9.2 Working with File Streams

There are two ways to create a file stream¹¹: you can create an empty file stream, open a file, and connect it to the stream later on; or you can open the file and connect it to a stream at construction time. These two procedures are demonstrated in the two following examples, respectively:

ifstream file;                                                //1
;
file.open(argv[1]);                                           //2
if (!file) // error: unable to open file for input

or:

ifstream source("src.cpp");                                   //3
if (!source) // error: unable to open src.cpp for input

Generally you can check whether the attempt to open a file was successful by examining the stream state afterwards; failbit will be set in case of failure.

There is also a function called is_open()that indicates whether a file stream is connected to an open file. This function does not mean that a previous call to open() was successful. To understand the subtle difference, consider the case of a file stream that is already connected to a file. Any subsequent call to open() will fail, but is_open() will still return true, as shown in the following code:

void main(int argc, char* argv[])
{
  if (argc > 2)
  {  ofstream fil;                                            //1
     fil.open(argv[1]);
     // ...
     fil.open(argv[2]);                                       //2
     if (fil.fail())                                          //3
     {  // open failed  }
     if (fil.is_open())                                       //4
     {  // connected to an open file  }
  }
}

In the example above, it would be advisable to close the file stream before you try to connect it to another file. This is done implicitly by the file streams destructor in the following:

void main(int argc, char* argv[])
{
  if (argc > 2)
  {  ofstream fil;
     fil.open(argv[1]);
     // ...
  }                                                           //1
  {  ofstream fil;
     fil.open(argv[2]);
    // ...
  }
}

You can explicitly close the connected file. The file stream is then empty, until it is reconnected to another file:

ifstream f;                                                   //1
for (int i=1; i<argc; ++i)
{
    f.open(argv[i]);                                          //2
    if (f)                                                    //3
    {
        process(f);                                           //4
        f.close();                                            //5
    }
    else
        cerr << "file " << argv[i] << " cannot be opened.\n";
}

©Copyright 1998, Rogue Wave Software, Inc.
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