Obtaining error messages in C++

Hi! How does one get access to the standard error messages in C++?

In C you can get the error message from strerror(), provided you have the error code. How can something similar be achieved using the iostreams library in C++? I thought that the exception messages could be useful but doesn't seem like it. "Underflow error reading the file" isn't too helpful :-/ And looking for "c++ error message" on the internet gets only pages on compiler error messages :rolleyes:

I'm looking for the equivalent of:

Code:

---

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    // pretend a directory is a file
    FILE* f = fopen("/", "r");
    getc(f);

    // print the resulting error message
    char* error_message = strerror(errno);
    // output: Oopsie: Is a directory
    printf("Oopsie: %s\n", error_message);

    return 0;
}

---

Code:

---

cout << "Output.";  // stdout
cerr << "Error.";  // stderr unbuffered
clog << "Log.";    // stderr buffered

---

I may be wrong about what file descriptor clog uses, but whatever.

Besides, the more common way to output to stderr in C would be:

Code:

---

fprintf(stderr,"Error");

---

EDIT:: SORRY MISUNDERSTOOD THE PROBLEM. CONTINUE TO MY OTHER REPLIES.

I'm not asking about how to print to cerr or clog. I'm asking what to print there, that is, from where I can get the error message---preferrably localized. That is, the string that strerror of C returns.

I think its the same
Just include "cerrno"

Hows this?

Code:

---

#include <iostream>
#include <fstream>
#include <string>
#include <cerrno>

using namespace std;

int main()
{
    ifstream inFile("", ios_base::in);
   
    string error = strerror(errno);
    cerr << error;
    cin.get();
    return 0;
}

---

I believe that C++ dropped the errono setup, and instead uses exceptions for passing error information. Mind you if you use the C libraries, they will still use it.

Or as Xipher said, with exceptions:

Code:

---

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
  int errorVal;
  try {
    ifstream inFile("", ios_base::in);
    throw errorVal;
  }
  catch(int error) {
      if (error == 8)
        cerr << "No such file or directory.";
      else
        cerr << "Unknown Error.";
  }
  return 0;
}

---

Thanks SlyMaelstrom and MadCow, strerror does seem to work even though I had doubts about it.

It doesn't seem to co-operate with exceptions though. Is there a way to get a meaninful error message when you are using exceptions?

That is, using something like

Code:

---

    try
    {
        ifstream in;
        in.exceptions(ifstream::badbit | ifstream::failbit);
        in.open(filename);

        string line;
        while (getline(in, line))
        {
            process(line);
        }
    }
    catch (ios_base::failure &ex)
    {
        // how to access the error message here?
        cout << "Problem: " << strerror(errno) << endl;
        cout << "Problem: " << ex.what() << endl;
    }

---

ex.what seems to give "basic_ios::clear" when the file doesn't exist and "basic_filebuf::underflow error reading the file" when it's a directory. Calling strerror in the catch block gives the real error message when the file doesn't exist (yay!), but "success" when it's a directory.

Oh well, I guess you can't have everything, C-style error handling isn't that bad :p