#include <iostream>
#include <string>
#include <map>

int main()
{
    std::string xml("a < > & ' \" string");
    std::cout << xml << "\n";

    // Characters to be transformed.
    //
    std::map<char, std::string> transformations;
    transformations['&']  = std::string("&amp;");
    transformations['\''] = std::string("&apos;");
    transformations['"']  = std::string("&quot;");
    transformations['>']  = std::string("&gt;");
    transformations['<']  = std::string("&lt;");

    // Build list of characters to be searched for.
    //
    std::string reserved_chars;
    for (auto ti = transformations.begin(); ti != transformations.end(); ti++)
    {
        reserved_chars += ti->first;
    }

    size_t pos = 0;
    while (std::string::npos != (pos = xml.find_first_of(reserved_chars, pos)))
    {
        xml.replace(pos, 1, transformations[xml[pos]]);
        pos++;
    }

    std::cout << xml << "\n";

    return 0;
}