Question

I want to write a file_handle class like this

#include <stdio.h>
#include <iostream>
#include <string>

//===========================================================================
class File_handle {
    FILE* p;
    public:
    File_handle(const char* pp, const char* r) {
    p = fopen(pp, r);
    }
    File_handle(const std::string& s, const char* r) {
    p = fopen(s.c_str(), r);
    }
    ~File_handle() {
    if (p) 
    fclose(p);
    }
    // what is this ?????    i found it in stroustrups c++ on page 389
    // ---> Mark A
    operator FILE*() { return p;} 


    // I have tried something like this, but without any success
    // ---> Mark B
    //friend std::ostream& operator<<(std::ostream&, const File_handle&); 
};

//===========================================================================
int main() {
    const std::string filename("test.txt");
    File_handle fh(filename, "w");
    // doesn't work
    // fh << "hello\n";
    // *fh << "hello\n";
    // this works now
    File_handle fA("A.txt", "w");
    fprintf(fA, "say hello to A.txt");
    File_handle fB("B.txt", "w");
    fputs("say hello to B.txt", fB);
    return 0;
}

I don’t know, how I can use the file stream outside the class.
I have tried it with overloading the operator<<, how you can see in the code example above. (Indicated as: mark B in the code above.)

And I found the line operator FILE*() { return p; } in the book of Bjarne Stroustrup.
But which operator is overloaded in this line? (Indicated as: mark A)

Answer 1

You should use std::fstream instead of your File_handle class.

But if you need to specify some additional behaviour, you can derive your class from std::fstream. So this kind of solution could look like this:

#include <iostream>
#include <fstream>

class File_handle : public std::fstream
{
public:
    File_handle(const char* filename, _Openmode o) { open(filename, o); }
    ~File_handle() { close(); }
};

int main()
{
    File_handle fh("test.txt", std::ios::out);
    fh << "aa";
    return 0;
}

Hope this helps.