启动时清除文件内容，然后追加

Question

I want to append some text in a file with std::fstream . I wrote something like this

class foo() {
   foo() {}
   void print() {
      std::fstream fout ("/media/c/tables.txt", std::fstream::app| std::fstream::out);
      // some fout
   }
};

Problem with this structure is that every time I run my program, the texts are appended to my previous run. For example at the end of the first run the size of the file is 60KB. At the beginning of the second run, the texts are appended 60KB file.

To fix that I want to initialize the fstream in the constructor and then open it in append mode. Like this

class foo() {
   std::fstream fout;
   foo() {
      fout.open("/media/c/tables.txt", std::fstream::out);
   }
   void print() {
      fout.open("/media/c/tables.txt", std::fstream::app);
      // some fout
   }
};

Problem with this code is a 0 size file at during the execution and at the end of run!!

Answer 1

you only need to open the file once :

class foo() {
    std::fstream fout;
    foo() {
        fout.open("/media/c/tables.txt", std::fstream::out);
    }
    void print() {
      //write whatever you want to the file 
    }
    ~foo(){
        fout.close()
    }
};

Answer 2

Your class should look more like this:

#include <fstream>

class Writer
{
public:
    Writer(const char* filename) { of_.open(filename); }
    ~Writer(){ of_.close(); }
    void print() {
        // writing... of_ << "something"; etc.
        of_.flush();
    }
private:
    std::ofstream of_;
};

Note that file stream is being open only once at the moment the Writer object is being constructed and in the destructor close() is called, which also automatically writes any pending output to the physical file. Optionally after each time something is written to the stream, you can call flush() to make sure the output goes to your file ASAP.

Possible usage of this class:

{
    Writer w("/media/c/tables.txt");
    w.print();
} // w goes out of scope here, output stream is closed automatically

Answer 3

ofstream out; // output file object

out.open (fname1,ios::out); // open file

    **out.clear();** // clear previous contents

///////////// code to write file

Eg out<<"hello";