error_code.md

std::error_code

std::error_code in C++ is a part of the <system_error> header and represents an error condition in a program, typically used to report and handle error situations in a more flexible and extendable way compared to traditional methods like using integer error codes or exceptions.

• It combines error codes with an error category, making it possible to understand and manage errors across different domains and libraries.
• It is particularly useful in performance-critical code or when working in environments where exceptions are not suitable.

Key Concepts:

1. Error Code and Error Category:

   • std::error_code consists of two parts: an error value (an integer) and an error category (std::error_category).
   • The error value represents the specific error.
   • The error category represents the domain or context in which the error value is meaningful, such as system errors, application-specific errors, etc.

2. Error Handling:

   • std::error_code provides a way to handle errors without throwing exceptions. This is particularly useful in contexts where exceptions are either not allowed or not preferred, such as in real-time systems or performance-critical code.

3. Comparison:

   • You can compare std::error_code objects directly to check if two error codes represent the same error.

4. Conversion to std::error_condition:

   • std::error_code can be converted to a more generic std::error_condition, which represents a portable error condition, independent of the platform-specific error codes.

Example:

Let's walk through an example to understand how std::error_code can be used in practice.

Example 1: Basic Usage

#include <iostream>
#include <system_error>
#include <fstream>

void open_file(const std::string& filename, std::error_code& ec) {
    std::ifstream file(filename);
    if (!file) {
        ec = std::make_error_code(std::errc::no_such_file_or_directory);
        return;
    }
    // Process file...
    ec.clear();  // No error
}

int main() {
    std::error_code ec;
    open_file("non_existent_file.txt", ec);
    
    if (ec) {
        std::cout << "Error opening file: " << ec.message() << " (Error code: " << ec.value() << ")\n";
    } else {
        std::cout << "File opened successfully.\n";
    }

    return 0;
}

Explanation:

1. std::errc::no_such_file_or_directory:

   • This is an enumerator from the std::errc enumeration, which contains platform-independent error codes.

2. std::make_error_code:

   • This function converts the std::errc value to a std::error_code.

3. Error Message:

   • ec.message() returns a human-readable error message corresponding to the error code.
   • ec.value() returns the actual error value.

In this example, if the file "non_existent_file.txt" does not exist, the function open_file sets the std::error_code to indicate that the file was not found, and the error is reported in main.

Example 2: Using std::system_category

std::system_category() represents the operating system's error codes, allowing you to work directly with native error codes.

#include <iostream>
#include <system_error>
#include <cerrno>
#include <cstdio>

void delete_file(const std::string& filename, std::error_code& ec) {
    if (std::remove(filename.c_str()) != 0) {
        ec = std::error_code(errno, std::system_category());
    } else {
        ec.clear();  // No error
    }
}

int main() {
    std::error_code ec;
    delete_file("non_existent_file.txt", ec);
    
    if (ec) {
        std::cout << "Error deleting file: " << ec.message() << " (Error code: " << ec.value() << ")\n";
    } else {
        std::cout << "File deleted successfully.\n";
    }

    return 0;
}

Explanation:

1. errno:

   • This global variable is set by certain system calls and library functions in the C standard library when an error occurs.

2. std::system_category():

   • It is used here to associate the error code with the system's error category.

In this example, if the file "non_existent_file.txt" does not exist, std::remove sets errno to ENOENT, and we set the std::error_code accordingly. This code is then used to report the error.

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