C++ - static const variable initialization from .ini file with boost::property_tree

Question

For reading INI configuration file boost:property_tree can be used in the way as follows:

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/ini_parser.hpp>

int main()
    boost::property_tree::ptree pt;
    boost::property_tree::ini_parser::read_ini("config.ini", pt);
    int N = pt.get<int>("Section.ValueOfN");

However I have a class Class which has a static const variable N. The variable is static because there are millions of instances of this class and I do not need each Class to remember the value of N while it is constant and the same for each of them.

If variable Class::N is static const int, then I declare it in Class.hpp and define it in Class.cpp this way:

//Class.hpp
class Class
{
    static const int N;
}

//Class.cpp
const int Class::N = 42;

But what if I want to load it from a configuration file? N is static variable and as such it has to be defined out of any function. On the other side I have to define property_tree inside of function, but if it is inside of function I can't use it to define static const variable.

If it would be this way:

//Class.hpp
class Class
{
    static const int N;
}

//Class.cpp

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/ini_parser.hpp>

boost::property_tree::ptree pt;
boost::property_tree::ini_parser::read_ini("config.ini", pt);
const int Class::N = pt.get<int>("Section.ValuOfN");

I would get an error on the line with read:ini:

error: expected constructor, destructor, or type conversion before ‘(’ token

I can solve it this way:

//Class.hpp
class Class
{
    static const int N;
}

//Class.cpp

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/ini_parser.hpp>

namespace {
int getFromINI(const std::string& section, const std::string& value)
{
    boost::property_tree::ptree pt;
    boost::property_tree::ini_parser::read_ini("config.ini", pt);
    return pt.get<int>(section + "." + value);
}
}

// Define constant static variables
const int Class::N = getFromINI("Section","ValueOfN");

As you can see, this is not an ideal solution. I have to make an property_tree for each static const variable I want to define and the tree is used only once. And of course I have to improve getFromINI function to be able to read other types than int.

Any suggestions?