std::list iterator: get next element

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.e开发者_如何转开发nd(); 
        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?

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();
}

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.

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();
}

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.

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).

Note: Since C++11 you can use std::next and std::prev.