Return errors as values with std::expected

std::expected<T, E> carries either a value or an error, so error paths stay visible in the type instead of hiding in exceptions or out-parameters. transform maps the value and passes errors through untouched.

#include <expected>
#include <iostream>
#include <string>

std::expected<int, std::string> parse(const std::string& s) {
    try {
        size_t pos = 0;
        int v = std::stoi(s, &pos);
        if (pos != s.size()) return std::unexpected("trailing characters");
        return v;
    } catch (...) {
        return std::unexpected("not a number");
    }
}

int main() {
    auto ok = parse("21").transform([](int v) { return v * 2; });
    std::cout << (ok ? std::to_string(*ok) : "error: " + ok.error()) << '\n';
    auto bad = parse("21x").transform([](int v) { return v * 2; });
    std::cout << (bad ? std::to_string(*bad) : "error: " + bad.error()) << '\n';
}

Observed output:

42
error: trailing characters

Try this: chain a second transform and watch the error skip both steps.

Facts verified 2026-11-04. The code on this page was compiled and run on the box with GCC 16.2.0 at -std=c++23 -Wall -Wextra; the outputs quoted are the observed outputs. MSVC was not tested.

Back to the daily tips index