std::list iterator: get next element

Question

I'm trying to build a string using data elements stored in a std::list, where I want commas placed only between the elements (ie, if elements are {A,B,C,D} in list, result string should be "A,B,C,D".

This code does not work:

typedef std::list< shared_ptr<EventDataItem> > DataItemList;
// ...
std::string Compose(DataItemList& dilList)
{
    std::stringstream ssDataSegment;
    for(iterItems = dilList.begin();
        iterItems != dilList.end(); 
        iterItems++)
    {
        // Lookahead in list to see if next element is end
        if((iterItems + 1) == dilList.end())  
        {
            ssDataSegment << (*iterItems)->ToString();
        }
        else
        {
            ssDataSegment << (*iterItems)->ToString() << ",";
        }
    }
    return ssDataSegment.str();
}

How do I get at "the-next-item" in a std::list using an iterator? I would expect that it's a linked-list, why can't I get at the next item?

Answer 1

You cannot do it + N because you have no random access for list iterators. You can only do one step at a time with list iterators (these are bidirectional iterators).

You can use boost::next and boost::prior

// Lookahead in list to see if next element is end
if(boost::next(iterItems) == dilList.end())  
{

Or you can print the comma before:

std::string Compose(DataItemList& dilList)
{
    std::stringstream ssDataSegment;
    for(iterItems = dilList.begin();
        iterItems != dilList.end(); 
        ++iterItems)
    {
        if(iterItems != diList.begin())
            ssDataSegment << ",";
        ssDataSegment << (*iterItems)->ToString();
    }
    return ssDataSegment.str();
}

Answer 2

I believe that a list iterator is bidirectional, but not random access. That means that you can do ++ and -- to it but not add or subtract.

To get the next iterator, make a copy and increment it.

Answer 3

You could avoid this problem altogether by using:

std::string Compose(DataItemList& dilList)
{
    std::stringstream ssDataSegment;
    for(iterItems = dilList.begin(); iterItems != dilList.end(); iterItems++)
    {
        ssDataSegment << (*iterItems)->ToString() << ","; // always write ","
    }
    std::string result = ssDataSegment.str();
    return result.substr(0, result.length()-1); // skip the last ","
}

You first write the "," for all elements (even for the last one). Than afterwards, you remove the unwanted last "," using substr. This additionally results in clearer code.

Answer 4

Another solution is to have the first entry be the special case, instead of the last entry:

std::string Compose(DataItemList& dilList)
{
    std::stringstream ssDataSegment;
    for(iterItems = dilList.begin();
        iterItems != dilList.end(); 
        ++iterItems)
    {
        // See if current element is the first
        if(iterItems == dilList.begin())  
        {
            ssDataSegment << (*iterItems)->ToString();
        }
        else
        {
            ssDataSegment << "," << (*iterItems)->ToString();
        }
    }
    return ssDataSegment.str();
}

Answer 5

Yet another possibility:

#include "infix_iterator.h"
#include <sstream>

typedef std::list<shared_ptr<EventDataItem> > DataItemList;

std::string Compose(DataItemList const &diList) {
    std::ostringstream ret;
    infix_ostream_iterator out(ret, ",");

    for (item = diList.begin(); item != diList.end(); ++item)
        *out++ = (*item)->ToString();
    return ret.str();
}

You can get infix_iterator.h from Google's Usenet archive (or various web sites).