Convert String Number

#include <sstream>
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cctype>
using namespace std;

string convert_to_string(float number)
{
    stringstream ss;
    ss << number;
    return ss.str();
}


float convert_to_number(string s)
{
    float number;
    stringstream ss (s);
    ss >> number;
    return number;
}

int check_to_number(string s)
{
    int check = 1;
    for (int i = 0; i<s.length(); i++)
        if (!isdigit(s[i]) && s[i] != '.')
        {
            check = 0;
            break;
        }
    return check;
}

int main()
{
    string s;
    float number;

    cin>>number;
    s = convert_to_string(number);
    cout<<s<<endl;

    fflush(stdin);
    getline(cin,s);


    if (check_to_number(s))
    {
        number = convert_to_number(s);
        cout<<number*2;
    }
    else cout<<"This isn't a number !";
    return 0;
}