How? Could you point out how to do this? Many thanks.

the attribute without ~ should not be printed right???

Thanks a lot for the clarification!

if '~' is replaced with '.' in query string, we can modify processing of query like this:

void process_queries(int q, std::map <std::string, std::string> &attributes)
{
    std::string str;
    for (int i = 0; i < q; i++)
    {
        getline(std::cin, str);
        if (attributes.find(str) == attributes.end())
        {
            size_t pos = str.find("~");
            if (pos != std::string::npos)
            {
                str[pos] = '.';
                if (attributes.find(str) != attributes.end())
                {
                    std::cout << attributes[str] << "\n";
                }
            }
            std::cout << "Not Found!\n";
        }
        else
        {
            std::cout << attributes[str] << "\n";
        }
    }
}

It clears the value of the string 'tag_preamble' :p Are you curious to know, what is the function of 'tag_preamble'? Try to figure out what are the keys in the map 'attributeDB'. You can analyse, how this map is built, or you can first take a look, how it's used:

for (int i=0;i<q;i++) {
    getline(cin,inputstr); 
    if (attributeDB.find(inputstr)==attributeDB.end())
        cout << "Not Found!" << endl;
    else
        cout << attributeDB[inputstr] << endl;
}

Yes, it still works. Because it resets "tag_preamble" when the closed tag is encountered:

pos=tag_preamble.find("."+tag);
if (pos==string::npos) tag_preamble="";
else tag_preamble=tag_preamble.substr(0,pos);

So, if it doesn't find the opening part of the tag in the format of "something.tag" (meaning it is a sub-tag) in the first line (so pos==string::npos), that means this is the primary tag, so it resests it to nothing (tag_preamble="").

If it does find the tag in that format ("something.tag"), that means this is a sub-tag, so it erases the ".tag" part (leaving only the "something", which will be retained through future line iterations, as it is declared outside of the main for-loop), removing it from the pathway hierarchy for future elements when they are added to the map.

The elements are added to the map based on the string generated from tag_preamble. This is how they are pulled for printing later. So any final values would have the correct, full, corresponding index "pathway" via the [string] (tab_preamble) used to access the map.

Here

When you call string::find and when it doesn't match anything, it returns string::npos which equal to literally -1. The type of npos here is comparable to size_t.

On second thought, a vector would probably be better here because the head to tail to traversal would have better locality. However, this would come with the downside that push_back would be only amoritized O(1) instead of strict O(1) unless capacity can be guessed from the maximum nesting level of tags (+1 for the attribute)