Re: ifstream error

Jack wrote:


Bad idea, see FAQ at parashift.com.


Irrelevant, please make minimal examples.


#include <fstream>


The 'ios_base::in' is implicitly given when you have an 'istream', same as
the 'ios_base::out' is given for an 'ostream'.

You are also missing #include <string>, BTW.


Now, this is a question that requires some longer answer. In general, the
best resource for C++ questions is comp.lang.c++.moderated, BTW.

Now, the general advise is to use exceptions for exceptional cases and error
handling. Taking your example of reading a few values from a file and doing
something with it, here just for comparison the two versions using
returnvalues and exceptions.

ifstream in(filename.c_str());//1
if(!in) {
  cerr << "failed to open '" << filename << "'\n";
  return EXIT_FAILURE;
}

datatype data;//2
if( !read_datatype( data, in)) {//3
  if(in.eof())
    cerr << "unexpected end of file\n";
  else
    cerr << "format error in file\n";
  return EXIT_FAILURE;
}

if( !data.transmogrify()) {//4
  cerr << "failed to transmogrify data\n";
  return EXIT_FAILURE;
}

You see that you have around four lines of 'real' code, mixed with five
mostly empty lines and 9 lines of error-checking code. Using exceptions,
this can be made much clearer:

try {
  ifstream in(filename.c_str());//1
  if(!in)
    throw std::runtime_error("failed to open file '" + filename + "'");

  datatype data;//2
  read_datatype(data, in);//3

  data.transmogrify();//4
} catch( std::exception const& e) {
  cerr << "error: " << e.what() << '\n';
  return EXIT_FAILURE;
}

Here, you have the same four lines, with two lines of error handling code in
between and four more wrapping it. The more operations you wrap in one
try{} block, the more error-handling code you concentrate in one place.
Note that if ifstream's constructor would throw exceptions to signal
failure it would have been even shorter. However, failure to open a file is
not really such an exceptional condition which is why the designers chose
to not use exceptions to signal it - opinions vary on that decision.

Uli