How to get detailed fail messages when using ifstream.fail() in C++

Saviz

Sorry for asking this simple C++ question here in the Qt forum, but I often find I get better answers to my questions here than anywhere else.

I'm working on creating a function in C++ that uses the ifstream object to open a file. However, file operations don't always proceed smoothly. Hence, I'm interested in understanding how I can provide accurate and descriptive error messages to pinpoint precisely what went wrong during this operation. Here's my current code snippet:

void open_file(std::ifstream& ifstream, const std::string& file_name)
{
	ifstream.open(file_name);

	bool success = ifstream.fail();

	if (!success)
	{
	        // This is not really helpful...
		std::cout << "Failed to open file" << std::endl;

		return;
	}

	std::cout << "File was opened successfully!" << std::endl;
};

While this code currently achieves its purpose, it falls short in terms of providing detailed error messages to pinpoint the exact issue that happens. I'm looking for a way to obtain descriptive messages that will precisely reveal what went wrong. How can I achieve this?

JonB

@Saviz
Start by reading e.g. How to get error message when ifstream open fails, it also references other SO topics. Or Google c++ std::ifstream error message.

Saviz

@JonB Thank you for sharing the link to the post. However, it appears that the post is not only outdated but also contains significant ongoing debates about the preferred method between "errno" and "e.what()". Furthermore, some developers suggest that "errno" may only function effectively on Unix-like systems. Do you happen to have a more up-to-date or comprehensive resource that addresses this topic?

James Gallegos

In C++, you can use the std::ifstream::fail() function to check if there was a failure during file operations. However, to get more detailed error messages, you can use the std::ifstream::rdstate() function in combination with std::strerror() or std::perror().

Here's an updated version of your code with more detailed error messages:

#include <iostream>
#include <fstream>
#include <cstring>

void open_file(std::ifstream& ifstream, const std::string& file_name)
{
    ifstream.open(file_name);

    if (ifstream.fail())
    {
        // Get the error code
        std::ios_base::iostate state = ifstream.rdstate();

        // Check for specific error bits
        if (state & std::ios_base::eofbit)
        {
            std::cout << "End of file reached." << std::endl;
        }
        if (state & std::ios_base::failbit)
        {
            std::cout << "Non-fatal I/O error occurred." << std::endl;
        }
        if (state & std::ios_base::badbit)
        {
            std::cout << "Fatal I/O error occurred." << std::endl;
        }

        // Print system error message
        std::perror("Error: ");

        return;
    }

    std::cout << "File was opened successfully!" << std::endl;
}

In this code, we use std::ios_base::iostate to retrieve the error state of the ifstream object. Then, we check specific error bits like std::ios_base::eofbit, std::ios_base::failbit, and std::ios_base::badbit to provide more detailed information about the failure.

The std::perror() function is used to print the system error message corresponding to the error code. This will give you additional information about what went wrong.

Remember to include the necessary headers for this code to work:

#include <iostream>
#include <fstream>
#include <cstring>