Error handling in C and C++

Return codes, errno, exceptions, std::optional and std::expected, and how to choose between them.

There are several ways to report that something failed:

• Return codes (C style): return -1 or NULL and set errno. Simple, but easy to ignore.
• Exceptions (throw / try / catch): errors can't be silently ignored, and RAII cleans up as the stack unwinds. Standard C++ uses them for truly exceptional failures.
• std::optional<T>: "a value or nothing", when the reason doesn't matter.
• std::expected<T, E> (C++23): a value or an error explaining why.

Whatever you pick, be consistent within a codebase, and never let a failure pass silently.

Example

#include <charconv>
#include <iostream>
#include <optional>
#include <string_view>

std::optional<int> parse_int(std::string_view s) {
    int value = 0;
    auto [end, ec] = std::from_chars(s.data(), s.data() + s.size(), value);
    if (ec != std::errc() || end != s.data() + s.size()) return std::nullopt;
    return value;
}

int main() {
    std::cout << parse_int("42").value_or(-1) << " " << parse_int("4x2").has_value() << "\n";
    return 0;
}

Output:

42 0

Practice it

• Lesson: Handling errors