There's no facility to query multi-valued keys in Property Tree. (Partly because most of the supported backend formats do not officially support duplicate keys).
However, you can iterate through child elements, so you can implement your own query, like so:
for (auto& child : pt.get_child("Clients"))
if (child.first == "Client")
std::cout << child.second.get<std::string>("<xmlattr>.name", "No name") << "\n";
See fully working sample Live On Coliru:
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>
#include <sstream>
#include <iostream>
using boost::property_tree::ptree;
int main()
{
std::stringstream ss("<Clients>\n"
" <Client name=\"Alfred\" />\n"
" <Client name=\"Thomas\" />\n"
" <Client name=\"Mark\" />\n"
"</Clients>");
ptree pt;
boost::property_tree::read_xml(ss, pt);
for (auto& child : pt.get_child("Clients"))
{
if (child.first == "Client")
std::cout << child.second.get<std::string>("<xmlattr>.name", "No name") << "\n";
}
};