Simplest way to create a file for writing within a function?

Question

I have a function like so:

void my_func(unordered_map<std::string, std::string> arg){

    //Create/open file object on first call and append to file on every call

    //Stuff
}

and inside this function I wish to write to a file. How can I achieve this without having to create the file object in the caller and pass it in as a parameter? Each time the function is called I would like to append the latest write to the end of the file.

`std::fstream`. Try to open in append mode; if it fails, open in output mode. — ChronoTrigger, May 15 '14 at 18:34

R Sahu · Accepted Answer · 2014-05-15T18:59:20.067

2

void my_func(unordered_map<std::string, std::string> arg){

    static std::ofstream out("output.txt");
    // out is opened for writing the first time.
    // it is available for use the next time the function gets called.
    // It gets closed when the program exits.

}

edited May 15 '14 at 18:59

answered May 15 '14 at 18:35

R Sahu

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@user997112 Use `std::string::c_str()` to get the `char const*` and use it in the call to `write()`. – R Sahu May 15 '14 at 18:46
and just use out << to_write ? – user997112 May 15 '14 at 19:00

score 1 · Answer 2 · answered May 15 '14 at 18:34

Pass two strings/char arrays. One is the file path, the other is the data to be written. Create the file using fstream myFile(fstream::out | fstream::app)

Need anymore explanation? I can write a full example if you would like.

Edit

Forgot to mention, this will do what you want, but you will be creating the file object everytime. You will not be creating a new file everytime though. That's what fstream::app is for. You open the file and start at the end.

score 1 · Answer 3 · answered May 15 '14 at 18:44

Another alternative is to use a functor. This will give you the potential to control the lifetime of the file object and even pass around the function object

#include <string>
#include <fstream>
#include <unordered_map>
struct MyFunc {
    MyFunc(std::string fname) {
        m_fobj.open(fname);
    };
    ~MyFunc() {
        m_fobj.close();
    };
    void operator ()(std::unordered_map<std::string, std::string> arg) {
       // Your function Code goes here
    };
    operator std::ofstream& () {
        return m_fobj;
    };
    std::ofstream m_fobj;
};

int main() {
    MyFunc my_func("HelloW.txt");
    my_func(std::unordered_map<std::string, std::string>());
    std::ofstream &fobj = my_func;
    return 0;
};

Simplest way to create a file for writing within a function?

3 Answers3

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