String--getline()

#include <string>
#include <sstream>
#include <iostream>

int main()
{
    std::wstring keyname = L"HKEY_LOCAL_MACHINE\\SOFTWARE\\Mozilla\\Firefox";
    std::wstring root, key;

    std::wistringstream iss(keyname);
    if (std::getline(iss, root, L‘\\‘) && std::getline(iss, key))
    {
        std::wcout<<root<<" "<<key;
    }
}

#include <string>

std::wstring keyname = ...; // "HKEY_LOCAL_MACHINE\\SOFTWARE\\Mozilla\\Firefox"

std::wstring::size_type pos = keyname.find(L‘\\‘);
if (pos != std::wstring::npos)
{
    std::wstring root = keyname.substr(0, pos);
    std::wstring key = keyname.substr(pos+1);
    // use root and key as needed...
}