c++ file handling issues

fuzzhead85 · 02-16-2005, 05:48 PM

i was under the assumption that in c++ when you use the open command with ios::in|ios:

ut, if the file does not exist, it will create one for you, and if you try and read in from that blank file, it should fill all the values of the record being read in, to nulls in a linux enviroment. here is a sample piece of code, that does not create a file and always returns read failed. any help would be great.

Code:

#include<iostream>
#include<fstream>
using namespace std;

class Block
{
public:
  int value1;
  int value2;
};

int main()
{
  fstream DVDM;
  Block test;
  DVDM.open("temp", ios::out|ios::in);
  test.value2 = -2;
  //idealy in this read, value2 will be set to null, and therefore 
  //the read would work
  DVDM.read((char*)&test, sizeof(Block));
  if(test.value2 == -2)
    cout << "read failed\n";
  else
    cout << "success\n";
  return 0;
}

dakensta · 02-16-2005, 06:03 PM

No, it won't create a file for you, try:

in | out | trunc

Secondly, read returns a reference to the stream - check the state fo this to see what has happened, either with:

if (your_stream) { /* oops error */ }

or with specific error checking functions e.g. good(), fail(), bad(), rdstate(), if you need to know more

fuzzhead85 · 02-16-2005, 06:23 PM

the ios::trunc worked wonders, but why doesnt value2 get changed?, is there a way to initalize the file so that it will get changed?

dakensta · 02-16-2005, 06:52 PM

Because there is nothing to read from!
You are asking to read some bytes from some underlying buffer which cannot provide them.

You would be better of checking the return and and then handling the error that way:

Code:

if (!DVDM.read((char*)&test, sizeof(Block)))
{
    test.value2 = 0; // or whatever ...
    DVDM.clear();    // need to clear the failbits before anything else can happen
}

Alternatively you might want to check how much is in the file with seekg(0, end) and tellg()