Using command line parameters outside of main function

I would like a program to read in a file entered by the user via the command line, which is then used in the main body of the code.

See example code below:

#include <iostream>
#include <fstream>

struct run_t {
    std::string file;
};

run_t run;

const POINTER* ptr = toy(run.file, 0);

// Here hardcoded the FILENAME as a string => code works.
// I am trying to get it to work when I read in the filename from the
// first entry on the command line upon program execution:
//
//const POINTER* ptr = toy("FILENAME", 0);

double Toy1(double a, double b, double c) {
    return ptr->func(a, b, c);
};

double Toy2(double d) {
    double factor = pow(d, 2); //some dummy prefactor
    return factor * Toy1(4, 5, 6);
};

int main(int argc, char* argv[])
{
    run_t run;
    run.file = argv[1];
    std::cout << Toy2(1) << std::endl;
    return 0;
}

The actual toy and Toy functions are software-specific and recognised by additional include-files. If I manually hardcode run.file in toy(run.file, 0) with the string FILENAME, compile and execute with a single command line parameter (./exe 1, say) then the program works.

My question is, how to modify the above code so that the value FILENAME entered in the command line is read in instead as run.file? That is, to make ./exe FILENAME work? I have tried with declaring argv[1] as the argument of toy but I have not yet got this to work.

Here is the code I have tested. Try this

#include <iostream>
#include <fstream>
#include <math.h>

struct run_t {
    std::string file;
};

run_t run;

class POINTER{
  public:
    POINTER(std::string file , int num){std::cout << "POINTER::POINTER(std::string file , int num)file " << file << std::endl;};
    double func(double a, double b, double c) {
        POINTER *toy(std::string file , int num);
        return (double)0;
    }
};


POINTER *toy(std::string file , int num)
{
    POINTER* ptr = new POINTER(file , num);
    std::cout << "POINTER *toy(std::string file , int num) file " << file << std::endl;
    return ptr;
}

const POINTER * ptr;

double Toy1(double a, double b, double c) {
    POINTER * ptrCst = const_cast<POINTER *>(ptr);
    return ptrCst->func(a, b, c);
};

double Toy2(double d) {
    double factor = pow(d, 2); //some dummy prefactor
    return factor * Toy1(4, 5, 6);
};

int main(int argc, char* argv[])
{
    run.file = argv[1];
    POINTER * ptrCst = const_cast<POINTER *>(ptr);
    ptrCst = toy(run.file, 0);
    std::cout << Toy2(1) << std::endl;
    while(1);
    return 0;
}