ifstream creates file if it doesn't exist

Question

I'm having some trouble writing a Linux console app which reads apache logs.

I need to handle bash script arguments, the last one being a path to the log file. My problem is that if the file doesn't exist, I would like to throw an exception.

But when I try to open the file in read-only mode, instead of failing it creates the file !

Here's the code :

// logreader.h

#include <string>
#include <sstream>
#include <iostream>
#include <fstream>
#include <stdexcept>


class LogReader
{
    public:
        LogReader(int, const char **);
        virtual ~LogReader();

        // ...

    private:
        std::ifstream log_;
};

// logreader.cpp

#include <logreader.h>

LogReader::LogReader(int argc, const char ** argv):
    log_()
{
    log_.exceptions(std::ifstream::failbit | std::ifstream::badbit);
    for (int i = 1; i < argc; ++i)
    {
        std::string arg(argv[i]);
        if (i == argc - 1)
        {
            try
            {
                log_.open(arg.c_str(), std::ifstream::in);
            }
            catch (std::ifstream::failure)
            {
                throw std::runtime_error("The file " + arg + " wasn't opened");
            }
        }
    }
}

LogReader::~LogReader()
{
}

// main.cpp

#include <logreader.h>

int main(int argc, const char ** argv)
{
    LogReader(argc, argv);
    return 0;
}

Script call:

jmcomets $ ./test -g -l
jmcomets $ ls -l
-rw-rw-r-- 1 jmcomets jmcomets     0 Nov 14 22:41 -l 

Answer 1

Since you are opening an std::ifstream it is necessary to add std::ios_base::in (or any other spelling of the std::ios_base::openmode) according to 27.9.1.9 [ifstream.members] paragraph 4: The flag is automatically added by the call to open(). Note that an std::ofstream or an std::fstream would automatically add std::ios_base::out (27.9.1.13 [ofstream.members] paragrpah 3) or std::ios_base::in | std::ios_base::out (27.9.1.17 [fstream.members] paragraph 3), both of which resulting in a new file being created if it doesn't exist (and there are write permissions, etc.).

If the code you posted creates a new file, the implementation of the standard C++ library is wrong: when only the flag std::ios_base::in is specified, the file is open "as if" using the open mode "r" with fopen() (27.9.1.4 [filebuf.members] paragraph 5). fopen() in turn doesn't create a new file when it gets an open mode of "r" (7.21.5.3 paragraph 3).

Answer 2

You can set the failbit in the exceptions flag for the ifstream:

std::ifstream log;
log.exceptions ( std::ifstream::failbit );
try {
    log.open ("test.txt");
}
catch (std::ifstream::failure e) {
    std::cout << "Exception opening/reading file\n";
}

Source

I've tested, and ifstream will throw a failure exception if the file cannot be opened, e.g. file not found, no read permissions. It will open read-only.