ofstream doesn't open, or write to files

问题

I've been looking at this for hours and I just know the answer is simple. It seems no matter what I do I cannot open a file. It's a multi-class program so in the header I have

#include <iostream>
#include < fstream>
class A{
  string path;

  A(string p): path(p){}
  ...
  ...
  void PrintToFile();
  void PrintBase();
  void PrintNext();
  ...
  ...
};

and in the cpp file I have

#include "A.h"

void A::PrintToFile(){

  ofstream f(path.c_str(), ios::out);
  assert(f.is_open);

  f << "markuptext" << endl;
  PrintBase();
  f << "endtag" << endl;
  f.close();
}


void A::PrintBase(){

  ofstream f(path.c_str(), ios::app);
  assert(f.is_open);

  f << "markuptext" << endl;
  f << somevale << endl;
   PrintNext();
  f << "endtag" << endl;
  f.close()
}

void A::PrintNext(){

  ofstream f (path.c_str(), ios::app);
  assert(f.is_open);

  f << "markuptext" << endl;
  f << somevalue << endl;
  f << "endtag" << endl;
  f.close()
}

I've played around with the flags on the constructors and with the open commands as well. And once it managed to open a file, but it never wrote anything to the file. If you have any insights I'd much appreciate it.

edit

Thanks for all the help guys, looks like I was trying to open a file with "". But even now after I've got that straightened out, my code is not writing to that open file. I checked my permissions and I'm doing chmod a+rwx... well here's the code in more detail.

#ifndef XML_WRITER_H
#define XML_WRITER_H

#include "WordIndex.h"
#include "PageIndex.h"
#include "StringUtil.h"
#include "CS240Exception.h"
#include <iostream>
#include <fstream>



/* prints out the wordIndex to an xml file
*/

class XMLWriter{
private:
    WordIndex * wIndex;
    PageIndex * pIndex;
    URL baseurl;
    //const char * file;
    ofstream f;
public:

  XMLWriter();
  XMLWriter(string base);
  XMLWriter(XMLWriter & other){
      assert(&other != NULL);
      Init(other);
  }
  XMLWriter & operator =(XMLWriter & other){
      Free();
      Init(other);
  }
  ~XMLWriter(){
      Free();
  }

  void Load(WordIndex & wi, PageIndex & pi);


  //prints to the file
  void Print(char * ofile);

private:
  void Init(XMLWriter & other){
    baseurl = other.baseurl;
    wIndex = other.wIndex;
    pIndex = other.pIndex;

  }
  void Free(){
  }

  void PrintWebsite();
  void PrintStartURL();
  void PrintPages();
  void PrintIndex();
  void PrintWord(OccurenceSet ocs);
  void PrintValue(string s);
  void PrintOccurence(Occurence o);
  void PrintPage(Page & page );
  void PrintDescription(string dscrptn );
  void PrintValue(int n );
  void PrintURL(URL url );

};
#endif

.cpp file

#include "XMLWriter.h"

XMLWriter::XMLWriter(){
}

XMLWriter::XMLWriter( string base): baseurl(base){
//cout << "filename : " << filename << endl;
//file =  filename.c_str();
//cout << "file : " << *file << endl;
}


void XMLWriter::Load(WordIndex & wi, PageIndex & pi){
wIndex = &wi;
pIndex = &pi;
wIndex->ResetIterator();
pIndex->ResetIterator();
}


void XMLWriter::Print(char * filename){

    cout << filename << endl;
    ofstream f(filename);
    if(!f){
      cout << "file : " << filename;
      throw CS240Exception("could not open the file for writing");
    }
    PrintWebsite();
    f.close();

}
//private methods
//
void XMLWriter::PrintWebsite(){


    f <<"<website>\n";
    PrintStartURL();
    PrintPages();
    PrintIndex();
    f << "</website>" << endl;
}

// startURL
//
void XMLWriter::PrintStartURL( ){

    f << "\t" << "<start-url>"<< endl;
    string val = baseurl.Value();
    StringUtil::EncodeToXml(val);
    f << "\t\t" << val << endl;
    f << "\t" << "</start-url>"<< endl;


}

//pages
//
void XMLWriter::PrintPages(){

    f << "\t" << "<pages>"<< "\n";
    while(pIndex->HasNext())
    PrintPage(*(pIndex->Next()));
    f << "\t" <<"</pages>"<<  '\n';

}
void XMLWriter::PrintPage(Page & page ){

    f << "\t\t" <<"<page>"<< endl;
    PrintURL(page.Value());
    PrintDescription(page.Description() );
    f << "\t\t" <<"</page>"<< endl;
}
void XMLWriter::PrintURL(URL url){
    f << "\t\t\t<url>"<< endl;
    f << "\t\t\t\t" << StringUtil::EncodeToXmlCopy(url.Value()) << endl;
    f << "\t\t\t</url>"<< endl;

}
void XMLWriter::PrintDescription(string dscrptn){
    f << "\t\t\t<description>";
    f << StringUtil::EncodeToXmlCopy(dscrptn);
    f << "</description>"<< endl;
}

//index
//
void XMLWriter::PrintIndex(){

    f << "\t<index>"<< endl;
    while(wIndex->HasNext())
        PrintWord(*(wIndex->Next()) );
    f << "\t</index>"<< endl;

}
void XMLWriter::PrintWord(OccurenceSet ocs ){
    f << "\t\t<word>" << endl;
    PrintValue(ocs.Value());
    ocs.ResetIterator();
    while(ocs.HasNext())
        PrintOccurence(*(ocs.Next()) );
    f << "\t\t</word>"<< endl;
}
void XMLWriter::PrintValue(string s ){
    f << "\t\t\t<value>";
    f << StringUtil::EncodeToXmlCopy(s);
    f << "</value>"<< endl;

}

void XMLWriter::PrintOccurence(Occurence o ){

    f << "\t\t\t<occurence>" << endl;
    PrintURL(o.Value()->Value());
    PrintValue(o.NumOfOccur());
    f << "<\t\t\t/occurence>"<< endl;

}
void XMLWriter::PrintValue(int n ){

    f << "\t\t\t\t<count>";
    f << n;
    f << "</count>"<< endl;
}

it won't write anything to the file :( but now it is creating a file so thats a step :-D. obviously I have a data structures and other things backing this up, but I just need to get it writing. Thanks in advance

回答1:

The most obvious problem is that you are opening the file several times. Each instance of the open file will have its own file position and its own buffer. In addition, depending on the system, either all open's but the first will fail (Windows, I think), or the open will truncate the file, effectively erasing any information that might have been written to it. What you should do is have PrintToFile pass the open stream to the functions it calls (recursively); each of these functions should take a std::ostream& (not std::ofstream&) to receive it.

回答2:

One thing I see is that you are opening the file several times. Maybe that would cause trouble. You open it first in PrintToFile then while it is still open there you open it for append in PrintBase. Then while in PrintBase you open it yet again in the call to PrintNext.

Put the ofstream as a member of the class, open it once and refer to it from all three functions.

回答3:

I would use a simple if :

ofstream f (path.c_str(), ios::app);
if(!f)
{
   throw std::exception(std::string("Could not open file : " + path + "for writing"));
}
//write to file here
f.close();

This way you can be sure if the file was opened succesfully or not.

来源：https://stackoverflow.com/questions/7970420/ofstream-doesnt-open-or-write-to-files