help with fstream

I am very new to c++ and need some help with fstream.h
Could someone please tell me how to ask the user for a name and make that the name of a .txt please? I imagine it would be something like this...

int num;
cin>>num;
ofstream a_file(num, ".txt");

etc...

If someone could help with this, or direct me to a advanced tutorial, it would be most appreciated.

Code:

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#include <fstream>
#include <iostream>
#include <string>



int main(){

        std::ofstream out;
       
        std::string str;
       
        std::cout << "Enter the name of the file" << std::endl;
       
        std::cin >> str;
       
        str.append(".txt");
       
        out.open(str.c_str(),std::ios::out);
       
        if(!out.is_open()){
                std::cout << "Could not open" << std::endl;
                return 1;
        }
       
        std::cout << "Success!!! Created " << str << std::endl;
       
        out << "This is text";

}

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