How Can I Retrieve Detailed Error Messages from Failing `ifstream` Opens?

Retrieving Error Messages in Failing ifstream Opens

When using the ifstream class to open a file, it's essential to handle potential errors. The fail() method can be used to check for open failures, but it doesn't provide any information about the cause of the failure, such as "File not found."

Fortunately, we can access the system's error message to get a more detailed description of the failure. Each system call that fails updates the global errno variable. By utilizing this error, we can gather more information about the failure by using:

cerr << "Error: " << strerror(errno);

In multithreaded applications, this approach requires cautious usage, as any other system call may trigger an error and update errno before the strerror function is called.

For systems adhering to the POSIX standard, the errno value is thread-local, ensuring that setting it in one thread doesn't affect its value in others.

While e.what() may initially appear to be a more C -specific method, the returned string is implementation-dependent. In G 's libstdc , this string doesn't often provide useful failure information.

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