::find.

My code is below:

/*
* QueryDDex.cpp
*
*  Created on: Aug 13, 2013
*      Author: Zach Graceffa
*/

#include <zorba/store_manager.h>
#include <zorba/xquery_exception.h>
#include <zorba/zorba.h>
#include <zorba/iterator.h>
#include <zorba/xquery.h>
#include <zorba/item.h>

#include <tr1/unordered_map>
#include <string>
#include <fstream>
#include <list>

using namespace zorba;
using namespace std;
using namespace tr1;

void runQuery (char * inFile) throw(ZorbaException)
{
//create return variable
unordered_map< string, unordered_map<string, list<string> > > nodeContainer;

//open file
ifstream myFile;
const char * ext = ".xq";
myFile.open(strcat(inFile, ext), ifstream::in);

//Instantiate the Zorba Object
void* lStore = zorba::StoreManager::getStore();
Zorba* lZorba = Zorba::getInstance(lStore);

//Feed file into string
string line;
string xqDoc;

if (myFile.is_open())
{
    while (myFile.good())
    {
      getline (myFile, line);
      xqDoc += (line + "\n");
    }
    myFile.close();
}
else
    xqDoc = "err";

//Compile the Query
XQuery_t lQuery = lZorba->compileQuery(xqDoc);

//Create an Iterator and open it so it can be used
Iterator_t parentIterator = lQuery->iterator();
parentIterator->open();

//Create an empty Item for future use
Item lItem;

while (parentIterator->next(lItem))
{
    //Create an iterator to iterate over all the child nodes that belong to the parent
    Iterator_t childIterator = lItem.getChildren();

    //Open the iterator for future use
    childIterator->open();

    //Create an empty item, which will be used to store the child nodes.
    Item child;

    //Select the first child node
    while(childIterator->next(child)){
        unordered_map<string, list<string> > childOne;

        Iterator_t grandChildIterator = child.getChildren();
        grandChildIterator->open();

        Item grandChild;

        //Create an empty item to hold the section tag name.
        Item sectionName;
        child.getNodeName(sectionName);
        nodeContainer.insert(pair<string, unordered_map<string, list<string> > >(sectionName.getStringValue(), childOne));

        while(grandChildIterator->next(grandChild)){

            list<string> grandChildren;

            //Create an empty Item to hold the contents of tag name
            Item tagName;

            //Put the tag name in variable tagName
            grandChild.getNodeName(tagName);

            unordered_map<string, list<string> > temp;

            unordered_map< string, unordered_map<string, list<string> > >::const_iterator got = nodeContainer.find(sectionName.getStringValue());

            if (temp.key_eq(tagName.getStringValue())){
                list<string> s = temp.find(tagName.getStringValue());
                s.insert(grandChild.getStringValue());
                temp.put(sectionName.getStringValue(), s);
                }else{
                    grandChildren.add(grandChild.getStringValue());
                    temp.insert(tagName.getStringValue(), grandChildren);
                }
            nodeContainer.insert(pair<string, unordered_map<string, list<string> > >(sectionName.getStringValue(), temp));

            //Release any memory consumed by tagName
            tagName.close();
            //free tagName;

            }//grandchild-loop
            //Release any memory consumed by Item grandChild
            grandChild.close();
            //delete grandChild;
    }//child-loop
}//end parent-loop
}

I give you the entire file that I am currently working on. There are a lot of errors as I pasted the java code directly into my c++ ide and am simply working at it line by line. Please focus on this line of code:

unordered_map< string, unordered_map<string, list<string> > >::const_iterator got = nodeContainer.find(sectionName.getStringValue());

Another thing I should add is that I am rusty at c++ so if there is a better way to accomplish this functionality than a

unordered_map< string, unordered_map<string, list<string> > >

I am all ears.

Thank you for reading this far:)

Answer 1

To get a more specific error message you could break down your problematic line into:

const string &keyToTemp(sectionName.getStringValue());
unordered_map< string, unordered_map<string, list<string> > >::const_iterator got = nodeContainer.find(keyToTemp);

Then once that's working the next steps could be along these lines:

With the minimum changes to your code I suppose this is what you're missing:

temp = got->second;

find gives you an iterator to the element and the value_type of a map element is a pair<KeyType, ValueType> hence the use of second. That though will copy the nested map.

Maybe it would be better to use a reference to it instead. In which case the line you asking us to look at would become:

 unordered_map<string, list<string> > &temp(nodeContainer.find(sectionName.getStringValue())->second);