C++ templates reading type from configuration file at runtime

I've been trying to write a piece of code that reads a configuration file at runtime, and this file contains the names of different variables, and their type. These types are almost all builtin (int, unsigned int, char*, unsigned long long, float), with the exception of one class which stores information as binary flags.

Example configuration file contains:

Name std::string Measurement

PositionX float 0.004

PositionY float 0.002

Time unsigned long long 1521479000

Function which needs this information:

std::vector<float> xPos;
...
while( loops_over_a_file_containing_values );
...
xPos.push_back(get_information["PositionX"].extract<float>());

However, I cannot for the life of me seem to find a method in which to define these in a C++ code, most of my googling leads me to "This doesn't work" or answers that are so complicated to me that I cannot implement it in a reasonable way.

The main reason behind this is because there are a rather large number of variables with different types, and their type is needed as this function comes from another user written C++ package (which I have no way to modify in any shape or form).

So far I've looked at boost::any, boost::variant but I can't see how this could be done with those. And I'm unsure how exactly to go about this with templates either, I've tried:

template <int ModeDecider> class TypeDecider{ };
template <> class TypeDecider<0>{ public: typedef int Type; };
template <> class TypeDecider<1>{ public: typedef unsigned int Type; };
template <> class TypeDecider<2>{ public: typedef unsigned long long Type; };
template <> class TypeDecider<3>{ public: typedef float Type; };

...

std::string test1 = "unsigned long long";
std::string test2 = "1521479000";

switch( test1 ){
...
   std::vector<TypeDecider<2>::Type> testvec;
   testvec.push_back(static_cast<TypeDecider<2>::Type>(test2));
...}

But this gives me:

cannot convert 'std::__cxx11::string' (aka 'basic_string<char>') to 'TypeDecider<2>::Type' (aka 'unsigned long long') without a conversion operator

Which I'm not exactly sure how to go about fixing. And so far my googling-fu has yielded very little. As such, I'm here asking for help before I go to the awful step of writing a Python script in order to write the C++ script.