isNumber with auto

#include <iostream>

bool toNumberConv(std::string str, auto &x)
{
    int dots = 0;
    bool num = true;
    unsigned int i = 0;
    while(i < str.length() && num)
    {
        if ((str[i] == '.') && dots == 0)
        {
            dots = 1;
        }
        else if (!isdigit(str[i]))
        {
            num = false;
        }
        i++;
    }
    if(num && dots == 0)
    {
        x = stoll(str);
        if(x <= 2147483647)
        {
            x = stoi(str);
            std::cout << "Ez egy int\n";
        }
        else
        {
            std::cout << "Ez egy long long\n";
        }
    }
    else if(num && dots == 1)
    {
        x = stod(str);
        std::cout << "Ez egy double\n";
    }
    else
    {
        x = -1;
        std::cout << "Nem sikerult a konvertalas\n";
    }
    return num;
}
/*
int             stoi        -2,147,483,648 to 2,147,483,647
long long int   stoll       -(2^63) to (2^63)-1
double          stod
*/
int main()
{
    std::string str;
    std::cin >> str;

    long double x;

    bool isNum = toNumberConv(str, x);

    if(isNum) std::cout << x << std::endl;
    else std::cout << "Ez nem egy szam\n";

    return 0;
}