algorithm - Most efficient way to escape XML/HTML in C++ string?

Question

I can't believe this question hasn't been asked before. I have a string that needs to be inserted into an HTML file but it may contain special HTML characters. I want to replace these with the appropriate HTML representation.

The code below works but is pretty verbose and ugly. Performance is not critical for my application but I guess there are scalability problems here also. How can I improve this? I guess this is a job for STL algorithms or some esoteric Boost function, but the code below is the best I can come up with myself.

void escape(std::string *data)
{
    std::string::size_type pos = 0;
    for (;;)
    {
        pos = data->find_first_of(""&<>", pos);
        if (pos == std::string::npos) break;
        std::string replacement;
        switch ((*data)[pos])
        {
        case '"': replacement = "&quot;"; break;   
        case '&':  replacement = "&amp;";  break;   
        case '<':  replacement = "&lt;";   break;   
        case '>':  replacement = "&gt;";   break;   
        default: ;
        }
        data->replace(pos, 1, replacement);
        pos += replacement.size();
    };
}

Answer 1

Instead of just replacing in the original string, you can do copying with on-the-fly replacement which avoids having to move characters in the string. This will have much better complexity and cache behavior, so I'd expect a huge improvement. Or you can use boost::spirit::xml encode or http://code.google.com/p/pugixml/.

void encode(std::string& data) {
    std::string buffer;
    buffer.reserve(data.size());
    for(size_t pos = 0; pos != data.size(); ++pos) {
        switch(data[pos]) {
            case '&':  buffer.append("&");       break;
            case '"': buffer.append(""");      break;
            case ''': buffer.append("'");      break;
            case '<':  buffer.append("&lt;");        break;
            case '>':  buffer.append("&gt;");        break;
            default:   buffer.append(&data[pos], 1); break;
        }
    }
    data.swap(buffer);
}

EDIT: A small improvement can be achieved by using an heuristic to determine the size of the buffer. Replace the buffer.reserve line with data.size()*1.1 (10%) or something similar depending of how much replacements are expected.