Efficient way of extracting specific numerical attributes from XML

Question

The application I work uses XML for save/restore purposes. Here's an example snippet:

<?xml version="1.0" standalone="yes"?>
<itemSet>
<item handle="2" attribute1="30" attribute2="blah"></item>
<item handle="5" attribute1="27" attribute2="blahblah"></item>
</itemSet>

I want to be able to efficiently pre-process the XML which I read in from the configuration file. In particular, I want to extract the handle values from the example configuration above.

Ideally, I need a function/method to be able to pass in an opaque XML string, and return all of the handle values in a list. For the above example, a list containing 2 and 5 would be returned.

I know there's a regular expression out there that will help, but is it the most efficient way of doing this? String manipulation can be costly, and there may be potentially 1000s of XML strings I would need to process in a configuration file.

Answer 1

I'd guess a regex of some sort is going to be your best option for efficiency. It's going to be faster than parsing the XML into any sort of structural construct, and as long as you can extract all the information you will need in one pass, it's likely the most efficient method.

Answer 2

You are looking for a stream oriented XML parser that reads each node in your XML one at a a time rather then loading the whole thing into memory.

One of the best known is the SAX - Simple API for XML

Here's a good article describing why to use SAX and also specific of using SAX in C++.

You can think of SAX as a parser of XML that only loads the bare minimum into memory and so works well on very large XML documents. As compare to the Regex or DOM approach that will require you to load the entire document into memory.

Answer 3

It would be hard to beat something like:

/* untested code */
using std::string;
size_t pos = 0;
vector<int> handles;
while ((pos = xmlstr.find("handle=\"", pos)) != string::npos) {
  handles.push_back(atoi(xmlstr.data() + pos + 7));
}

It would be more efficient if handles.reserve() were called with the proper size, or perhaps if handles were a deque or list, depending on how it needs to be used later. This is unsafe code if the xml string might be malformed (xmlstr.data() is not null-terminated, so atoi might go off the end of the array). It also doesn't check that handle isn't the end of a longer attribute name, or indeed whether it is actually an attribute.

Using a regex library for a regular expression like "\\bhandle=\"\\d+\"" is likely to get you results nearly as fast with less likelihood of error. It still doesn't confirm that handle is an attribute; you have to judge if that's likely to be a problem.